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Review
Systematic Literature Review of Challenges in
Blockchain Scalability
Dodo Khan 1, * , Low Tang Jung 1,2 and Manzoor Ahmed Hashmani 1,2
Citation: Khan, D.; Jung, L.T.;
Hashmani, M.A. Systematic
Literature Review of Challenges in
Blockchain Scalability. Appl. Sci. 2021,
11, 9372. https://doi.org/10.3390/
app11209372
Academic Editor: Gianluca Lax
Received: 10 August 2021
Accepted: 3 October 2021
Published: 9 October 2021
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1Department of Computer and Information Science, Universiti Teknologi PETRONAS (UTP),
Seri Iskandar 32610, Malaysia; lowtanjung@utp.edu.my (L.T.J.); manzoor.hashmani@utp.edu.my (M.A.H.)
2High Performance Cloud Computing Center (HPC3), Universiti Teknologi PETRONAS (UTP),
Seri Iskandar 32610, Malaysia
*Correspondence: dodo_18001633@utp.edu.my
Abstract:
Blockchain technology is fast becoming the most transformative technology of recent
times and has created hype and optimism, gaining much attention from the public and private
sectors. It has been widely deployed in decentralized crypto currencies such as Bitcoin and Ethereum.
Bitcoin is the success story of a public blockchain application that propelled intense research and
development into blockchain technology. However, scalability remains a crucial challenge. Both
Bitcoin and Ethereum are encountering low-efficiency issues with low throughput, high transaction
latency, and huge energy consumption. The scalability issue in public Blockchains is hindering
the provision of optimal solutions to businesses and industries. This paper presents a systematic
literature review (SLR) on the public blockchain scalability issue and challenges. The scope of this
SLR includes an in-depth investigation into the scalability problem of public blockchain, associated
fundamental factors, and state-of-art solutions. This project managed to extract 121 primary papers
from major scientific databases such as Scopus, IEEE explores, Science Direct, and Web of Science.
The synthesis of these 121 articles revealed that scalability in public blockchain is not a singular term.
A variety of factors are allied to it, with transaction throughput being the most discussed factor. In
addition, other interdependent vita factors include storages, block size, number of nodes, energy
consumption, latency, and cost. Generally, each term is somehow directly or indirectly reliant on the
consensus model embraced by the blockchain nodes. It is also noticed that the contemporary available
consensus models are not efficient in scalability and thus often fail to provide good QoS (throughput
and latency) for practical industrial applications. Our findings exemplify that the Internet of Things
(IoT) would be the leading application of blockchain in industries such as energy, finance, resource
management, healthcare, education, and agriculture. These applications are, however, yet to achieve
much-desired outcomes due to scalability issues. Moreover, Onchain and offchain are the two major
categories of scalability solutions. Sagwit, block size expansion, sharding, and consensus mechanisms
are examples of onchain solutions. Offchain, on the other hand, is a lighting network.
Keywords:
blockchain; distributed ledger technology scalability; consensus model; scalability solu-
tion; throughput
1. Introduction
Many online transactions between individuals or organizations are based on a central-
ized controlled system (or controlled by a third-party organization). For example, a bank
or a credit card vendor is acting as a third-party entity in executing a digital payment or
money transfer process between two organizations (or individuals). The third-party vendor
takes a fee for every successful transaction. In this centralized mechanism, the third-party
controls and manages almost all the information of the stakeholders that are involved in
the online transaction. This approach requires the third party to uphold the transaction’s
security. In contrast, blockchain is an immutable distributed ledger of cryptographically
Appl. Sci. 2021,11, 9372. https://doi.org/10.3390/app11209372 https://www.mdpi.com/journal/applsci
Appl. Sci. 2021,11, 9372 2 of 27
signed transactions maintained by a peer-to-peer network, where no third party is required
to manage the information, and trust is no longer an issue among the network participants.
Blockchain technology is one of the most hyped decentralized innovations, with an
enlightening future. Blockchain was introduced by Haber and Stornetta [
1
] and later
gained intense attention because of the Bitcoin principle by Namakoto in 2008 [
2
]. Bitcoin
is highly successful in the cryptocurrency arena. Many similar currencies have were
launched following Bitcoin. There were 2017 crypto currencies available on the internet by
2019 [
3
], with different business models. Besides global crypto currency hype, Bitcoin holds
the highest market capitalization up to 53%. Blockchain is serving as the fundamental
technology behind Bitcoin. The survey conducted by World Economic Forum [
4
] showed
that blockchain will be soaring to 10% of global GDP by 2027.
It has been two decades since the launch of Bitcoin as the first public blockchain appli-
cation. Till then, blockchain technology had been restricted to cryptocurrency (Bitcoin and
Ethereum) public blockchains settings. It has hardly been accepted in other industries since.
Among the many hindrances, scalability is found to be the key hurdle in implementing
public blockchains in many real business environments. In general, scalability has not been
well-defined in the literature.
Basically, the scalability issue arises with the increasing number of nodes and transac-
tions in blockchain. This issue is indeed present in major public blockchain applications
(e.g., Bitcoin and Ethereum) because every node needs to store and execute a computational
task to validate every transaction. The public blockchains are therefore always demand-
ing a huge amount of computational power, a high bandwidth internet connection, and
massive storage space. Transaction throughput and transaction latency are the two most
discussed performance metrics in blockchain, and both have not reached a satisfactory
QoS level in many recent popular public blockchain systems. For instance, Bitcoin and
Ethereum are able to process 7 [
5
,
6
] to 20 [
7
] transactions-per-second (TPS), but they also
face high consensus processing time delays (the average time required to mine a block) at a
magnitude of up to 10 min. Besides efficiency, the current size of Bitcoin, Ethereum, and
Litecoin are, respectively, 305.23 GB, 667.10 GB, and 28.45 GB [
8
], causing great demand on
storage spaces. The time needed to download the whole blockchain is considerable.
Several studies deliberated the concept of scalability trilemma [
5
,
8
]. Initially, it was
described by Vitalik Buterin, the co-founder of Ethereum [
9
]. Vitalik stated that trade-offs
are inevitable between three important blockchain properties: decentralization, scalability,
and security (see Figure 1). Decentralization is the core and the nature of blockchain.
Security is an essential propriety, whereas scalability is the main challenge. In other
words, the scalability trilemma states that trade-offs are almost inevitable among these
characteristics of blockchain [10,11].
Appl. Sci. 2021, 11, 9372 2 of 27
transaction’s security. In contrast, blockchain is an immutable distributed ledger of cryp-
tographically signed transactions maintained by a peer-to-peer network, where no third
party is required to manage the information, and trust is no longer an issue among the
network participants.
Blockchain technology is one of the most hyped decentralized innovations, with an
enlightening future. Blockchain was introduced by Haber and Stornetta [1] and later
gained intense attention because of the Bitcoin principle by Namakoto in 2008 [2]. Bitcoin
is highly successful in the cryptocurrency arena. Many similar currencies have were
launched following Bitcoin. There were 2017 crypto currencies available on the internet
by 2019 [3], with different business models. Besides global crypto currency hype, Bitcoin
holds the highest market capitalization up to 53%. Blockchain is serving as the fundamen-
tal technology behind Bitcoin. The survey conducted by World Economic Forum [4]
showed that blockchain will be soaring to 10% of global GDP by 2027.
It has been two decades since the launch of Bitcoin as the first public blockchain ap-
plication. Till then, blockchain technology had been restricted to cryptocurrency (Bitcoin
and Ethereum) public blockchains settings. It has hardly been accepted in other industries
since. Among the many hindrances, scalability is found to be the key hurdle in imple-
menting public blockchains in many real business environments. In general, scalability
has not been well-defined in the literature.
Basically, the scalability issue arises with the increasing number of nodes and trans-
actions in blockchain. This issue is indeed present in major public blockchain applications
(e.g., Bitcoin and Ethereum) because every node needs to store and execute a computa-
tional task to validate every transaction. The public blockchains are therefore always de-
manding a huge amount of computational power, a high bandwidth internet connection,
and massive storage space. Transaction throughput and transaction latency are the two
most discussed performance metrics in blockchain, and both have not reached a satisfac-
tory QoS level in many recent popular public blockchain systems. For instance, Bitcoin
and Ethereum are able to process 7 [5,6] to 20 [7] transactions-per-second (TPS), but they
also face high consensus processing time delays (the average time required to mine a
block) at a magnitude of up to 10 min. Besides efficiency, the current size of Bitcoin,
Ethereum, and Litecoin are, respectively, 305.23 GB, 667.10 GB, and 28.45 GB [8], causing
great demand on storage spaces. The time needed to download the whole blockchain is
considerable.
Several studies deliberated the concept of scalability trilemma [5,8]. Initially, it was
described by Vitalik Buterin, the co-founder of Ethereum [9] . Vitalik stated that trade-offs
are inevitable between three important blockchain properties: decentralization, scalabil-
ity, and security (see Figure 1). Decentralization is the core and the nature of blockchain.
Security is an essential propriety, whereas scalability is the main challenge. In other
words, the scalability trilemma states that trade-offs are almost inevitable among these
characteristics of blockchain [10,11].
Figure 1. Scalability trilemma.
Figure 1. Scalability trilemma.
Appl. Sci. 2021,11, 9372 3 of 27
For example, in Bitcoin, minimizing latency may increase the transaction throughput,
but it would make the security vulnerable due to the high chance of the forks forming in
public blockchains. It is therefore essential to find a balance between all those three aspects
of the blockchain and to consider the requirements of public blockchain applications.
In this article, we performed SLR on the latest research works on the scalability of
public blockchains. The SLR started off with a deep investigation into the scalability issue in
major public blockchain applications to identify the impacts of the adoption of blockchain
technology in areas/fields other than cryptocurrency, after which the potential factors
associated with challenges in transaction throughput, energy consumption, number of
nodes, latency, storage, etc., were explored and tracked. All the scalability-interdependent
elements were systemically scrutinized and linked to the public blockchain consensus
mechanism. Many researchers have attempted to address this issue in one of two ways:
on-chain or off-chain. Sagwit, block size increases, sharding, and consensus mechanisms
are examples of on-chain solutions. Off-chain, on the other hand, is a lighting network.
Researchers also studied these examples’ impact on blockchain implementations.
The main contribution is two-fold. First, we provide a comprehensive review of public
blockchain scalability with a special focus on the critical factors causing the scalability issue,
and the related mitigation approaches. Many surveys and reviews have been released on
the blockchain scalability challenge in the context of cryptocurrencies, as well as studies
that only look at how to address scalability problems with different implementations;
however, the problem still exists. Secondly, this SLR is attempting to build a comprehensive
knowledge base in the field of public blockchain scalability that could be beneficial to those
researchers interested in seeking ways to solve the public blockchain scalability problem in
the context of time-critical applications.
This SLR (systematic literature review) consists of nine sections and is structured as
follows: Section 1introduces blockchain technology. Section 2discusses the key features
and the structure of blockchain. Section 3discusses related work. Section 4presents
the research methodology, the research question, and the data collection procedure. The
findings of this SLR are discussed in Section 5. Sections 6–8exemplify the relevant research
on blockchain scalability based on our research questions, and Section 9concludes this
paper.
2. Blockchain Overview
Blockchain technology is considered “revolutionarily” that it is highly likely to disrupt
technology ecosystem in offering feasible solutions for securing data due to these strengths:
decentralized features, secure data storing capability, lack of trust, data transaction au-
ditability, and transparent data processing. It offers data immutability against different
attacks, and provides more advanced data privacy, data security, and data integrity. It is
believed that blockchain technology will potentially disrupt every industry that exists and
drastically change all aspects of our lives [11–13].
A blockchain is a decentralized shared database maintained by a computer node in a
peer-to-peer network. The records in the original bitcoin blockchain include transactions
between parties concerned with crypto-currency transfer. Both the parties are assigned a
private key and public key as per the public key infrastructure (PKI) [
12
]. The identity or
transaction address of the parties is established by the public key hash value. Transaction
parties use their private keys to sign transactions, and other parties can then verify them
with the public key of the signator. The transactions are sent to all peer nodes in the network
for validation purposes [
11
,
14
–
16
]. Peer nodes agree on the validated transactions by means
of a distributed consensus technique and the sequence in which they should occur. The
transactions are recorded in a data format called a “block” [
12
] and are then committed
to a shared database to form a linked chain. Each block in the blockchain has a separate
timestamp and a cryptographic hash connecting it to the previous block. Blocks cannot be
removed but can be added to a chain [
14
]. A block of information may be manipulated
Appl. Sci. 2021,11, 9372 4 of 27
by peer nodes on the blockchain, and a common database with an ever-growing list of
immutable and irreversible records can be created [17].
Distributed consensus mechanism is critical for blockchain since it determines which
block can be accepted and inserted into the chain. This is similar to agreeing on distributed
power allocation because the node authoring the accepted block can change the state of the
database shared by every other peer. To prevent abuse, the power distribution mechanism
must be linked to cost and resources. The proof-of-work technique used by the initial
Bitcoin blockchain requires nodes to compete by solving a cryptographically complicated
puzzle. This puzzle feature ensures three properties: a node must invest a commensurate
amount of processing resources to complete it, the next node to successfully solve the
puzzle is chosen at random, and a node’s claim to having found the puzzle’s answer can
be easily validated by any other peer nodes. One further concern, however, is the random
choice of malicious nodes controlled by an attacker as the official validator, provided they
are in line with the same procedure. Once selected, a rogue node might still be able to insert
itself into the blockchain blocks of fake transaction data. Thus, after a peer node receives
the block proposed by the official validator, there is an implicit consensus follow-up. In that
stage, pair nodes can verify the transactions in the new block received and can maintain
the previous condition of the blockchain without accepting a new block, if there are any
anomalies (e.g., discrepancy of related hash values or missing transaction signing and
identity). Otherwise, the node will confirm the new block and accept an updated blockchain
if all goes properly. With the amount of acceptance confirmations it receives from several
nodes, the probability of a block being rejected decreases exponentially. After a certain
number of confirmations, “5–6 in the case of Bitcoin”, the block is deemed permanent
(occasionally it may take over an hour for this process to be completed). The cost of a
blockchain relies on the computer resources needed for the mining process. The miners,
who produce new blocks in combination with unrecorded transactions, receive a blockchain
of this kind to create new blocks. Mining companies are competing to solve a mathematical
riddle for gaining a price, and substantial investment in computer resources is needed for
mining in blockchain. The prices of electricity for trading in P2P energy typically fall below
the prices of energy purchases from a business of utilities [18].
The blockchain is basically made up of a series of cryptographically linked blocks
carrying a list of transactions such as the traditional public ledger [
12
]. Figure 2illustrates
a simplified example of a blockchain structure. Every block points to a similar related block
by a relation that is conceptually the hash value of the previous block called the parent
block. The first block in the chain is referred as Genesis block, and it has no prior block [
17
].
Appl. Sci. 2021, 11, 9372 4 of 27
cannot be removed but can be added to a chain [14]. A block of information may be ma-
nipulated by peer nodes on the blockchain, and a common database with an ever-growing
list of immutable and irreversible records can be created [17].
Distributed consensus mechanism is critical for blockchain since it determines which
block can be accepted and inserted into the chain. This is similar to agreeing on distributed
power allocation because the node authoring the accepted block can change the state of
the database shared by every other peer. To prevent abuse, the power distribution mech-
anism must be linked to cost and resources. The proof-of-work technique used by the in-
itial Bitcoin blockchain requires nodes to compete by solving a cryptographically compli-
cated puzzle. This puzzle feature ensures three properties: a node must invest a commen-
surate amount of processing resources to complete it, the next node to successfully solve
the puzzle is chosen at random, and a node’s claim to having found the puzzle’s answer
can be easily validated by any other peer nodes. One further concern, however, is the
random choice of malicious nodes controlled by an attacker as the official validator, pro-
vided they are in line with the same procedure. Once selected, a rogue node might still be
able to insert itself into the blockchain blocks of fake transaction data. Thus, after a peer
node receives the block proposed by the official validator, there is an implicit consensus
follow-up. In that stage, pair nodes can verify the transactions in the new block received
and can maintain the previous condition of the blockchain without accepting a new block,
if there are any anomalies (e.g., discrepancy of related hash values or missing transaction
signing and identity). Otherwise, the node will confirm the new block and accept an up-
dated blockchain if all goes properly. With the amount of acceptance confirmations it re-
ceives from several nodes, the probability of a block being rejected decreases exponen-
tially. After a certain number of confirmations, “5–6 in the case of Bitcoin”, the block is
deemed permanent (occasionally it may take over an hour for this process to be com-
pleted). The cost of a blockchain relies on the computer resources needed for the mining
process. The miners, who produce new blocks in combination with unrecorded transac-
tions, receive a blockchain of this kind to create new blocks. Mining companies are com-
peting to solve a mathematical riddle for gaining a price, and substantial investment in
computer resources is needed for mining in blockchain. The prices of electricity for trad-
ing in P2P energy typically fall below the prices of energy purchases from a business of
utilities [18].
The blockchain is basically made up of a series of cryptographically linked blocks
carrying a list of transactions such as the traditional public ledger [12]. Figure 2 illustrates
a simplified example of a blockchain structure. Every block points to a similar related
block by a relation that is conceptually the hash value of the previous block called the
parent block. The first block in the chain is referred as Genesis block, and it has no prior
block [17].
Figure 2. An example of blockchain consisting of a continuous linked block.
A block is fundamentally divided into 2 sections, the block body and the block
header. Block header carries these metadata: block version, Prev-hash, timestamp, Merkle
tree, bits, and Nonce value, whereas the block body comprises a list of transactions. The
quantity of the transactions is entirely dependent on the size of the block.
Figure 2. An example of blockchain consisting of a continuous linked block.
A block is fundamentally divided into 2 sections, the block body and the block header.
Block header carries these metadata: block version, Prev-hash, timestamp, Merkle tree, bits,
and Nonce value, whereas the block body comprises a list of transactions. The quantity of
the transactions is entirely dependent on the size of the block.
In general, there are two types of blockchain: the permissioned and the permission-
less. The permissionless blockchain is open for anyone to join and perform transactions.
It permits all participants to take part in the consensus process in forming the chain.
Bitcoin [
2
] and Ethereum [
13
] are the classic examples of permissionless blockchain. Per-
missioned blockchain is a private chain allowing only proprietary users to join and perform
Appl. Sci. 2021,11, 9372 5 of 27
transactions. Users may belong to one or more organizations. In case of more than one
organization, the chain is referred as consortium-based blockchain. Hyperledger fabric [
14
]
and multichain [
15
] are examples of private and consortium-based blockchain in which the
number of participations is restricted.
2.1. Digital Signature
A digital signature [
7
] is an asymmetric cryptography deployed in a trustless environ-
ment. Every block in the blockchain is assigned a public and private key. The private key
is like a password that no one has access to except the owner, and it is needed to sign-in
to blockchain to perform the transaction. The public key is visible to every node in the
blockchain network, and it is used to access the signed transaction that was broadcasted
across the entire network. The digital signature works in two phases for signing into a
transaction.
2.2. Consensus Mechanism
The consensus mechanism specifies how to have every node to agree on the veri-
fied state of the ledger. It is a process of approving/verifying transactions to avoid the
double-spending issue. The consensus mechanism is characterized as either proof-based
or voting-based mechanism. Proof-based is mostly utilized in public/permissionless
blockchain. Proof-of-work (PoW) protocol is well recognized as the proof-based consensus
mechanism. However, it suffers from high energy consumption and required specialized
hardware and resources. It consumes more computation energy in verifying blockchain
transactions. Proof-of-stake (PoS) is another example of proof-based mechanism. It can
process transactions much faster than PoW but is vulnerable to other possible risks such as
the Agency issues. Ethereum is known to gradually implement the PoS because of reduced
energy consumption and better scalability. The voting-based consensus model is typically
used in private Blockchain. PBFT is one good example of voting-based consensus model.
2.3. The Key Characteristics of Blockchain
This section discusses the key characteristics of blockchain on decentralization, persis-
tency, and auditability.
2.3.1. Decentralization
Decentralization: in the traditional centralized system, a trusted authority is required
to validate every occurring transaction in the network. However, the decentralized envi-
ronment does not support any governing authority or single entity to control the whole
network. All the nodes in the network collectively manage the network, i.e., decentralized
governance. The transaction in blockchain can therefore be accomplished between 2 peers
(P2P) without the approval of a central agency.
2.3.2. Persistency
Each transaction occurring in the blockchain network is spread and stored across the
network, which can amount to 100 s and 1000 s of nodes. It is therefore not possible to
tamper or alter the data in the blockchain [
16
]. In addition, each block must be validated by
every other node in the network and the transaction in those blocks must also be verified.
Data tempering is thus impossible.
2.3.3. Auditability
Every confirmed transaction in blockchain is stored on the Block with a timestamp.
Consequently, it is extremely convenient for other nodes to verify and track the prior
transaction in the distributed network. For example, in Bitcoin, every transaction is
connected to prior transaction through a hashed link that proves the auditability of the
stored data.
Appl. Sci. 2021,11, 9372 6 of 27
3. Related Work
It is noticed that blockchain technology has somehow been restricted to cryptocurren-
cies (Bitcoin and Ethereum). It is yet to be widely accepted in other industries. Among
the many hindrances, scalability is found to be the key hurdle in implementing public
blockchains in many real business environments. In this section, we concisely discuss the
importance of scalability in blockchain. Moreover, we also present the related literature on
works that have been conducted in the past that focused on scalability in blockchain.
There are claims that blockchain is the technology that will disrupt the technology
eco-system. However, it is yet to achieve this outcome due to its scalability issue. Many
researchers have attempted to address the scalability issue by proposing different solu-
tions, but the problem persists. Many surveys and reviews have been published on the
blockchain scalability challenge and on the evaluation of the proposed solutions with dif-
ferent implementations. Recently, the authors of the survey published in [
19
] evaluated the
blockchain scalability challenges in the healthcare domain and went on to provide potential
solutions for those challenges. Similarly, another author in [
4
] published a survey that
broadly classified and compared the blockchain scalability solutions. The review published
in [
17
] sees the author discussing different consensus protocols in blockchain scalability
perspective. Other studies related to blockchain scalability can be found in [
5
–
8
,
20
], and
perhaps more will come. This paper presents a comprehensive novel study that none of
the available surveys and reviews have discussed. This survey evaluates public blockchain
scalability with special focus on the critical factors that are causing the scalability problem.
This survey also comprehensively evaluates the related blockchain scalability mitigation
approaches. We attempt to build a comprehensive knowledge base for public blockchain
scalability due to deal with the scarcity of such comprehensive research in public blockchain
domain. Table 1lists some currently available blockchain scalability surveys and reviews.
Table 1. Some recent survey and review articles on blockchain scalability.
Reference Year Cite Title 1st Authors
[21] 2020 24
Scalability challenges in
healthcare blockchain
system—a systematic review
Ahmad
Akmaluddin
Mazlan
[5] 2020 107 Solutions to scalability of
blockchain: a survey Qiheng Zhou
[8] 2020 28 Scaling blockchains: a
comprehensive survey Abdelatif Hafid
[22] 2019 63 A Survey on the scalability of
blockchain systems Junfeng Xie
[23] 2016 1559
Where is current research on
blockchain technology?—a
systematic review.
Jesse Yli-Huumo
[7] 2018 83 Blockchain and scalability Anamika
Chauhan
[8] 2018 105 A survey of scalability
solutions on blockchain Soohyeong Kim
[24] 2018 305 A survey about consensus
algorithms used in blockchain
Nguyen,
Giang-Truong
[25] 2019 45 Survey: sharding in
blockchains Guangsheng Yu
4. Survey Methodology
This systematic literature review was conducted based on the Preferred Reporting
Items for Systematic Review and Meta-Analyses (PRISMA) [
26
]. The standard guidelines
by Kitchenham [
27
] were applied to align this SLR with the computer science domain.
Figure 3illustrates the complete methodology adopted in this SLR.
This methodology section lays out the methods for this SLR. It includes the formation
of research questions, the eligibility criteria for selecting the most relevant articles, the
Appl. Sci. 2021,11, 9372 7 of 27
knowledge sources, paper search, study collections, and data extraction. Table 2illustrates
the method designed for this SLR.
Table 2. Method for this SLR based on PRISMA protocol.
Title Description
Abstract It provides a concise overview of this paper, which includes the
background of the research, the methodology, and the key findings.
Methodology Research question
Selection criteria
Information sources
Screening process
Data extraction process
Introduction This section presents the existing knowledge base as well as a
straightforward problem statement, and the finding of the study.
Result Discussion This section provides the findings and analyses for the research works
Conclusion Concludes the outcomes of the entire research and provides some
relevant future directions.
This study deeply investigates the public blockchain scalability issue, the factors
involved with it, and the available solutions. A knowledge base is eventually created
based on the issues identified in the investigation. The following research questions are
entirely focused on the scalability of public blockchains scope and are designed through a
methodical discussion among the members in this research team.
Appl. Sci. 2021, 11, 9372 8 of 27
4. Survey Methodology
This systematic literature review was conducted based on the Preferred Reporting
Items for Systematic Review and Meta-Analyses (PRISMA) [26]. The standard guidelines
by Kitchenham [27] were applied to align this SLR with the computer science domain.
Figure 3 illustrates the complete methodology adopted in this SLR.
This methodology section lays out the methods for this SLR. It includes the formation
of research questions, the eligibility criteria for selecting the most relevant articles, the
knowledge sources, paper search, study collections, and data extraction. Table 2 illustrates
the method designed for this SLR.
Table 2. Method for this SLR based on PRISMA protocol.
Title Description
Abstract It provides a concise overview of this paper, which includes the
b
ackground of the research, the methodology, and the key findings.
Methodology Research question
Selection criteria
Information sources
Screening process
Data extraction process
Introduction This section presents the existing knowledge base as well as a
straightforward problem statement, and the finding of the study.
Result Discussion This section provides the findings and analyses for the research
works
Conclusion Concludes the outcomes of the entire research and provides some
relevant future directions.
This study deeply investigates the public blockchain scalability issue, the factors in-
volved with it, and the available solutions. A knowledge base is eventually created based
on the issues identified in the investigation. The following research questions are entirely
focused on the scalability of public blockchains scope and are designed through a me-
thodical discussion among the members in this research team.
Figure 3. Paper search and selection process.
Figure 3. Paper search and selection process.
Appl. Sci. 2021,11, 9372 8 of 27
4.1. Research Questions
•RQ1: How can the scalability issue impact blockchain implementation?
The motive of this study is to investigate and deeply review the public blockchain
scalability issue. RQ1 is therefore aimed to review all relevant papers/information from
the academic research works that are directly correlated to scalability issue to understand
its vital impact on public blockchain implementation.
•RQ2: What vital root factors are causing scalability issue in blockchain?
This question is correlated to the probable reasons and their connections, which are
creating hindrances in leveraging blockchain technology for large-scale implementation.
RQ2 is dependent on RQ1 because it shall lead to the creation of knowledge based on
public blockchain scalability issues and pairing to the intended research directions.
•RQ3: How researchers address the scalability issues in blockchain?
This question seeks to understand the state-of-art solutions pertaining to public
blockchain scalability issues. RQ3 seeks to uncover how other researchers have addressed
scalability in public blockchains. This SLR is looking at studies that have proposed specific
solutions that were either implemented, simulated, or formally proven instead of mere
idea/vision presented in published papers.
4.2. Inclusion and Exclusion Criteria
The inclusion and exclusion criteria in this SLR are structured to solely accept the
documents or articles that directly dealt with the scalability of public blockchains as a
problem. The main scope is the factors that are becoming a hindrance to implement
blockchain on a large scale. In addition, this SLR is also targeting articles that attempt to
solve the scalability issue. The solution should have been implemented or simulated and
is feasible instead of just highlighting the trend or idea without considering the feasible
implementation of the solution. Five key eligibility criteria are designed for this SLR. The
criteria are shown in Table 3.
Table 3. Inclusion and exclusion criteria for this SLR.
Criteria Quantity
1 The study must be original research work instead of a review or a survey paper.
2
The papers focusing on the blockchain scalability issue (directly or indirectly) and
highlighting the relevant reasons/factors.
3Papers proposing a feasible solution aiming to solve the blockchain scalability
(method, technique, model, and framework).
4The proposed solutions have been evaluated (implemented, simulated,
and formal proof).
5 The papers are published in peer-reviewed journals/conference journals.
6 The papers should only be in English language.
4.3. Information and Data Sources
The relevant information and data sources were determined after exhaustive deliber-
ations between research team members based on the literature reviewed. As mentioned,
this study focuses on the fundamental issue of scalability in public blockchains; as such,
we converged on computer science literature in academic journals and conference proceed-
ings for a quality SLR research. The data sources searched include both significant and
focused computer sciences and multidisciplinary databases. Articles were obtained from
the following sources. These sources have maximum coverage of quality articles in our
domain, such as ISI- and Scopus-indexed articles [16,27].
•Scopus;
•IEEE explore;
•Science Direct;
Appl. Sci. 2021,11, 9372 9 of 27
•Web of Science.
4.4. Search Process
As per the PRISMA activity guidelines [
26
], predefined search protocols are essential
to avoid any bias during the article search. As such, our search protocols were designed to
undertake the specific literature search from the above-mentioned publication portals via
their internal search engines.
The keywords used in our search strings were identified after several exhaustive
test searches, and viable keywords were tested exhaustively. Initially, keywords such
as “Blockchain” and “scalability” were applied to different databases. Unfortunately,
it revealed that these keywords are rather too limited in scope. Eventually, different
combinations of keywords were manipulated in the search strings to discover papers
containing explicit technical synonym of “Blockchain” and “scalability”. This approach
was adopted to conduct the search for articles between years 2010 and 2019. That is, all
relevant articles published during these years are included in this SLR.
It was observed that in using the “scalability process” search string, such search string
did not return a uniform hit on every database. By searching in IEEE Explore by using
“Blockchain” and its relevant technical synonyms such as “Bitcoin” and “cryptocurrency”,
it turned out that the search yielded papers well matched to the search requirements.
However, in other databases, these keywords returned a huge number of irrelevant papers
mostly related to the economy and/or cryptocurrency domains. Hence, different search
strings must be designed for different databases. The complete search strings for this SLR
are listed in the Appendix A.
4.5. Screening Process
To ensure the relevancy of every searched article to the research questions, incremental
approach has been used. Therefore, our first step is to screen the downloaded papers
to remove duplicated articles obtained from the different data sources. Then, the title
of every paper was carefully filtered to eliminate irrelevant papers, i.e., those with no
relevancy to the research questions. For example, the search string returned articles related
to blockchain but not discussing the scalability issue; the scope is thus out of this SLR.
However, occasionally it could be rather hard to decide on the relevancy by just reading the
title of the paper. This demanded a more careful reading of the abstract of every paper to
decide the eventual acceptance of the paper. As a matter of fact, our predefined inclusion
and exclusion criteria were vital to screen each article for its relevancy to the RQs.
4.6. Data Extraction
A Microsoft excel form was designed for executing the data extraction process based
on the PRISMA activity guidelines. This was to extract the required information from the
papers with respect to the research questions. The form had three main parts, namely,
the characteristic/demographics of the selected papers, the technical aspect of blockchain
(including its scalability) issue, and the quality assessment of the selected papers. Data
were extracted from all the papers that passed through the quality assessment. The
objective was to record accurately only the needed information from the papers. The quality
assessment was conducted based on the standard PRISMA guidelines. Demographics of
the papers included the title of the article, author(s) of the paper, country of the author,
publication type/place, and publication year. The technical aspects of blockchain included
the information about blockchain and its scalability issue, blockchain type, performance
efficiency, application areas, and factors involved in scalability (throughput, latency, storage,
and bandwidth power efficiency).
It should be highlighted here that the validity of the form is mandatory to ensure
authenticity of the data gathered. The form was therefore rigorously tested on 25 randomly
selected papers and was revised iteratively per each validating process.
Appl. Sci. 2021,11, 9372 10 of 27
5. Discussion on Consolidated Paper
This section discusses the analysis of the 121 selected papers published between 2011
and 2019. It provides insight into the research trend in the last decade on scalability issue
and the available solutions for public blockchains. The discussion focuses on the following:
1.
The distribution of blockchain based publications concerning scalability issue over
time.
2. The distribution of types of blockchain publication.
3. The distribution of countries of publication.
4. The distribution of application areas of blockchain.
To appropriately answer the research questions, the data collected during the data
extraction process were properly compiled, and the demographic data were analyzed for
the mentioned years of publications.
Figure 4illustrates the year-wise analysis of the selected papers. The increasing interest
of academic research on public blockchain scalability is observed in a rising number of
publications over the years. It is noted that majority of the academic research on public
blockchains concerning scalability were in 2018–2019.
There was not much blockchain research until 2015, probably because time was
required for blockchain to gain momentum after the launching of bitcoin in 2008. The
research on blockchain scalability started to emerge in 2016, specially in public blockchains.
The research in scalability emerged in 2016, when there were 10 published articles; by
2019 there were over 60, a factor of 6
×
in 3 years. Over these years, blockchain began to
disrupt more and more applications on a larger scale. It is therefore logical that the research
community should start addressing the burning scalability issue in public blockchains.
Appl. Sci. 2021, 11, 9372 11 of 27
It should be highlighted here that the validity of the form is mandatory to ensure
authenticity of the data gathered. The form was therefore rigorously tested on 25 ran-
domly selected papers and was revised iteratively per each validating process.
5. Discussion on Consolidated Paper
This section discusses the analysis of the 121 selected papers published between 2011
and 2019. It provides insight into the research trend in the last decade on scalability issue
and the available solutions for public blockchains. The discussion focuses on the follow-
ing:
1. The distribution of blockchain based publications concerning scalability issue over
time.
2. The distribution of types of blockchain publication.
3. The distribution of countries of publication.
4. The distribution of application areas of blockchain.
To appropriately answer the research questions, the data collected during the data
extraction process were properly compiled, and the demographic data were analyzed for
the mentioned years of publications.
Figure 4 illustrates the year-wise analysis of the selected papers. The increasing in-
terest of academic research on public blockchain scalability is observed in a rising number
of publications over the years. It is noted that majority of the academic research on public
blockchains concerning scalability were in 2018–2019.
There was not much blockchain research until 2015, probably because time was re-
quired for blockchain to gain momentum after the launching of bitcoin in 2008. The re-
search on blockchain scalability started to emerge in 2016, specially in public blockchains.
The research in scalability emerged in 2016, when there were 10 published articles;
by 2019 there were over 60, a factor of 6× in 3 years. Over these years, blockchain began
to disrupt more and more applications on a larger scale. It is therefore logical that the
research community should start addressing the burning scalability issue in public block-
chains.
Figure 4. Figure illustrates the year wise distribution of published paper in the blockchain domain.
Figure 5 displays the details on the types of publications for the selected articles in
this SLR. The following are the types of publications included (and identified) in this
study.
• Journals;
• Conference proceedings;
• Book chapters;
• Workshops;
• Symposiums.
Figure 4. Figure illustrates the year wise distribution of published paper in the blockchain domain.
Figure 5displays the details on the types of publications for the selected articles in
this SLR. The following are the types of publications included (and identified) in this study.
•Journals;
•Conference proceedings;
•Book chapters;
•Workshops;
•Symposiums.
The findings revealed that most of the publications concerning public blockchain
scalability were published in conferences and journals, i.e., 54 papers out of the 121 articles
were published in conference proceedings, followed by 33 in journal publications. The
remaining articles were in book chapters (20), symposium (8), and workshops (6).
Appl. Sci. 2021,11, 9372 11 of 27
Appl. Sci. 2021, 11, 9372 12 of 27
The findings revealed that most of the publications concerning public blockchain
scalability were published in conferences and journals, i.e., 54 papers out of the 121 articles
were published in conference proceedings, followed by 33 in journal publications. The
remaining articles were in book chapters (20), symposium (8), and workshops (6).
Figure 5. Figure illustrates the analysis of selected paper types in the blockchain domain.
Figure 6 shows the statistics about the geographical distribution of the selected pa-
pers. It is noticed that China leads with 24 articles, followed by the USA with 13 papers
(published in academia or industries). Moreover, India is the 4th leading country with
eight papers, and closely followed by UK, Switzerland, and Australia with eight, seven,
and seven papers, respectively. Only seven countries have over five publications. The rest
are below five. This analysis reflects the interest of the research community on blockchain
scalability around the world.
Figure 6. Figure illustrate the analysis of geographic distribution of selected paper in the blockchain domain.
6. RQ1: How Scalability Issue Can Impact Blockchain Implementation?
Blockchain was made famous by Bitcoin in 2008 [2], and since then it has already
advanced into several industries. There are claims that blockchain is the technology that
will disrupt the technology eco-system. However, it is yet to witness much growth and
disruption apart from cryptocurrency. Figure 7 is about public blockchain applications
except cryptocurrency. From our SLR, cryptocurrency (Bitcoin and Ethereum) is noted as
the state-of-the-art application of public blockchains. Next to cryptocurrency, IoT is the
most discussed blockchain-related application, with 17 publications. Our findings also re-
vealed that although blockchain is very applicable to IoT applications, it is yet to achieve
the preferred outcomes due to scalability issues. Blockchain appeared to be influencing
Figure 5. Figure illustrates the analysis of selected paper types in the blockchain domain.
Figure 6shows the statistics about the geographical distribution of the selected pa-
pers. It is noticed that China leads with 24 articles, followed by the USA with 13 papers
(published in academia or industries). Moreover, India is the 4th leading country with
eight papers, and closely followed by UK, Switzerland, and Australia with eight, seven,
and seven papers, respectively. Only seven countries have over five publications. The rest
are below five. This analysis reflects the interest of the research community on blockchain
scalability around the world.
Appl. Sci. 2021, 11, 9372 12 of 27
The findings revealed that most of the publications concerning public blockchain
scalability were published in conferences and journals, i.e., 54 papers out of the 121 articles
were published in conference proceedings, followed by 33 in journal publications. The
remaining articles were in book chapters (20), symposium (8), and workshops (6).
Figure 5. Figure illustrates the analysis of selected paper types in the blockchain domain.
Figure 6 shows the statistics about the geographical distribution of the selected pa-
pers. It is noticed that China leads with 24 articles, followed by the USA with 13 papers
(published in academia or industries). Moreover, India is the 4th leading country with
eight papers, and closely followed by UK, Switzerland, and Australia with eight, seven,
and seven papers, respectively. Only seven countries have over five publications. The rest
are below five. This analysis reflects the interest of the research community on blockchain
scalability around the world.
Figure 6. Figure illustrate the analysis of geographic distribution of selected paper in the blockchain domain.
6. RQ1: How Scalability Issue Can Impact Blockchain Implementation?
Blockchain was made famous by Bitcoin in 2008 [2], and since then it has already
advanced into several industries. There are claims that blockchain is the technology that
will disrupt the technology eco-system. However, it is yet to witness much growth and
disruption apart from cryptocurrency. Figure 7 is about public blockchain applications
except cryptocurrency. From our SLR, cryptocurrency (Bitcoin and Ethereum) is noted as
the state-of-the-art application of public blockchains. Next to cryptocurrency, IoT is the
most discussed blockchain-related application, with 17 publications. Our findings also re-
vealed that although blockchain is very applicable to IoT applications, it is yet to achieve
the preferred outcomes due to scalability issues. Blockchain appeared to be influencing
Figure 6.
Figure illustrate the analysis of geographic distribution of selected paper in the blockchain
domain.
6. RQ1: How Scalability Issue Can Impact Blockchain Implementation?
Blockchain was made famous by Bitcoin in 2008 [
2
], and since then it has already
advanced into several industries. There are claims that blockchain is the technology that
will disrupt the technology eco-system. However, it is yet to witness much growth and
disruption apart from cryptocurrency. Figure 7is about public blockchain applications
except cryptocurrency. From our SLR, cryptocurrency (Bitcoin and Ethereum) is noted as
the state-of-the-art application of public blockchains. Next to cryptocurrency, IoT is the
most discussed blockchain-related application, with 17 publications. Our findings also
revealed that although blockchain is very applicable to IoT applications, it is yet to achieve
the preferred outcomes due to scalability issues. Blockchain appeared to be influencing
Appl. Sci. 2021,11, 9372 12 of 27
and disrupting many other industries such as finance, resource management, healthcare,
education, and agriculture [17].
Every selected paper discusses and states that scalability is a critical issue in public
blockchains. These papers manifested the fact that blockchain is yet to be able to “scale-up”
at par to the centralized infrastructure, be it in cryptocurrency or some other applica-
tions. Scalability in public blockchains is recognized as the vital issue that is affecting the
performance and efficiency in the blockchain associated applications.
The following section discusses the scalability issue based on the consolidated data
from the articles reviewed. This is to justify the impact of scalability in the top three public
blockchain-associated applications, namely, Bitcoin, Ethereum, and IoT.
Appl. Sci. 2021, 11, 9372 13 of 27
and disrupting many other industries such as finance, resource management, healthcare,
education, and agriculture [17].
Every selected paper discusses and states that scalability is a critical issue in public
blockchains. These papers manifested the fact that blockchain is yet to be able to “scale-
up” at par to the centralized infrastructure, be it in cryptocurrency or some other applica-
tions. Scalability in public blockchains is recognized as the vital issue that is affecting the
performance and efficiency in the blockchain associated applications.
The following section discusses the scalability issue based on the consolidated data
from the articles reviewed. This is to justify the impact of scalability in the top three public
blockchain-associated applications, namely, Bitcoin, Ethereum, and IoT.
Figure 7. Blockchain application domain per selected papers.
Scalability Issue in Major Public Blockchains Application (Bitcoin and Ethereum)
The high acceptance of cryptocurrency is escalating the burning scalability issue in
public blockchains [6,7,28]. The number of transactions with Bitcoin and Ethereum is in-
creasing every day [5,7]. More 13,000 transactions take place with Bitcoin everyday [7].
This high number of transactions is making Bitcoin transactions bulky. One of the pro-
cesses in public blockchains is to verify the source of the transaction, and every node needs
to store and validate every transaction. It is therefore a challenge to the miners to verify
every transaction in Bitcoin as it scales up in volume. This is happening because of the
inefficient proof-of-work (PoW) [29] consensus mechanism deployed in the Bitcoin in ver-
ifying and validating every single transaction. PoW is unfortunately the most frequently
used consensus protocol [17]. It is worth to notice that the total number of transactions in
Ethereum has continuously grown over the years [7]; many thousands of transactions
have occurred every day in recent years. In Ethereum, the limited block size cannot ac-
commodate all the transactions submitted by the miners. It is therefore challenging for the
miners to verify every transaction. The consequence is that the miners tend to select trans-
actions with more rewards for the sake of securing more rewards. The transactions with
Figure 7. Blockchain application domain per selected papers.
Scalability Issue in Major Public Blockchains Application (Bitcoin and Ethereum)
The high acceptance of cryptocurrency is escalating the burning scalability issue in
public blockchains [
6
,
7
,
28
]. The number of transactions with Bitcoin and Ethereum is
increasing every day [
5
,
7
]. More 13,000 transactions take place with Bitcoin everyday [
7
].
This high number of transactions is making Bitcoin transactions bulky. One of the processes
in public blockchains is to verify the source of the transaction, and every node needs to
store and validate every transaction. It is therefore a challenge to the miners to verify
every transaction in Bitcoin as it scales up in volume. This is happening because of the
inefficient proof-of-work (PoW) [
29
] consensus mechanism deployed in the Bitcoin in
verifying and validating every single transaction. PoW is unfortunately the most frequently
used consensus protocol [
17
]. It is worth to notice that the total number of transactions
in Ethereum has continuously grown over the years [
7
]; many thousands of transactions
have occurred every day in recent years. In Ethereum, the limited block size cannot
accommodate all the transactions submitted by the miners. It is therefore challenging for
the miners to verify every transaction. The consequence is that the miners tend to select
transactions with more rewards for the sake of securing more rewards. The transactions
Appl. Sci. 2021,11, 9372 13 of 27
with fewer rewards are left in the queue, leading to longer transaction latency [
30
]. It is
estimated that more than 10 k transactions are waiting to be verified.
The highest transaction throughput in Bitcoin is capped at 7 TPS (Transaction-per-
second). In contrast to the VISA counterpart, 400 TPS is the norm [
5
,
6
,
29
–
31
] The block
interval latency (in public blockchains) is in a magnitude of up to 10 min [
32
] for a trans-
action to be verified. Ethereum was to be theoretically verified at the rate of 1k TPS, but
due to certain limitations in its structure, is more likely to be verified at 20 TPS. This is
far less than other electronic payment methods such as PayPal—at 193 TPS. Besides the
inefficiency in the transaction verification process, another crucial factor of importance is
storage, which needs to be seriously considered [
5
]. As transactions grows, the required
storage capacity for blocks needs to scale up at tandem. It is reported that currently, Bitcoin
storage is more than 305.23 GB [
8
], Ethereum is at 667.110 GB [
8
], and LiteCoin at 28.45 GB.
It should be mentioned here that energy consumption is also a crucial issue in public
blockchain implementation. When comparing the consumption of electricity by Bitcoin
with other cryptocurrencies, Bitcoin was in the 49th position [
5
]. It is interesting to note
that actual consumption of electricity by Bitcoin is less than the predicted scale, which may
suggest that the Bitcoin could not scale well per expectations and predictions.
The combination of all the limitations mentioned above is apparently degrading
the performance of public blockchain decentralized applications. The low throughput,
high latency, high storage, and high energy consumption cannot satisfy the large-scale
implementation of blockchain in time-mission-critical or real-time applications.
The Internet of Things (IoT) is a technology that is growing aggressively, and it is
embracing blockchain as an integral component in IoT security applications. IoT was tagged
as “The Global Infrastructure of the Information Society” by ITU in 2015 [
14
]. Besides
the many benefits, IoT has some limitations. Public blockchains have been technically
considered to address those issues by decentralizing computation powers, processing,
and storage. Unfortunately, public blockchain is still suffering from scalability matters
in IoT applications [
33
,
34
]. Principally, public blockchain technology is not suitable for
lightweight IoT devices. In blockchain, a node is supposed to verify every transaction
and perform search in every block, likely an extremely heavy load for lightweight IoT
devices. As discussed earlier, public blockchains require massive resources to support their
operations and are highly constrained by consensus delay, making it almost impossible
to deploy them in small/low spec IoT devices. It would not be possible for IoT devices
to verify a transaction without a massive amount of historical data. IoT therefore needs
to either carry high storage by itself or rely on a centralized server. While considering
the large-scale storage requirement in public blockchains, it is worth to also examine the
financial aspects. For example, in Ethereum, it costs 2
×
105 US Dollars per gigabyte
of data storage, making it probably highly expensive to implement IoT networks with
blockchain [35].
The storage requirement for the IoT network is very much dependent on the types
of application. As such, the overall data storage size could be destructive in IoT-enabled
blockchain since each block would be replicated n times in the n-node public blockchain
networks. For example, in smart city application, vehicular traces of 700 cars for 24 h
demand a storage capacity of close to 4.03 GB, which is about 0.24 MB per hour per car [
36
].
In public blockchains, high latency can be due to transaction confirmation. This
behavior may cause inconsistency in a decentralized environment. The usual tolerated
latency in blockchain is not suitable in many IoT applications. For example, in Bitcoin,
the confirmation time is 10 min, which can be an extremely long delay for sensitive IoT
applications such as vehicular networks. In the light of all these limitations, it is obvious
that scalability issue is persisting and degrading the performance of IoTs enabled with
blockchain.
Appl. Sci. 2021,11, 9372 14 of 27
7. RQ2: What Vital Root Factors Are Causing Scalability Issue in Blockchain?
With the increasing popularity of Blockchain technology and its permissionless appli-
cation such as cryptocurrency-based applications, scalability issue has become a primary
focus in the blockchain research community. Many researchers are attempting to analyze
in details on this issue [
5
,
37
]. From our SLR, there seems to be no rule-of-thumb or state-of-
the-art research with good matrices to address scalability issue in public blockchains. This
section intends to discuss factors causing scalability in more detail.
Figure 8illustrates the factors causing scalability issue in public blockchains. Among
these factors, transaction throughput has received the most attention. Out of the 121
selected papers, 39 papers discussed transaction throughput as main factor/concern for
low scalability in public blockchains. Twenty papers highlighted the consensus mechanism
as the second most discussed factor concerning public blockchain scalability. Consensus
mechanism is somewhat related to throughput. There are nine papers talked about the
computational power involving scalability in public blockchains. Latency and storage are
discussed in six papers. The remaining factors such as block size, cost issue, number of
nodes, network load, and overall performance are discussed in 1–3 articles, respectively.
There are 23 papers clearly stating scalability as an issue, but reasons causing scalability
are somewhat unclear. The Table 4shows the factors related to scalability issues, along
with the relevant reference.
Appl. Sci. 2021, 11, 9372 15 of 27
7. RQ2: What Vital Root Factors Are Causing Scalability Issue in Blockchain?
With the increasing popularity of Blockchain technology and its permissionless ap-
plication such as cryptocurrency-based applications, scalability issue has become a pri-
mary focus in the blockchain research community. Many researchers are attempting to
analyze in details on this issue [5,37]. From our SLR, there seems to be no rule-of-thumb
or state-of-the-art research with good matrices to address scalability issue in public block-
chains. This section intends to discuss factors causing scalability in more detail.
Figure 8 illustrates the factors causing scalability issue in public blockchains. Among
these factors, transaction throughput has received the most attention. Out of the 121 se-
lected papers, 39 papers discussed transaction throughput as main factor/concern for low
scalability in public blockchains. Twenty papers highlighted the consensus mechanism as
the second most discussed factor concerning public blockchain scalability. Consensus
mechanism is somewhat related to throughput. There are nine papers talked about the
computational power involving scalability in public blockchains. Latency and storage are
discussed in six papers. The remaining factors such as block size, cost issue, number of
nodes, network load, and overall performance are discussed in 1–3 articles, respectively.
There are 23 papers clearly stating scalability as an issue, but reasons causing scalability
are somewhat unclear. The Table 4 shows the factors related to scalability issues, along
with the relevant reference.
Figure 8. Identified factors causing scalability issue in public blockchains.
Table 4. Factors related to blockchain scalability.
No. Facto
r
Description Ref. Source
1 Transaction
Throughput
This implies the total number of transactions that
the protocol may handle in one second. [7,14,36,38–73]
2 Latency
This applies to the time it takes for a transaction to
b
e initiated to achieve a consensus on it. It is also
regarded as a finality.
[33,59,69,74,75]
3 Storage
It refers to the total space/capacity a blockchain
network can consume. [36,71,76–81]
4 Block Size
This is total storage capacity of a block to be uti-
lized by the transactions. The network will reject
the block if it exceeds the storage capacity.
[77,82]
5 Computation en-
ergy
This indicates if the algorithm (or the utilizing
system) consumes a significant amount of energy
for block mining.
[42,79,83–90]
6 Network load
This implies the number of transactions being car-
ried by the network. [91–94]
Figure 8. Identified factors causing scalability issue in public blockchains.
Table 4. Factors related to blockchain scalability.
No. Factor Description Ref. Source
1Transaction
Throughput
This implies the total number of
transactions that the protocol may handle in
one second.
[7,14,36,38–73]
2 Latency
This applies to the time it takes for a
transaction to be initiated to achieve a
consensus on it. It is also regarded as a
finality.
[33,59,69,74,75]
3 Storage It refers to the total space/capacity a
blockchain network can consume. [36,71,76–81]
4 Block Size
This is total storage capacity of a block to be
utilized by the transactions. The network
will reject the block if it exceeds the storage
capacity.
[77,82]
Appl. Sci. 2021,11, 9372 15 of 27
Table 4. Cont.
No. Factor Description Ref. Source
5Computation
energy
This indicates if the algorithm (or the
utilizing system) consumes a significant
amount of energy for block mining.
[42,79,83–90]
6 Network load This implies the number of transactions
being carried by the network. [91–94]
7 Cost issue This implies the total cost associated with
verifying a transaction in blockchain. [13,50,55,64]
8 Number of nodes This refers to the total number of nodes
available in the blockchain network. [95–97]
9 Consensus model
Consensus mechanism represents the
process of approving/verifying blockchain
transactions.
[6,45,47,49,54,56,
57,59,60,63,71,73,
83,84,90,98–115]
It should be mentioned here that public blockchain scalability is not a singular term.
It is a combination of various parameters, and these parameters are interdependent. In
many papers, every parameter is discussed somehow directly or indirectly to the consensus
model deployed. For example, transaction throughput, latency, and computational energy
are dependent on the efficiency/performance of the consensus model. Block size and
storage are also interdependent. Block size can affect transaction throughput and latency,
which in turn can be indirectly linked to consensus model. A larger block can store more
transactions, thus directly raising the throughput, but it also causes an increase in block
propagation time. In Bitcoin, the block interval is about 10 min with a block size of around
1 MB, to illustrate the limits on the number of transactions that can be stored in each block.
The consensus mechanism is a process in which the nodes in the public blockchains
network agree with each other about the ledger that they hold. The consensus protocol is
thus the most fundamental and indispensable component in Blockchain. It provides the
essential process flow for the nodes to verify and validate every single transaction and
hence to append a new block to a blockchain. It follows that a high network bandwidth
would be consumed when the whole network updates its chain. For example, Bitcoin
uses PoW as its core consensus model, and for the mining process, it requires specialized
hardware with stable internet connection. Collectively, this is resource-hungry. It may have
a high processing time, resulting in thousands of transactions (per second) waiting in the
queue to be verified. This constitutes a pressing latency factor in the public blockchain.
Consensus mechanism is considered a crucial challenge in public blockchain applications. It
is known to cause bottleneck and restrict transaction throughput [
115
] in the permissionless
blockchain (Bitcoin and Ethereum) [29].
7.1. Latency
Latency in blockchain is referring to the processing time for a transaction measured
starting from getting an input till the transaction is completed at the output [
24
]. In permis-
sionless blockchain, thousands of nodes need consensus to verify and process a transaction.
Transactions are buffered in a queue waiting to be verified, thus logically causing increased
latency. In public blockchains, every node verifies and stores every single transaction
for upholding data integrity and data security but unfortunately compromises the la-
tency [
35
,
116
]. High latency in blockchain, however, may be used to ensure consistency in
the decentralized public blockchain networks.
7.2. Number of Nodes
Entities connected to blockchain network are considered as nodes. The inter-nodes
latency increases when many nodes get connected to the network [
19
]. The number of
transactions increases with the increasing number of nodes, and so, more transactions
are getting involved in the consensus process. This is bound to affect the transaction
throughput and latency. Additionally, the growing number of nodes leads to increasing
Appl. Sci. 2021,11, 9372 16 of 27
computational energy [
21
]. Nodes in public blockchains care split into two categories:
partial node (lightweight nodes) and full nodes. The processing of partial nodes is com-
pletely dependent on the full nodes. Although the partial nodes are not needed to store
a whole blockchain, the processing workload on public blockchains grows as number of
nodes increases. This is of certain to affect the throughput. The involvement with many
participants may lead to performance degradation because of the higher number of nodes
on every stage of transaction. The threshold on the number of nodes (participants) may
also be a crucial issue.
7.3. Block Size
Typically, the size of one block is around 1 MB in Bitcoin [
5
,
117
]. This is considered
very small and limits the number of transactions that can be stored. It is apparent that large
block size is able to accumulate more transactions and directly increase the throughput and
lower the latency [
5
], but the larger block would prompt an increase in block propagation
time because heavier block needs more time to be transmitted over the network. In
addition, handling more transactions in one block would demand more computational
resources [117].
7.4. Computational Cost/Energy
This implies the total cost associated in verifying a transaction [
118
]. It includes the
required amount of bandwidth in block propagation and, most importantly, the mining
process. The mining process is associated with the consensus mechanism in the permission-
less blockchain. Bitcoin uses PoW consensus mechanism that the mining process requires
specialized hardware to mine a block. The special high-end hardware consumes more
energy, and as a whole this higher energy consumption means more computation cost [
116
],
which in return may impinge on the implementation scale of a public blockchain.
7.5. Transaction Cost
Transaction fee plays a vital role in public blockchains, i.e., Bitcoin. Miner tends to
make selection of the transactions to be verified based on the fee associated with it. This
directly affects the confirmation time of a transaction, thus influencing the throughput and
latency. The transaction with a small fee may suffer massive confirmation delays, and this
logically happens because the consensus mechanism promises to reward miners based on
the associated fee in a transaction.
7.6. Storage
Public blockchain storage is another essential factor to be seriously considered in the
context of scalability. Public blockchain storage requirement grows in tandem with the
increasing number of nodes and transactions [
5
]. As such, full nodes that store complete
block data demand high storage capacity. Moreover, high storage requirements can be
relieved by increasing partial nodes in public blockchains [
35
]. Although partial nodes do
not store the whole blockchain, they may considerably increase the workload. Throughput
would therefore be affected, and addressing high storage requirements is non-trivial.
8. RQ3: How Researchers Address the Scalability Issue in Blockchain?
Some significant solutions to public blockchains scalability are discussed in this
section based on the literature reviewed. Scalability challenges are also mentioned in this
discussion. At macro level, researchers have attempted to address this issue in two ways:
on-chain or off-chain. On-chain solutions tend to address scalability issues by working on
elements within the blockchain, whereas off-chain prefers to process transactions outside
of the chain. Along with these classifications, there are several application-based solutions
designed to achieve the required scalability. For example, DAG [
119
] and NormaChain [
86
]
for IoT, BlockTrail [40] for auditing, e-commerce EBCM [86,109] and supply chain [55].
Appl. Sci. 2021,11, 9372 17 of 27
8.1. On-Chain Solution
On-chain solutions tend to address scalability issues by working on elements within
the blockchain. This section will discuss on-chain approaches to address the scalability
issue of blockchain.
8.1.1. Block-Data-Related Approaches
1. Block size increase
The big block size is typical of on-chain solutions. In this approach, the public
blockchain scalability issue has some significant linkages to the block size [
118
]. Obviously,
the big block can accommodate more transactions, which would undoubtedly improve
the overall throughput. Bitcoin unlimited [
8
] is an example of a big block. This method
increases transmission limit and decreases cost related to transmission as compared to
the conventional method. However, it downgrades block prorogation efficiency (in time)
and may increase the blockchain forking chances, leading to the probability of orphan
block to incur higher maintenance cost. The full node would be more expensive to operate
through this approach. Miners prefer this approach because the increased block size can
accommodate more transactions in one block, and they incur a larger transaction fee when
mining that block.
8.1.2. Segwit
In Segwit method [
120
], the block size is kept constant while adding more transactions
to the block. This approach aims to extract the signature data from the transaction and store
it outside the base transaction block to effectively allow more spaces for new transactions.
In this approach, the validating part is kept separated from actual data of the transaction.
As the digital signature contributes almost 70% to the transaction, the signature must be
stored in the data structure called the witness and is isolated from the transaction to resolve
the malleability of the transaction [
8
]. In addition, Segwit has launched a new transaction
size unit. A single transaction is divided into two sections. Non-witness (it should be
stored in the block as usual) and witness data (it will transfer to extended block). Moreover,
non-witness data bytes are counted as 4 WU each, whereas the witness data byte is counted
as 1 WU each. The highest storage space for a block is 4 WU, which is equivalent to the old
maximum block size of 1 MB, if no node uses Segwit.
8.1.3. Sharding
Sharding was traditionally proposed in the field of database for storage optimization
in commercial databases. It was later adopted in public blockchains to address the scala-
bility issue. It is considered to be one of the effective methods. In the sharding technique,
the nodes are broken into several chunks called shards [
121
]. Each shard possesses a small
part of nodes. Every shard is responsible for processing small portions of a transaction.
Therefore, the transaction is processed in parallel. The parallel processing of the transaction
improves the authentication mechanism that ultimately maximizes the throughput of the
entire blockchain network. The Byzantine consensus algorithm is used in between the
nodes within shards to agree on the state of the transaction [
25
]. There is an immense
need for the inter-shard communication protocol for cross-shard transaction. The total
computation energy increases with the increasing number of shards. In this technique,
every shard process transaction with the same throughput, and by increasing the number
of shards, results in a linear increase in throughput. Elastico [
122
] and OmniLedger [
123
]
implemented the sharding technique to increase the throughput. The only difference be-
tween them is that Elastico is unable to process inter-shared transactions while OmniLedger
processes them atomically using an atomic Commit Protocol. Another approach utilizing
sharding is the rapid chain [79].
Sharding technique is the only viable solution if transaction stays in the same shard,
which is considered to be the most significant limitation. As a matter of fact, inter-shard
transaction cannot be automatically executed by Elastico. OmniLedger supports inter-
Appl. Sci. 2021,11, 9372 18 of 27
shard transactions but high shard size is hard to select. However, large shards are not
scalable because whole network executes the Byzantine consensus, and small shards are
not scalable because they would result in a huge number of shards. Furthermore, small
shards are more vulnerable to security risk. Each shard is fault-tolerant, up to, at the
most, a third of the shard’s size. Moreover, the shard size would decrease if the number of
malicious nodes remained constant, making the shard highly likely to fail.
8.1.4. Consensus-Protocol-Related Approaches
In this section, some major consensus techniques are discussed. These techniques
have been implemented and applied in different applications for the sake of improving
scalability in public blockchains. Based on this SLR findings, the consensus protocol is
found to be the second-most-discussed factor. The inefficiency in the consensus protocol
is the main cause of scalability issue in public blockchains. The research community
has therefore tried to address the scalability issue with different innovative consensus
approaches.
1. Proof of work
In 2008, Satoshi Nakamoto (an individual or a group) proposed the initial idea of
Proof-of-Work (PoW) model with Bitcoin [
2
]. Since then, it has been widely used in public
blockchains, especially in cryptocurrencies like the Bitcoin. In this model, the miners
(nodes) on every transaction compete to solve an intensive mathematical puzzle (hashing),
to win a chance to add the next Block to the chain and to earn a reward in the form of Bitcoin
for their energy consumed and work done in the mining process [
24
,
124
]. For getting a
chance to append a new Block, every miner (node) must justify that it has accomplished
sufficient work done. Therefore, it is referred to as proof-of-work consensus protocol. It is
important to mention here that the Bitcoin dynamically controls the difficulty level of the
cryptographic puzzle [125].
2. Proof of stake
Proof-of-stake (PoS) [
126
] consensus model is considered an energy-efficient version
of PoW because, comparatively, it saves more energy than PoW. In this model, miners
(nodes) are supposed to affirm the ownership on the currency they have. Therefore, the
miner (node) possessing the highest number of currencies would be given a chance for
adding next block to the chains. It is believed that the miners (nodes) possessing more
currency would not attack the blockchain network or it would be less likely they would
attack. Furthermore, the node associated with higher monetary reward will dominantly
control the network [
124
] because it will always secure chance to publish the block. This is
not fair to newer nodes with less currency. There are various PoS variants available with
the deployment of the appropriate stake size for selecting a node to append the next block
in public blockchains, e.g., peercoin and Blackcoin.
3. Delegated Proof-of-Stake consensus
The Delegated PoS (DPoS) [
18
] is considered a variant of the proof-of-stake. It does not
constitute a significant improvement, but the discrepancy between the proof-of-stake (PoS)
and the DPoS is mainly based on direct democracy, while the other is based on a democratic
representative [
107
]. In DPoS model, the miners (nodes) are given the right to find their
representative, which is called delegate. The delegate is required to perform three tasks,
including creating, validating, and verifying the block. The process of validating would
be much faster if a limited miner (nodes) performed the validation process instead of the
whole network. Therefore, it would directly affect the transaction throughput. Furthermore,
the delegates are accountable for controlling and managing the block size. The dishonest
delegate should not be a concern because every node has the right to vote for delegate of
their choice. Bitshares is an example of DPoS implementation.
4. Practical byzantine fault tolerance
Appl. Sci. 2021,11, 9372 19 of 27
PBFT consensus protocol is typically used to accept Byzantine faults [
18
] and is often
used by permissioned blockchain, i.e., Hyperledger, because it is able handle up to 1/3
malicious byzantine replica. This model works in rounds process. There are predefined
steps to follow in every round in selecting a primary node. The processing of PBFT
consensus protocol is divided into three stages: pre-prepared, prepared, and commit. The
node should get the majority of 2/3 votes from all the nodes to change the state of the
Blockchain. This will confirm that majority of the nodes are identifiable and known to every
node. Some practical variants of PBFT have been proposed, such as the Steller consensus
model [
34
]. There is not much difference between PBFT and Steller. In PBFT, every node
requires one to inquire about other nodes, whereas in Stellar Consensus Model other nodes
can choose which set of participants to rely on.
5. Proof of authority/proof of identity consensus model
Proof of identity/proof of authority consensus model [
8
] rely on the complete pub-
lishing nodes. This protocol only involves nodes whose identity is linked to real-life
information. In the PoA/PoID model, nodes need to reveal their real-life identities to
gain a chance to publish a new Block. The defined identity should be identifiable and
confirmable in the blockchain network. Furthermore, the nodes are required to put their
real-life identity at stake along with their reputation to earn the chance to become a publish-
ing node. However, the reputation of these nodes is a concern based on their actions and
activities performed in the network, which means any malicious activity in the publishing
can ruin their reputation in the public blockchain networks. However, their reputation
would improve if the node performed in the way that other nodes agreed on. There is
much less of a chance for a node with a poor reputation to become a publishing node. It is
therefore essential for publishing to retain high status. It is not preferred to implement this
model in permissionless blockchain because it requires trust between nodes.
6. Proof of elapsed time
The PoET consensus protocol randomly chooses the leader by running an election
protocol [
18
]. It was often named as chance-based SGX-based election model. In this model,
the leader is chosen to be responsible for adding the next block to the blockchain. In the
processing of random leader votes, this model would concern thousands of entrusted
nodes and free participation. Therefore, for experiencing the efficiency of this model, it is
essential that the election of leader should be distributed between the most available nodes,
while the remaining nodes would be kept responsible for ensuring transparency in the
selection process and confirm that there is no manipulation in the whole leader selection
process. The need for a transparent mechanism in the selection of the leader could be
fulfilled by the trustworthy execution environment (TEE), in which security would remain
constant during the process. Intel SGX and TEE are supposed to execute the validation
and mining process. Every miner needs permission to execute the code within the TEE for
waiting time, and miner with the shortest wait-time becomes leader node. Any internal
and external tampering could be avoided by the TEE function. This model requires special
equipment or hardware, which is considered a drawback.
7. Bitcoin-NG
Bitcoin-NG [
127
] is new generation consensus model derived from PoW Consensus
model [
2
]. It splits time into different epochs, and a leader is responsible for transaction
sterilization in each epoch. This model introduces two new types of Block: the critical
block and the microblock. The critical block does not carry transaction data, and it is
generated by the miner (nodes) via PoW model and is only used for the leader selection.
In contrast, the leader is responsible for creating a microblock containing transaction data.
The transaction can therefore be processed continuously unless the next leader ensures to
decrease transaction confirmation time which improved the throughput.
8. Proof of reputation consensus mechanism
Appl. Sci. 2021,11, 9372 20 of 27
In PoR [
56
] protocol, a node’s reputation is made on the basis of its assets, transactional
activities, and participation in the consensus process. A leader who possesses the best
reputation will be able to generate a new Block. Voting process will be initiated for new
block validation. There is another scalable protocol on the similar idea of reputation, known
as delegated proof of reputation [
99
]. The protocol’s novelty is that it replaces coin staking
with a reputation-based method.
8.2. Off-Chain Solutions
The off-chain blockchain scalability solution is designed to increase the transaction
throughput by executing the transaction outside of the main blockchain.
Lightening Network
In the lightening network, it is possible that in the Bitcoin network two nodes would
be able to create an off-chain trading channel, where those two nodes would be able to
process the transaction with low latency [
128
]. There are three main phases in the lightening
network, namely, establishing, trading, and closing the channel. Firstly, it is extremely
important to open the payment channel with the nodes sharing the payment channel. After
the opening of the channel, some coins need to be placed on the multi-signature address
for sharing with other nodes. In this way, nodes are prevented from scamming with the
coins arbitrarily. Opening the channel would be accomplished by an on-chain exchange,
which would charge a transaction fee to the main chain. Now, once the channel is opened,
the transaction will be an off-chain transaction. So, those transactions are not supposed
to be stored on the main chain, and it may charge 0 waiting time and transactional fee.
The channel would be closed after the process of payment was completed. The final stage
would be to notify all nodes and append this new block to the main chain, which would
appear on the on-chain. So, in this way, the multiple transactions could take place off the
chain and the complete process would create two transaction records on the main chain [
8
].
Let us suppose A and B are maintaining a channel and B and C are also maintaining a
channel each. This enables A and C to contact each other, which increases the throughput.
This technique can reduce transaction costs, wait/standby time, and reduce the load on
the main chain. However, as fee for transaction is decreasing or has disappeared, there
would be no or less benefit for miners, so their ecosystem may change. Raiden network is
another example that has been implemented in Ethereum. It is known as Ethereum version
of Lightening Network [
18
]]. It follows the same process and protocol for operation as
Lightening network except for the transaction details. Its state channels transfer smart
contract details as well.
8.3. Other Potential Attempts to Address the Blockchain Scalability Issue
Some other potential attempts to address blockchain scalability are briefly presented
here:
TrustChain [
111
] is designed for permissionless/public blockchains and the data
structure is entirely tamper-proof and used by agents to store their transactional record.
It enables the design of a separate immutable chain of temporally organized interactions
with other agents. It is inherently in parallel that every agent creates its own genesis
block. Concept Superlight [
76
] is mainly used in public blockchains and seeks to address
the problem of scalability. It is not mandatory for all nodes to store the whole blockchain
locally for verification of each transaction. So, the nodes in this framework may validate the
transaction using their header. Block Summarization [
78
] decreases the additional storage
for transferable transactions. This approach allows resource-constrained lightweight nodes
to store blockchain shaped in such a way that transactions can be independently verified
to eventually decrease full node dependency. FRChain [
100
] consensus model is mostly
utilized in permissioned blockchain. It is immune to multiple nodes and blockchain
network failures. For block propagation and block validation, FRChain utilizes mutual
signing over multicast trees. Fast BFT [
64
] is a faster and scalable consensus protocol.
Appl. Sci. 2021,11, 9372 21 of 27
The inventive idea of this protocol is a message grouping technique that uses hardware-
based, protected execution environments (TEEs) and lightweight secret sharing. Satellite
chains [
96
] affect the notion of satellite chains that can run many consensual protocols in
private at the same time, thereby significantly increasing the scalability of the system’s
premises to meet industrial standards.
9. Conclusions
Blockchain has grown rapidly in the last two decades after the immense success of
public blockchain networks such as Bitcoin and Ethereum. However, it has not disrupted as
many industries as was expected because of the fundamental issue of scalability, which has
become a major concern, especially when applying blockchain to the real-world business
environment. As a matter of fact, major crypto currencies are also facing the same scalability
problem. As such, public blockchain scalability is fast becoming an active research topic
in academia and in industries, where many sectors are trying to adopt the blockchain in
their practical applications. In this study, we found that scalability is not a singular term.
There are a number of factors attached to it, including transaction throughput, number of
nodes, storage, block size, high communication, latency, cost, and the verification process.
Out of these, transaction throughput is the most discussed factor and is strongly linked
to a consensus mechanism. It is found that most of the factors are interdependent and
are somehow directly or indirectly linked to a consensus mechanism. It is also noticed
that the contemporary available consensus models are not efficient enough to address
the scalability issue and fail to provide required throughput and latency for industrial
applications, specifically for those demanding time mission-critical (or real-time) responses
such as IoT. IoT is high on the list of technologies adopting Blockchain. Other than
IoT, blockchain seems to be impacting other industries such as energy, finance, resource
management, healthcare, education, and agriculture; however, it is yet to achieve desired
outcomes due to scalability issues, especially in public blockchain settings. The research
community has attempted to address the scalability issue with different techniques. In this
study, we discussed the major scalability solutions along with their challenges with respect
to blockchain technology. It is foreseeable that within the next few years, blockchain will
transform a lot of applications and the transformation will be driven by scalability balanced
with decentralization and security requirements. In this paper, we have highlighted several
potential research open issues such as the huge amount of public blockchain data storage
that needs to be considered, huge bandwidth consumption, and consensus approaches
aimed at addressing scalability in public blockchain systems.
Author Contributions:
Manuscript preparation, study concept, and design D.K., L.T.J.; research
methodology, design, data analysis, and review and visualization, D.K., M.A.H.; critical comparison,
D.K., L.T.J.; survey deductions, D.K.; validation framework design, D.K., L.T.J. and M.A.H.; proof-
reading, editing, and formatting, L.T.J., M.A.H. All authors have read and agreed to the published
version of the manuscript.
Funding:
This study is conducted in Universiti Teknologi PETRONAS (UTP) under the Fundamental
Research Grant Scheme (FRGS) from the Ministry of Higher Education (MOHE) Malaysia.
Institutional Review Board Statement: Not Applicable.
Informed Consent Statement: Not Applicable.
Data Availability Statement: Not Applicable.
Acknowledgments:
The authors extend their deep regards and acknowledgement to Universiti
Teknologi PETRONAS for provision of resources and materials for the completion of this research
work.
Conflicts of Interest: The authors declare no conflict of interest.
Appl. Sci. 2021,11, 9372 22 of 27
Appendix A
Search Strings: EEE Explore
(Blockchain OR Block Chain OR Distributor Ledger Technology OR DLT OR Cryp-
tocurrency OR Crypto-currency OR Bitcoin OR Ethereum) AND (scalability OR scalable OR
Scaling OR extensible) AND (Problem OR Issue OR challenge OR Solution OR Framework
OR protocol OR Model OR Algorithm)
Search Strings: Science Direct
1. (“Blockchain” OR “Block Chain” OR “Distributor Ledger Technology” OR “Bitcoin”)
(“Scalability” OR “Scalable” OR “Scaling” OR “extensible”) (“Problem” OR “issue” OR
“challenge”)
2. (“Blockchain” OR “Block Chain” OR “Distributor Ledger Technology” OR “Bitcoin”)
(“Scalability” OR “Scalable” OR “Scaling” OR “extensible”) (“Solution” OR “Framework”
OR “protocol”)
Search Strings: Web of Science
ALL = (Blockchain* OR Block chain OR Bitcoin OR “Distributor Ledger Technology”
OR DLT OR Cryptocurrency) AND ALL = (Scalability OR Scalable OR Scaling OR extensi-
ble) AND ALL = (Problem OR Issue OR challenge OR Solution OR Framework OR protocol
OR Model OR Algorithm) AND PY = (2010–2019)
Search Strings: Scopus
+(“Blockchain scalability” (+(Blockchain bitcoin Block Chain) +(scalability Scalable
Scaling) +(Problem issue challenge) +(Solution Framework protocol Model Algorithm))
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