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A Survey of IoT Stream Query Execution Latency Optimization within Edge and Cloud


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IoT (Internet of Things) streaming data has increased dramatically over the recent years and continues to grow rapidly due to the exponential growth of connected IoT devices. For many IoT applications, fast stream query processing is crucial for correct operations. To achieve better query performance and quality, researchers and practitioners have developed various types of query execution models—purely cloud-based, geo-distributed, edge-based, and edge-cloud-based models. Each execution model presents unique challenges and limitations of query processing optimizations. In this work, we provide a comprehensive review and analysis of query execution models within the context of the query execution latency optimization. We also present a detailed overview of various query execution styles regarding different query execution models and highlight their contributions. Finally, the paper concludes by proposing promising future directions towards advancing the query executions in the edge and cloud environment.
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Review Article
A Survey of IoT Stream Query Execution Latency Optimization
within Edge and Cloud
Fatima Abdullah , Limei Peng , and Byungchul Tak
Department of Computer Science and Engineering, Kyungpook National University, Daegu 41566, Republic of Korea
Correspondence should be addressed to Byungchul Tak;
Received 1 October 2021; Accepted 29 October 2021; Published 16 November 2021
Academic Editor: Xiaojie Wang
Copyright © 2021 Fatima Abdullah et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
IoT (Internet of Things) streaming data has increased dramatically over the recent years and continues to grow rapidly due to the
exponential growth of connected IoT devices. For many IoT applications, fast stream query processing is crucial for correct
operations. To achieve better query performance and quality, researchers and practitioners have developed various types of
query execution modelspurely cloud-based, geo-distributed, edge-based, and edge-cloud-based models. Each execution model
presents unique challenges and limitations of query processing optimizations. In this work, we provide a comprehensive review
and analysis of query execution models within the context of the query execution latency optimization. We also present a
detailed overview of various query execution styles regarding dierent query execution models and highlight their
contributions. Finally, the paper concludes by proposing promising future directions towards advancing the query executions
in the edge and cloud environment.
1. Introduction
Recent advancements in the Internet of Things (IoT)
domain have led to the production of a large number of
internet-connected devices. These IoT devices emit millions
of streaming events [1]. The stream processing applications
analyze the streaming events for the extraction of meaning-
ful insights. These real-time insights serve as the decision-
making points for many IoT and E-commerce applications
in various domains such as online business, retail, stock
markets, manufacturing, and healthcare. Streaming analytics
has emerged as a real-time analytics paradigm that goes well
with the processing of latency-sensitive IoT applications.
These applications span various IoT domains such as smart
healthcare, smart trac management system, smart cities,
energy, and retail sectors. Streaming analytics utilizes the
concept of windowing for the real-time analysis of data. Data
analysis is done on the y by executing the queries on the
windowed data. Prompt execution of analytics on the
streaming data is critical for the latency-sensitive applica-
tions since the results may lose their value if the query exe-
cution time exceeds the required time bound [2].
The stream query processing comprises two major exe-
cution modelscentralized and distributed. The centralized
query execution model (CQEM) refers to the style of query
processing entirely performed in the cloud, whereas the dis-
tributed query execution model (DQEM) spreads the query
processing into computing nodes beyond the central cloud
nodes. Figure 1 shows the categorization of query execution
models. Conventional Stream Processing Applications
(SPAs) follow the in-cloud query processing style [3]. In this
scenario, all data items are transmitted to the cloud data center
for processing. The cloud-based stream processing can handle
well the resource and storage demands, but falls short for the
latency demands of IoT applications. Networking delay is
the primary weak point of the centralized query processing
architecture. As such, a centralized data processing scenario
is becoming infeasible or insucient to keep up with the strin-
gent requirement of latency-sensitive applications.
To mitigate the networking delay issue, the geo-distributed
and edge computing-based query processing approaches have
emerged as a promising direction. Most of the cloud compa-
nies are deploying their data centers at the network edge for
timely provision of services to the end-users [4]. The geo-
Wireless Communications and Mobile Computing
Volume 2021, Article ID 4811018, 16 pages
distributed data analytics systems perform the query execution
in a distributed fashion byutilizing near-the-edge data centers.
These geo-distributed systems have contributed signicantly
in cutting down the networking delays [413]. Nevertheless,
challenges still exist in distributed stream processing in the
context of a heterogeneous network-compute resources and
wide area network (WAN) dynamics. The compute and net-
work resource management is a challenging issue in ensuring
the low response time for the data analytics jobs. In contrast to
the intracluster analytics jobs, the geo-distributed analytics
jobs have to overcome the challenge of limited and heteroge-
neous network and compute resources [7, 14]. To this end,
several recent works have incorporated WAN-awareness in
their geo-distributed query processing schemes to address
these challenges [4, 5, 7, 9, 10].
Similarly, the edge-cloud-based query processing is also
playing a vital role in the eort of reducing the query execu-
tion latency [1, 3, 1520]. Edge and fog computing have
emerged as promising paradigms for meeting stringent pro-
cessing demands of latency-sensitive applications [2025].
Likewise, vehicular edge computing is also playing a vital
role in enhancing the computing services for vehicular users
[26, 27]. These paradigms extend the cloud services to the
network edge to ensure the quality of service (QoS), security,
and low latency requirements of IoT applications [2836].
We are starting to see success stories of these paradigms in
meeting the low latency requirement of IoT-enabled applica-
tions [37, 38]. Among them, stream processing applications
weigh more because they need to perform data analytics over
short time windows. Their processing is more valuable
within the context of result provisioning under the require-
ment of short response times [3]. The edge-cloud-based query
processing approaches utilize the edge nodes for the partial
computations [1, 3, 15, 17, 18]. In this scenario, the stream
processing (SP) application logic spans both edge nodes and
the cloud data center. The edge-cloud-based query processing
techniques perform optimal placement of SP operators on
edge nodes in a resource-aware manner [1, 3]. The edge nodes
perform initial computations on the streaming data and trans-
mit the partial results to the cloud for aggregation.
To further cut down the networking delay, some of the
recent work has also utilized the edge nodes for the complete
query execution [19, 39, 40]. These edge-based approaches
perform whole computations on the edge, thus, refraining
themselves from excessive WAN bandwidth usage. These
schemes perform latency optimization by ooading the
query processing tasks within the edge network instead of
cloud. The edge-based query processing techniques opt for
networking delay reduction. Therefore, their query execu-
tion plans (QEPs) strive for the optimal cost in terms of
minimum networking delay [19, 39].
The aforementioned query processing techniques depict
that majority of the geo-distributed analytics schemes have
performed query execution latency optimization in a
network-aware manner to deal with the heterogeneous
WAN resources [48, 10, 11]. The edge-cloud-based
schemes have optimized the query execution latency by
mainly focusing on the networking delay reduction along
with ecient bandwidth utilization [1, 3, 15, 17, 18]. Like-
wise, the edge-based latency optimization schemes have also
focused on network delay reduction by reducing the hop
count between the source node and the query computation
node [19, 39, 40]. In contrast to this, the cloud-based query
execution schemes have emphasized compute time reduc-
tion along with ecient utilization of system resources
[4143]. The current trend in the stream query processing
shows that the latency optimization techniques tend to put
more emphasis on the networking aspect. Also, a few geo-
distributed analytic schemes have taken further steps by
considering the network and compute constraints together
in their latency optimization schemes [7, 8]. The network-
compute aware latency optimization is crucial and more
eective for edge-based schemes since edge nodes have lim-
ited and variable compute capacity as compared to the cloud
This research survey includes papers from stream query
processing literature that mainly focus on latency optimiza-
tion. It highlights the contribution of query processing
schemes regarding latency optimization. There exist some
other research surveys that have also reviewed the stream
processing literature [2, 14]. These research surveys are
inclined towards the challenges regarding distributed stream
processing frameworks, whereas this research survey is
focused on query latency optimization schemes. Isah et al.
[2] surveyed the distributed stream processing frameworks.
In this survey, authors reported the guidelines for the selec-
tion of appropriate stream processing framework according
to the specic use case. Bergui et al. [14] surveyed the
WAN-aware geo-distributed data analytics frameworks.
The authors highlighted the challenges regarding heteroge-
neous intersite bandwidth in the geo-distributed data analyt-
ics domain.
This survey paper reviews the query processing latency
optimization schemes under centralized and distributed
query execution models (QEMs). QEMs we consider are
cloud-based query processing, geo-distributed query pro-
cessing, edge-cloud-based, and edge-based query processing.
The main goal of the paper is to comprehensively perform a
comparative analysis of centralized and distributed query
execution techniques in view of latency optimization.
Our contributions are as follows:
(i) Detailed overview of the query processing
approaches within the context of latency
Query execution models
Geo-distributed query processing
Edge-cloud based query processing
Edge-based query processing
Centralized Distributed
query processing
Figure 1: Categorization of query execution models.
2 Wireless Communications and Mobile Computing
(ii) The categorization of query execution techniques
based on their execution models. Comparison of
query execution latency optimization techniques in
several aspects including the network-awareness,
network-compute awareness, multiquery execution,
data reduction, WAN adaptive, privacy awareness,
cost-awareness, and partial/full computations on
(iii) Thorough analysis of how they overcome the exist-
ing challenges with their query processing architec-
tures and what needs to be researched further in
order to advance the state-of-the-art
This paper is organized as follows. Section 2 gives back-
ground and brief overview of stream query execution regard-
ing dierent execution models. Section 3 highlights the
current research challenges regarding centralized and dis-
tributed query execution models. Section 4 reviews and com-
pares the query processing techniques. Section 5 discusses
the query processing techniques related to their key features.
Section 6 presents the future work, and nally, Section 7
concludes the paper.
2. Background
This section provides an overview of streaming data genera-
tion and dierent query execution models.
2.1. Streaming Data. Streaming data is the unbounded data
generated continuously from the data-producing sources.
These data sources include IoT sensors, smartphones, smart
security systems, smartwatches, wearable devices, tness
trackers, social media platforms, click-streams, and online
business. The stream processing system is characterized by
its real-time computation capability. It typically applies the
time-based windows for the real-time computation of data.
The stream processing systems typically processes the
streaming data in following three steps. First, it ingests the
input data by the specied window size. Second, it performs
various aggregation functions (e.g., Sum, Count, AVG, MIN,
and MAX) on the windowed data [44, 45]. Lastly, it trans-
mits the output results to the user dashboard. There exist
several types of windowing functions applicable to the query
computation scenarios. They include tumbling, sliding, hop-
ping, session, and snapshot window. (
window-functions). The tumbling window groups the input
tuples in a single time-based window. There is no overlap-
ping of windows, and each tuple belongs to only one time
window. In the hopping window, the hopping function is
forwarded in time by a xed interval known as hop size. It
consists of three parameterstimeunit, hopsize, and win-
dowsize. The sliding window is characterized by the sliding
interval and the window size. It slides over the data stream
according to the particular sliding interval. It outputs the
aggregated events upon the completion of the sliding inter-
val. The aggregation function computes over all the tuples
included in the current time window. In contrast to the tum-
bling window, sliding and hopping windowed functions can
overlap, and one tuple can belong to multiple windows.
Window size is the key element of stream query process-
ing system. The window size determines the time duration of
stream data ingestion. Stream query performs computations
over windowed data items. Authors in [17] have mentioned
the importance of window size regarding query computation
latency. The query computation latency increases with the
increased window size. Therefore, it is advisable to use small
size time windows to cope with the low latency requirement
of applications. Furthermore, as discussed above, there exist
dierent window types. Each window type comprises unique
functionality, and its selection depends upon the query com-
putation scenarios.
2.2. Cloud-Based Query Processing. Stream query processing
consists of three main components which include data
sources, processing operators, and output visualization
dashboard [1]. The processing operators are functional
units that perform aggregation operations on the windowed
data. The cloud-based query execution plan (QEP) places all
query processing operators on the central node. Figure 2
shows the simplied view of cloud-based query processing
In the cloud query execution, the streaming data items
are transmitted through WAN links to the central stream
processing system located in the cloud data center. The
input data items are then ingested by the data ingestion sys-
tem. After that, the data ingestion system forwards the win-
dowed data items to the query processing module. The query
processing operators then perform the aggregation opera-
tions on the windowed data. Finally, the query results are
forwarded to the user dashboard after query processing.
2.3. Geo-Distributed Query Processing. The geo-distributed
query processing system spans multiple geo-distributed sites.
Each site varies in its compute capacity (cores and storage)
and uplink, downlink bandwidth [5]. Each sites local
Analytics output
Smart cars
Streaming data sources
Window size = t seconds
Cloud data center
Figure 2: Simplied view of the cloud query processing model.
3Wireless Communications and Mobile Computing
manager is responsible for the continuous monitoring of its
computational and network resources. The local manager
updates the global manager about resource information
periodically [4]. This network monitoring is crucial for
WAN-aware query optimization.
The geo-distributed streaming data spans multiple data
sources, including IoT and E-commerce applications. Like-
wise, query output transmission may also span multiple
locations depending on the query scenario. The geo-
distributed query processing system translates the query to
the directed acyclic graph (DAG) of stages comprising graph
vertices as query operators and edges as data ows between
query operators [10]. Figure 3 depicts the query execution
model which is employed by the geo-distributed query pro-
cessing schemes in the reviewed literature [48, 10].
Geo-distributed query execution model comprises one
global site and multiple local sites. For the sake of simplicity,
Figure 3 represents just two local sites. Figure 3 shows the
geo-distributed query execution model. It consists of one
global site and two local sites (site-1 and site-2).
The global site is responsible for the optimal QEP gener-
ation and query scheduling. The QEP optimization is per-
formed according to the current WAN conditions. The
global manager analyzes the WAN condition by examining
the network-compute resource updates. These resource
updates are transmitted to the global-site manager by the
local-site managers on a periodic basis. After QEP genera-
tion, the job scheduler places the query tasks on dierent
sites in a resource-aware manner. The geo-distributed query
task assignment spans multiple geo-distributed sites to
achieve the goal of distributed processing. The resource het-
erogeneity incorporation is crucial for ecient task place-
ment in geo-distributed query processing [5, 11, 46].
Therefore, most of the geo-distributed query optimization
techniques have incorporated WAN-awareness in the gener-
ation of their optimal QEPs [6, 8, 9].
2.4. Edge-Cloud-Based Query Processing. In the edge-cloud-
based query processing, the query operators span both edge
nodes and cloud layer. The edge-cloud query execution
model comprises the cloud tier and the edge computing tier.
These two tiers dier in their compute-storage capacity, net-
work latency, and communication cost. The cloud tier is
enriched with abundant compute and storage resources as
compared to the edge, but falls short for network communi-
cation cost and latency. On the other hand, the edge com-
puting tier has limited computational and storage
resources. But, it succeeds in providing low latency, ecient
bandwidth utilization, and reduced network cost. Therefore,
the edge-cloud-based query processing takes full benetof
both tiers by placing the query operators on both the edge
and the cloud according to their resource-latency demands.
The stream query processing comprises three compo-
nents: the source node, processing operators, and the sink
node [1]. A query may consist of multiple processing opera-
tors depending on the query scenario. The compute-storage
demand of the query operator depends on the data ingestion
amount and the type of processing operation performed by
it on the windowed data. The query operators vary in their
compute storage and latency demand. Thus, the edge-
cloud-based passes cloud data center.
Operator placement scenario places the query operators
in a resource-latency-aware manner. The edge-cloud-based
query processing divides the query operators into two
groupsedge executable and cloud executable. The edge-
executable query operators (local operators) span edge
nodes, and cloud executable query operators (global opera-
tors) encompass the central data center.
Figure 4 shows the execution model of edge-cloud-based
query processing, which is employed by most of the edge-
cloud-based query processing techniques in the reviewed lit-
erature [1, 3, 15]. The edge-cloud architecture comprises
three layers which include the end device layer, edge, and
Status update
Global manager
Local manager (Site-1)
Local manager (Site-2)
Local query
Query result
Status update
Global site
Distributed query
execution module
Distributed query
execution module
Geo-distributed local sites
= WAN network conditions
+ compute resource conditions
Figure 3: Geo-distributed query processing model.
4 Wireless Communications and Mobile Computing
cloud layer. The number of edge computing layers varies
depending upon the query processing scheme. Some
schemes have utilized a single edge layer, whereas others
have utilized multiple edge computing layers [17, 18]. In
Figure 4, L OP
to L OP
represents the local operators
whereas G-OP represents the global operator. The local
operators of edge-layer1 ingest streaming data items from
source nodes. After that local operators perform some calcu-
lations on the data items and transmit them to the upper
edge layer in the computing hierarchy. Finally, the edge-
layer-ntransmits the results of partial calculations to the
G-OP. The G-OP aggregates the partial calculations to com-
pute the complete query results. Upon the completion of
global calculations, these analytics results are transmitted
to the user dashboard for visualization.
2.5. Edge-Based Query Processing. In edge-based query pro-
cessing, the query processing operators span edge nodes.
Compared to the edge-cloud query processing, the operator
placement of edge query processing spans solely within the
edge. The edge query processing takes advantage of the com-
putational capability of edge nodes for some simple types of
query processing scenarios [19, 39, 40]. The edge query pro-
cessing initiates by the submission of a query on the edge
node. The query can be submitted to any edge node,
depending upon the location of the query-submission user.
The query is processed according to the optimal QEP. The
edge-based query processing emphasizes the latency aware-
ness incorporation in its optimal QEP due to the heteroge-
neous network-compute nature of edge environment.
The existing literature shows that some studies have uti-
lized edge resources to process some simple types of queries
which include distributed join and image recognition query
processing scenarios [39, 40]. Figure 5 shows the architectural
view of distributed join query processing within the edge envi-
ronment [39]. The distributed join query processing scenario
requires data shipment between dierent edge nodes (E-1, E-
2, and E-3), as shown in Figure 5. The data shipment direction
depends upon the placement location of the query processing
operator. The query can be processed on the same node where
it is submitted or on some other edge node depending upon
the optimal QEP. After complete query processing, the output
results are shipped to the user dashboard.
Similarly, in the case of the image recognition scenario,
application logic is divided into a sequence of small tasks
to make it adaptable to the edge environment [40]. In this
scenario, the computational tasks are ooaded to the group
of edge nodes. Figure 6 shows the task ooading scenario
within the edge environment [40]. It comprises four steps
which include cluster enabling, node discovery, node selec-
tion, and task placement. In the cluster enabling phase,
network-connected edge nodes send join requests to the net-
work manager. The network manager sets up the connection
with the edge nodes as shown in Figure 6. The initiator node
then discovers the physically and functionally reachable edge
nodes in the node discovery phase. The nodes opt for selec-
tion sends ack (acknowledgment) to the initiator node.
Finally, the initiator node selects the nodes for task place-
ment based on cluster suitability criteria.
Edge query processing is benecial in cutting down the
networking delay, excessive bandwidth usage, and complete
task ooading from the cloud. This type of query processing
scenario exploits the collaboration of edge nodes in perform-
ing the parallel execution of query tasks. But the edge-based
Partial calculations
Partial calculations
Aggregation of partial calculations
Input data sources
Streaming data items
Partial results
Query results
Cloud data center
Figure 4: Depiction of query operator placement within the edge-cloud environment. Local operators (L OP
to L OP
) spans edge
computing layers (edge-layer-1 to layer-n) and global operator placement (G-OP) encompasses cloud data center.
5Wireless Communications and Mobile Computing
query processing is feasible for some simple type of query
processing scenarios as depicted by the edge-based query
processing literature [19, 39, 40]. The feasibility for edge
query processing depends upon the query type, type of pro-
cessing operations, and the data ingestion amount.
3. Challenges
In this section, we discuss some of the key challenges of
query optimization in the centralized and distributed query
processing. These challenges include central data aggrega-
tions, network-compute-data heterogeneity, WAN dynam-
ics, WAN-data transmission cost, and privacy.
3.1. Central Data Aggregation. In the centralized query pro-
cessing scenario, the data is transmitted to the central node
for aggregation through WAN links. The centralized data
aggregation suers from networking delays and excessive
bandwidth consumption. These longer networking delays
aect the processing of latency-sensitive queries. Moreover,
the raw data transmissions to the central node may result
in excessive network trac and link congestion.
Query results
E1 - Data sources
E2 - Data sources
E3 - Data sources
Figure 5: Representation of distributed join query processing within the edge environment. Edge nodes (E-1, E-2) perform data shipment to
E-3 for query processing.
Step -III, IV-Node selection
and task placement
Step II-Node discovery
1-Ooad REQ
Step I-Cluster enabling
Figure 6: Illustration of query task ooading within the edge environment. The task ooading scenario comprises four steps; cluster
enabling, node discovery, node selection, and task placement.
6 Wireless Communications and Mobile Computing
3.2. Network-Compute-Data Heterogeneity and WAN
Dynamics. The distributed query processing spans a WAN
environment comprising multiple edge data centers. Geo-
distributed query suers from WAN resource dynamicity
and heterogeneity due to uctuating network conditions.
The WAN bandwidth is dened by its three propertiess-
carcity, variability, and expensiveness. Stable execution of
distributed queries is a challenging issue in the presence of
uctuating WAN resources. In addition to the network-
compute heterogeneity, the geo-distributed data also uctu-
ates due to the variable data emission rate. The data emis-
sion rate varies over time, thus, giving rise to workload
unpredictability in the WAN environment. Overall,
network-compute and data heterogeneity may lead to unex-
pected delays in distributed query processing which is intol-
erable for latency-sensitive queries.
3.3. WAN-Data Transmission Cost. In the distributed query
processing scenario, data needs to be transmitted between
dierent edge sites. These data transmissions contribute to
the cost bottleneck within the WAN environment because
WAN links are equipped with variable costs due to heteroge-
neous pricing policies. [9]. The existing literature also shows
that intersite data transmission cost is higher than intrasite
data transmission cost [15]. Therefore, the distributed query
processing needs to abide by some cost constraints to bal-
ance the query execution latency and data movement cost.
3.4. Privacy. Privacy is a major concern in distributed query
processing because its processing spans multiple geo-
distributed sites or edge data centers. The edge environment
also lacks ecient regulatory policies regarding privacy and
security as compared to cloud [39]. Moreover, the data
transmissions to the untrustworthy nodes in distributed
query processing may lead to the disclosure of sensitive
information. Therefore, it is crucial to take measures for
secure data movements and processing within the edge
4. Review and Comparison of Query
Processing Techniques
4.1. Literature Review. This subsection reviews the query
processing latency optimization schemes within the context
of their execution models. Table 1 categorizes the query pro-
cessing techniques regarding their execution models.
4.1.1. Cloud-Based Query Processing Techniques. Quoc et al.
[42] propose StreamApprox, an approximation-based query
processing system. StreamApprox attains low latency and
ecient resource utilization by executing the query on
reduced data (a subset of data). StreamApprox introduces
OASRS (online adaptive stratied reservoir sampling) algo-
rithm to achieve data reduction. The OASRS algorithm takes
as input the users specied query and query budget to per-
form operations on the windowed data. Query budget is
dened in terms of latency tolerance level, throughput, accu-
racy, and available compute resources. The system deter-
mines the sample size for each windowed interval
according to the query budget. Query budget is adaptive
towards users demand across windowing intervals. After
the sample size determination, the OASRS algorithm
Table 1: Categorization of query processing techniques within the context of their execution models.
Query processing techniques Year
Query execution models
Centralized Geo-distributed
query processing
Distributed Edge-query
Edge-cloud query
Iridium [4] 2015
Clarinet [10] 2016
SpanEdge [1] 2016
IncApprox [41] 2016
StreamApprox [42] 2017
Sana [5] 2018
SAQL [43] 2018
AWStream [6] 2018
Tetrium [7] 2018
Sajjad et al. [15] 2018
Amarasinghe et al. [3] 2018
QueryGuard [39] 2018
ApproxIoT [17] 2018
CalculIoT [18] 2019
WASP [8] 2020
Oh et al. [9] 2020
Dautov and Distefano [40] 2020
Fossel [19] 2020
7Wireless Communications and Mobile Computing
performs sample selection on the windowed data. Finally,
the system executes the query on the sampled data. The
OASRS algorithm takes benet of both sampling meth-
odsstratied and reservoir sampling. Stratied sampling
incorporates fairness by selecting data items fairly from the
dierent input data streams, and the reservoir sampling
technique holds good for the sampling of streaming data.
StreamApprox system repeats the whole process for each
time window, by performing query execution on sampled
data streams.
IncApprox [41] introduced the concept of incremental
and approximate computing for fast query processing. The
authors proposed an online-biased sampling algorithm to
combine incremental and approximate computing. The pro-
posed algorithm works by biasing the sample selection
towards the memorized data elements.
Gao et al. [43] proposed SAQL, the stream-based real-
time query system for the detection of anomaly events. The
SAQL system takes as input the system monitoring data in
the form of event streams. The SAQL system analyzes the
event streams and generates alerts upon the arrival of anom-
aly events. SAQL query engine includes four modulesmul-
tievent matcher, state maintainer, concurrent query
scheduler, and error reporter. Multievent matcher job is to
match the input stream events against the query specied
event patterns. The state maintainer maintains the state of
each sliding window based on the matched events. The con-
current query scheduler groups the concurrent queries for
execution to reduce the redundant data transmissions.
Finally, the error reporter reports the error during execution.
SAQL has also introduced the concept of concurrent query
execution for the ecient utilization of system resources.
The concurrent query scheduler divides the compatible
queries into separate groups leveraging MasterDependent-
Query-Scheme.This scheme reduces the data replications
by using one data copy for each group.
4.1.2. Geo-Distributed Query Processing Techniques. Jona-
than et al. [5] introduced a WAN-aware multiquery optimi-
zation scheme for the reduction of query processing latency
and ecient utilization of WAN bandwidth. This scheme
allows multiple queries to share their common executions
in a WAN-aware manner. The sharing types include IN
(input-only sharing), IN-OP (input-operator sharing), and
partial input sharing. The multiquery optimization scheme
generates the QEPs by exploiting those sharing opportuni-
ties which do not lead to WAN bandwidth contention. The
bandwidth contention may result in the performance degra-
dation of the system. This scheme outputs and submits the
set of WAN-aware QEPs to the job scheduler. The job
scheduler prioritizes the deployment of the QEPs, which
exhibit IN-OP sharing over IN-sharing because its main
focus is the minimum bandwidth utilization. Moreover, this
scheme is also adaptive towards the query requirements in
terms of minimum resource utilization or min-latency.
Zhang et al. [6] proposed AWStream, the WAN adaptive
scheme for the stable execution of queries in a dynamic
WAN environment. AWStream introduces data degradation
functions to adapt to the dynamic WAN bandwidth. It auto-
matically learns the pareto optimal strategy about the invok-
ing of degradation function according to the current
network conditions. For this purpose, the system builds an
accurate model based on the application accuracy and band-
width consumption under dierent combinations of data
degradation functions. To build the model, the system rst
uses oine training to build the default prole by utilizing
the data provided by the developer. The system performs
online training for the continuous renement of the prole
according to the changing network conditions. For runtime
adaptation, AWStream examines the available bandwidth.
It automatically adjusts the application data rate according
to the available bandwidth. The AWStream application con-
troller handles the level of degradation according to the cur-
rent situation. If the data generation rate exceeds the data
processing rate, then, the application controller inquires
the estimated bandwidth and regulates the data ow by uti-
lizing the most suitable degradation function.
Hung et al. [7] introduced the Tetrium technique for the
optimal placement and scheduling of query tasks in a
network-compute aware manner. Tetrium performed task
placement optimization by setting the task assignment prob-
lem across geo-distributed sites as LP (linear program). The
optimal scheduling of multiple analytics jobs is done by inte-
grating the LP with SPRT (shortest remaining processing
time) heuristic. The Tetrium system enhanced the overall
performance of the system by prioritizing the scheduling of
shortest remaining time jobs.
WASP system [8] performed WAN-aware adaptations
for low latency and stable executions of long-running
queries. It introduced multiple adaptation techniques, which
include task reassignment, operator scaling, and query
replanning. The implication of the adaptation technique
depends upon the type of query and optimization goals.
The adaptation technique is selected automatically at run-
time according to the type of query. The WASP system con-
tinuously monitors its task performance metrics such as
processing latency and input/output data stream rates. Each
operator performs runtime monitoring of its resources to
detect any bottlenecks and unstable executions. WASP sys-
tem recomputes the sites capacity for task reassignment.
The system ensures that the site has enough network-
compute resources for task assignment. For operator scaling,
WASP performs scaleup/out to mitigate the computational
and network bottlenecks and scale down to limit the waste-
ful consumption of system resources. To scale down, WASP
gradually reduces the degree of parallelism to avoid any of
the remaining running tasks being constrained. In case of
query replanning, the query planner creates the query execu-
tion plan by considering the runtime information.
Oh et al. [9] introduced the cost-aware task placement
strategy for a geo-distributed query processing. The cost-
aware task placement technique allows queries to explore
the trade-ospace according to their cost and performance
demarcations. The task-placement problem is formulated
as a cost-constrained optimization problem and solved opti-
mally by employing MIP (mixed-integer programming). It
takes as input (i) query trade-opreference, (ii) network
bandwidth, (iii) data transmission cost, and (iv) tasks data
8 Wireless Communications and Mobile Computing
size. The system calculates the target cost budget by consid-
ering the trade-opreference. After estimating the target
budget, the task placement optimization utilizes the cost
budget as a constraint for query task placement.
Pu et al. [4] proposed Iridium technique that jointly
optimizes the task and input data placement to reduce the
query execution time. The Iridium system performs task
placement formulation as LP (linear program). For ecient
task placement, the LP determines the number of tasks to
be placed on each site by considering its uplink, downlink
bandwidth capacity, and intermediate data transmission
size. The LP avoids bottlenecks by refraining excessive data
transmission on bandwidth-constrained links. In addition
to the task placement optimization, the Iridium system opti-
mizes the placement of input data for the further reduction
of query processing time. The input data placement strategy
ooads the input data from bottleneck sites to other sites.
The data placement approach iteratively selects the bottle-
neck sites and transfers the input data in chunks to other
sites. The Iridium system also optimizes the input data
movement of multiple queries. The system prioritizes the
data movement of the most frequently accessed datasets or
the datasets whose input data movement contributes
towards the reduction of intermediate data transmission
time. If the data movement of specic dataset results in per-
formance degradation of already running queries, then, the
system leaves this dataset and goes for the next dataset in
the ordered list. The system performs the data movement
of prioritized datasets in the descending order of their query
lag times.
Viswanathan et al. [10] proposed an ecient WAN-
aware query optimizer (Clarinet) for the latency optimiza-
tion of geo-distributed query. The Clarinets query optimizer
has incorporated WAN-awareness in its QEP by ltering out
the QEPs whose task placement can result in excessive band-
width usage. The QEP optimization involves optimal task
placement and scheduling. The optimal task placement
strategy deploys the independent tasks (map tasks) by fol-
lowing the concept of site locality. For intermediate task
(reduce tasks) placement, it takes into consideration (i) the
amount of data generated by its predecessor node, (ii) the
location of the predecessor node, and (iii) intersite band-
width. The main objective of the scheduling strategy is the
determination of the optimal start time of tasks that result
in minimum query execution time. The Clarinet system also
handles the scheduling of multiple queries. Clarinet adopts
the shortest job rst scheduling strategy to schedule the
QEPs of dierent queries. This scheme incorporates fairness
in the system by sorting the optimal QEPs of all queries in
descending order of their completion deadlines. In this
way, the system prioritizes the scheduling of queries whose
completion deadlines are near as compared to other queries.
4.1.3. Edge-Cloud-Based Query Processing Techniques. Sajjad
et al. [1] introduced SpanEdge, an edge-cloud-based query
processing scheme for the reduction of latency and band-
width consumption. In this scheme, the stream processing
application spans two tiersedge data centers and cloud.
The system denes two types of tasks named as local and
global tasks. Local tasks are the tasks that need to be placed
near the data-producing sources. Local tasks perform partial
calculations, which are fed as input to global tasks for com-
plete calculations. SpanEdge assigns local tasks to the spoke
workers and global tasks to the hub workers. The hub
workers reside in the cloud data center, whereas spoke
workers span edge data centers. The SpanEdge scheduler
examines the tasks attributes before deploying it on the
hub or spoke worker. Task assignment comprises two steps:
(i) the local tasks are assigned to the spoke-workers, and (ii)
global tasks are allocated to the hub-workers. For the local
task assignment, the scheduler iterates through every local
task to retrieve its input data sources. The scheduler deploys
each local task to that spoke worker, which resides in close
vicinity to its input data sources. For the global task assign-
ment, the scheduler iterates through the local tasks that are
adjacent to the global task and groups the spoke workers
of these local tasks in a set. Finally, it selects that hub worker
for global task assignment which resides in close vicinity to
the group of spoke workers. After the assignment of all tasks,
the runtime engine of SpanEdge performs distributed query
processing by utilizing the computational capability of both
edge and cloud data centers.
Amarasinghe et al. [3] introduced an optimization
framework for the optimal placement of query processing
operators in an integrated edge-cloud environment. The
optimization framework models the operator placement sce-
nario as a constraint satisfaction problem by considering the
operators compute and power consumption requirements.
The optimization system considers two types of con-
straintsresidency and resource constraint. Residency con-
straints are related to the operators location, whereas
resource constraints are linked with its resource utilization.
Residency constraints include (i) system utilizes the edge
nodes solely for source task placement, (ii) cloud executable
task placement encompasses only cloud data center, (iii) and
task assignment formulation must avoid the restricted nodes
for task allocation. Likewise, resource constraints include (i)
number of CPU cycles required by the query task must be
less than or equal to the hardness of the CPU constraint
and CPU clock speed of the node, (ii) tasks outgoing event
size and throughput must be less than or equal to its egress
link bandwidth capacity, and (iii) tasks energy consumption
must be less than or equal to the maximum power drawn
from the power source at that node. The query optimization
framework introduced in this study performs query operator
placement being mindful of these residency and resource
Sajjad et al. [15] proposed a low overhead coordination
algorithm for the reduction of data communication cost
across intersite links in an edge-cloud environment. This
scheme has focused on the windowed aggregation of data
streams. It utilizes intraregion links for the aggregation of
partial computations. The coordination algorithm selects
one edge data center from each region as the master data
center (DC). Each region comprises an nnumber of edge
data centers. All edge data centers of a region perform aggre-
gation operations on windowed data and transmit the aggre-
gation results to the master DC. The master DC then
9Wireless Communications and Mobile Computing
aggregates the partially aggregated results that are transmit-
ted from the other edge data centers. Finally, each regions
master DC transmits the aggregated results to the cloud data
center. The coordination algorithm has reduced the interre-
gion communication cost by reducing the amount of data to
be transmitted over interregion links.
The ApproxIoT scheme introduced the concept of
approximation and hierarchical processing for stream query
execution [17]. ApproxIoT achieved approximation by uti-
lizing edge nodes for data sampling. For this purpose,
authors utilized integrated edge-cloud-based architecture
comprising source nodes, two edge computing layers, and
one cloud layer. In ApproxIoT, edge nodes ingest data
streams from source nodes, perform sampling, and transmit
the sampled data items to the cloud node. The cloud node
again samples the data streams and then executes the query
on sampled data. ApproxIoT achieved low latency and min-
bandwidth consumption by reducing the query computation
time and transmission data size. Transmission data size is
reduced by transmitting the sampled data streams over
WAN links, whereas computation time is reduced by execut-
ing the query on sampled data.
Ding and Dan [18] propose CalculIoT, an edge-fog
cloud-based query processing scheme. CalculIoT utilizes
edge nodes for data calculations, fog layer for aggregation,
and cloud for query result transmission. In this scheme,
the edge computing layer ingests data items from source
nodes and performs calculations on the windowed data.
After performing calculations, it transmits the calculation
results to the fog layer. The fog layer then aggregates the cal-
culation results and transmits them to the cloud layer.
Finally, the cloud layer transmits the query results to the
user dashboard. CalculIoT attains low networking delay
and reduced bandwidth consumption by transmitting the
aggregated data to the cloud layer over WAN links.
4.1.4. Edge-Based Query Processing Techniques. Xu et al. [39]
proposed QueryGuard, an ecient privacy and latency-
aware query optimization technique for distributed query
processing on edge. QueryGuard ensures the sensitive data
transmissions to the secure edge nodes for intersite join
query processing. Additionally, it also optimizes query pro-
cessing latency by considering the network conditions of
the edge environment. QueryGuard incorporates privacy
awareness into the system by introducing few privacy con-
straints. These privacy constraints include privacy level and
privacy preference constraints. The privacy level constraint
states that one edge node can transmit data to another edge
node if both exhibit the same privacy level or the destination
edge node has a higher privacy level. The privacy preference
constraint is related to the users preference in terms of edge
node selection for data processing. The main goal of the
QueryGuard system is the incorporation of the privacy con-
straints and latency-awareness in its optimal QEP genera-
tion. QueryGuard incorporates latency-awareness by
selecting the min-latency QEP as optimal from the set of
possible QEPs.
Dautov and Distefano [40] introduced the concept of
dynamic horizontal ooading for distributed query process-
ing within the edge environment. This approach liberates
the stream query processing from centralized control by
deploying it within the edge environment. In order to enable
decentralized query processing on edge, the authors
extended the Apache NiFi middlewareby modifying and
adding some new modules to the existing architecture. The
newly added modules include task partitioner, node discov-
ery, and node selector. The crux of the horizontal ooading
scheme is the cluster formation of edge devices. It comprises
four steps which include cluster enabling, node discovery,
node selection, and task placement. The cluster enabling
process is initiated by the edge node that needs to ooad
the task to the peer devices. Prior to task ooading, the task
partitioner module breaks down the stream processing
application logic into the sequence of small atomic tasks to
make it adaptable to the horizontal ooading scenario. After
that, tasks computation requirement is identied in order to
initiate the node discovery and selection process. The node
discovery process is initiated for the integration of edge
devices into a common cluster. The node discovery process
includes network discovery and functional discovery. After
the node discovery, the initiator broadcasts the task ooad-
ing requests to the functionally and physically reachable
nodes. Upon receiving the broadcast requests, peer nodes
rst examine the task requirements and opt for selection if
they meet the required criteria. The initiator node then
examines the discovered nodes given the cluster suitability
criteria. After nal evaluations, the initiator node assigns
the task to the group of selected nodes.
Abdullah et al. [19] introduced Fossel, an ecient
latency reduction scheme for query processing. Fossel per-
formed path delay optimization for the reduction of query
execution latency. Path delay is optimized by introducing
the algorithm named as Optimal Fog Nodes Selection for
Sampling.The algorithm selects the set of optimal fog
nodes for sampling for path delay optimization. The Fossel
system sampled the input data streams on optimal fog nodes
and performed query execution on the root fog node. Fossel
achieved low latency and min-bandwidth consumption by
reducing the data size and executing the query within the
4.2. Comparison of Query Processing Techniques. This sub-
section compares dierent query processing techniques
regarding latency optimization. We compare query process-
ing techniques based on some key features. The key features
are their main focusing points in performing latency optimi-
zation. These key features include network-awareness,
network-compute awareness, multiquery execution, data
reduction, WAN adaptive, partial/full computations on
edge, and privacy awareness. Table 2 categorizes the query
processing techniques based on these key features. Table 3
presents the dierentiating points of query processing tech-
niques based on their key features.
4.2.1. Network-Awareness. The Iridium technique optimized
the geo-distributed query processing latency by incorporat-
ing the network awareness in input data and task placement
[4]. The geo-distributed query processing scenario executes
10 Wireless Communications and Mobile Computing
query tasks in parallel. Its query completion time is highly
aected by the transmission time of the bandwidth-
constrained link. The main goal of the Iridium scheme is
networking delay reduction. Therefore, its task placement
strategy incorporates network awareness by avoiding the
task placement on bottleneck link sites.
Keee et al. [47] reduced the query execution latency for
MANETs (mobile ad-hoc networks) by introducing the
network-aware query processing scheme. This scheme has
coped up with the dynamic network conditions by increas-
ing the network path diversity. Path diversity has enlarged
by deploying the query operator replicas on multiple nodes.
The selection of optimal operator replica is based on its net-
work cost. The system selects the operator replica that
results in the minimum network cost of the whole path. Net-
work awareness is incorporated by placing the query tasks
(map, reduce) in a network-aware manner [10]. Map task
placement followed the concept of data locality, whereas
reduce task placement formulation considered three metrics;
intersite bandwidth, location, and amount of data generated
by its predecessor node. This scheme has ltered out the task
placements that result in link congestion and excessive net-
working delay in query processing.
4.2.2. Network-Compute Awareness. The Tetrium approach
optimizes the query processing latency by placing the query
tasks in a network-compute-aware manner [7]. The main
aim of Tetrium task placement strategy is the optimization
of networking delay as well as compute time. The Tetrium
system performs latency optimization by setting the task
assignment problem across geo-distributed sites as an LP
(linear program). The Tetrium map-task placement strategy
aims for the reduction of the whole processing time of the
map stage, where reduce-task placement focuses on the
reduction of the jobs remaining compute time in the reduce
4.2.3. Multiquery Execution. SAQL system introduced the
master-dependent-query scheme for the ecient execution
of multiple queries [43]. In this scheme, each group of com-
patible queries comprises one master query and other
Table 2: Comparative analysis of query processing techniques.
Query processing
on edge
Iridium [4]
Keee et al. [47]
Clarinet [10]
Tetrium [7]
SAQL [43] ✓✓
Sana [5] ✓✓
StreamApprox [42]
ApproxIoT [17]
Fossel [19]
WASP [8] ✓✓
Kimchi [9]
AWStream [6] ✓✓
QueryGuard [39]
SpanEdge [1] ✓✓
Amarasinghe et al. [3] ✓✓
Sajjad et al. [15] ✓✓
CalculIoT [18] ✓✓
Dautov and Distefano [40]
Table 3: Query processing techniques dierentiating points.
Key features Dierentiating points
Network-aware input and data
task placement [4]
Network-aware operator selection
Network-aware task placement [10]
Network-compute aware task
placement [7]
Multiquery execution Sharing opportunities [5]
Master-dependent-query scheme [43]
Data reduction
Sampling [17, 19, 42]
Partial computations on edge
[1, 3, 15, 18]
Data degradation [6]
Task-reassignment, operator-scaling,
query-replanning [8]
Privacy-awareness Privacy-aware data transmissions [39]
Anomaly detection engine [43]
11Wireless Communications and Mobile Computing
dependent queries. The dependent queries share the execu-
tions with the master query. The SAQL system checks the
compatibility of a newly arrived query against all existing
master queries. If the new query does not exhibit any level
of compatibility with the existing master queries, then, the
system sets the newly arrived query as a master query. Oth-
erwise, the system adds the query to its compatible group.
The system repeats this process on the arrival of each new
query. The master query of each group can directly access
the data, whereas the execution of the dependent query relies
on the execution of the master query. If two queries exhibit
similarity, then, the query engine directly utilizes the execu-
tion output of the master query for the dependent query.
Otherwise, it checks the level of compatibility between two
queries and fetches the intermediate execution results for
the dependent query.
Sana system [5] performs multiquery optimization by
allowing the multiple queries to share their common execu-
tions in a WAN-aware manner. In this scheme, three shar-
ing types are introduced (i) IN-OP sharing, (ii) IN-sharing,
and (iii) partial input sharing. The IN-OP sharing type
groups the queries that perform the same processing function
on the same input data. Likewise, IN-only sharing groups the
queries that require common input data. The system selects
the sharing type for each query within the context of min-
bandwidth consumption. For instance, if the query can share
both types of sharing opportunities (IN-OP and IN-only),
the job scheduler ensures min-bandwidth consumption by
prioritizing the deployment of IN-OP sharing over IN-only
4.2.4. Data Reduction. StreamApprox [42] performs data
reduction to reduce the query processing latency. It reduces
the query computation time by performing query operations
on sampled data items. ApproxIoT [17] has also utilized
sampling for data reduction. But in ApproxIoT, sampling
is performed multiple times by two edge computing layers
and cloud layer before query execution. The query process-
ing latency is optimized by reducing both networking and
processing delay. Networking delay is reduced by reducing
the transmit data size over WAN links. Fossel scheme [19]
has performed data reduction by employing sampling in a
resource-latency-aware manner. Fossel scheme has reduced
latency by performing query execution on sampled data
and saved system resources by utilizing a subset of fog nodes
for sampling.
4.2.5. WAN-Adaptive. In the WASP system, the query pro-
cessing scheme adapts to the dynamic WAN conditions in
a network compute aware manner [8]. This scheme intro-
duces three adaptation techniques, namely, task reassign-
ment, operator scaling, and query replanning. WASP
system selects the adaptation technique on runtime by con-
sidering the query type and current network conditions.
AWStream performs runtime adaptation to the dynamic
WAN conditions by introducing the data degradation oper-
ators [6]. The system selects the data degradation level by
examining the application requirement and current band-
width conditions. If the data generation rate exceeds the data
processing rate, the AWStream then examines the band-
width and regulates the data ow by utilizing the most suit-
able degradation conguration.
4.2.6. Cost-Awareness. Oh et al. [9] introduced a cost-aware
query processing scheme for a geo-distributed environment.
The authors aimed to optimize the query execution time by
considering the query cost budget as a constraint in task
placement formulation. This scheme prioritizes the schedul-
ing of query tasks on the optimal data centers in a cost-
aware manner. Additionally, this scheme also adapts to the
dynamic WAN conditions. It examines the occurrence of
WAN dynamics by computing the dierence between the
estimated and expected latency. If WAN dynamics occur,
the scheduler adapts to the current network conditions by
performing the cost-aware task adjustment.
4.2.7. Partial/Full Computations on Edge. SpanEdge [1] per-
formed partial calculations on edge by deploying the stream
processing application in an integrated edge-cloud environ-
ment. In SpanEdge, local task placement spans edge data
centers, whereas global task placement encompasses cloud
data center. Local tasks perform the partial calculations on
the input data and transmit the partial results to the global
task for complete computations. SpanEdge has reduced the
query processing latency by performing partial computa-
tions on edge.
Amarasinghe et al. [3] performed partial computations
on edge by the optimal deployment of the query operators
in an integrated edge-cloud environment. The authors intro-
duced an optimization framework for the optimal placement
of query operators. The optimization framework optimizes
the query operator placement by considering the residency
and resource constraints. Dautov and Distefano [40] per-
formed complete query computations on edge by introduc-
ing a horizontal ooading scheme. This scheme performs
query processing in a distributed way by ooading the
query processing tasks to a cluster of edge devices. The initi-
ator node initiates the cluster formation process by discover-
ing the computationally and physically reachable edge
devices. After node discovery, the initiator node examines
the discovered nodes based on the cluster suitability criteria.
The cluster suitability criteria ensure that the selected node
would not lead to higher networking delay and exhibit su-
cient and relatively equal compute capacity with other clus-
ter nodes.
Sajjad et al. [15] reduced data transmission cost by utiliz-
ing intraregion links for the aggregation of partial calcula-
tions. This scheme selects master DC from each region
based on some attributes. The master DC collects the calcu-
lation results from other edge DCs residing within its
region. Each master DC then aggregates the calculation
results and transmits them to the central DC. In this scheme,
data transmission cost is optimized by allowing master DCs
to transmit data on intersite links instead of all edge DCs. Cal-
culIoT [41] reduced the query processing latency by reducing
the transmission data size. It utilized edge-fog-cloud architec-
ture for stream query processing. CalculIoT performed data
calculations on edge computing layer and utilized fog nodes
12 Wireless Communications and Mobile Computing
for data aggregation. The networking delay is reduced by
sending the aggregated data to the cloud layer.
4.2.8. Privacy-Awareness. QueryGuard [39] introduced pri-
vacy awareness for distributed join query processing within
the edge environment. The QueryGuard system ensured pri-
vacy by allowing sensitive data transmissions to the trusted
edge nodes in intersite join query processing. In distributed
join query processing, data needs to be shipped between dif-
ferent edge nodes. As edge nodes comprise dierent privacy
levels, the data shipment to the untrustworthy nodes may
lead to the disclosure of sensitive information. Therefore,
the QueryGuard system introduced some privacy con-
straints to handle sensitive data transmissions within the
edge environment.
Gao et al. [43] designed SAQL, an anomaly detection
query system. SAQL system takes as input the system moni-
toring data and continuously queries it for the identication
of abnormal behaviour. SAQL system introduced four types
of anomaly detection queriesrule-based anomalies, time-
series, invariant-based, and outlier-based anomalies. The main
aim of the SAQL query engine is to save the system nodes
against cyberattacks by continuously querying the system
monitoring data. Since, in most cases, cyberattacks proceed
in a sequence of steps, and it is possible to detect the attack
in its initial stages by continuously monitoring the system
data. SAQLs query engine took advantage of the sequential
steps accomplishment property of cyberattacks for real-time
anomaly detection.
5. Discussion of Query Technique Key Features
and Execution Models
In this section, we discuss and analyze the query processing
techniques regarding their key features and execution
models. The query processing techniques related to the net-
work awareness category have performed latency optimiza-
tion by considering the heterogeneous WAN conditions.
They performed optimization within the context of optimal
task placement and scheduling, optimal operator selection,
and optimal QEP generation by considering the network
heterogeneity of the WAN environment [4, 10, 47]. How-
ever, Tetrium system has introduced network-compute
awareness in geo-distributed query processing [7]. The intu-
ition of Tetrium scheme is based on the concept that geo-
distributed environment exhibits heterogeneity in both net-
work and compute resources. The existing literature also
shows that the geo-distributed data centers vary in their sizes
and compute capacities. Additionally, the network-aware
task placement may lead to a computational bottleneck by
scheduling more tasks on a single site. Therefore, Tetrium
system has performed task placement optimization, being
mindful of the network-compute heterogeneity of the geo-
distributed environment.
Sana and SAQL system introduced multiquery optimiza-
tion schemes to reduce the redundant data transmissions
and ecient resource utilization [5, 43]. Sana system has
coped well with the heterogeneous WAN resources by allow-
ing the query system to select the most suitable sharing
opportunity according to the current network conditions
and query type. Moreover, it is also exible to cope up with
the latency and resource demands of dierent queries. On
the other hand, the SAQLs concurrent query execution
scheme has focused on the ecient utilization of memory
usage by utilizing one data copy for the group of compatible
queries instead of using separate data copy for each query.
StreamApprox, ApproxIoT, and Fossel performed sam-
pling to reduce the query execution latency [17, 19, 42].
They optimized the latency by reducing the transmit and
processing data size. Likewise, SpanEdge, CalculIoT, Amara-
singhe et al., and Sajjad et al. performed partial computa-
tions on edge to reduce the transmit data size over WAN
links. Both groups of approaches have reduced the network-
ing delay and bandwidth consumption by reducing the
transmit data size. However, they dier based on their data
reduction techniques. The former group has performed data
reduction by sampling, whereas the latter group performed
partial computations on edge to reduce the transmit data
size [1, 3, 15, 18]. The sampling-based techniques may result
in less accurate query results as compared to the partial com-
putations group. The intuition behind sampling-based tech-
niques is that some applications also go well with the
approximate results because they aim at fast processing of
query results compared to accurate results. Therefore, we
can switch to the sampling-based or partial computation
techniques according to the application demand.
WASP and AWStream introduced the WAN-adaptive
schemes for the stable running of analytic queries [6, 8].
WASP system has introduced three adaptation techniques
to handle the WAN dynamics. AWStream adapts to the
WAN dynamics by introducing the dierent data degrada-
tion operators. It adapts to the WAN dynamics by congur-
ing the data degradation operator according to the current
bandwidth conditions. AWStream system design is moti-
vated by the idea that a single degradation policy may not
go well for all applications because every application has dif-
ferent performance and accuracy demands. For instance, the
video streaming applications are more focused on QoE
(quality of experience), whereas video analytic applications
prefer image quality compared to QoE. Therefore, the sys-
tem should adapt to handle the processing of dierent appli-
cations eciently. In short, the AWStream system has
performed well in fullling the performance demands of
the heterogeneous applications in a network-aware manner,
whereas the WASP system has mitigated the network-
compute bottlenecks by introducing network-compute-
aware WAN adaptive techniques.
QueryGuard and SAQL systems introduced privacy-
awareness in stream query processing [39, 43]. QueryGuard
system has performed privacy-aware data transmissions for
distributed query processing within the edge environment.
On the other hand, SAQL has introduced an anomaly detec-
tion query engine for the identication of cyberattacks. Both
schemes are ecient for ensuring privacy, but they dier in
achieving privacy or security related to their functioning and
execution models. QueryGuard system aimed at ensuring
the privacy of edge query processing, whereas the SAQL sys-
tem introduced an anomaly query engine which can be
13Wireless Communications and Mobile Computing
utilized in business organizations to ensure the security of
system nodes against cyberattacks.
The above discussion shows how current query process-
ing techniques performed latency optimization by overcom-
ing the network-compute heterogeneity and privacy
challenges. These query execution models dier in terms of
their positive and negative features and latency reduction
rate. We estimated the latency reduction rate of distributed
query execution models in comparison with the centralized
execution model [3, 8, 19, 40, 42, 43]. The quantitative esti-
mation shows that geo-distributed, edge-cloud, and edge-
based query execution has reduced latency by 9.3, 21.4,
and 23.2 times compared to the cloud query execution.
Based on this estimation, we can classify the query execution
models as high medium and low latency query processing
models. We classify cloud query execution as high latency,
geo-distributed as a medium, and edge-cloud based and
edge-based query execution as low latency execution.
Table 4 depicts the main dierences between query exe-
cution models in terms of their strengths and weaknesses.
Cloud-based query processing does not suer from
network-compute heterogeneity but suer from longer net-
working delays. The cloud-based schemes have successfully
optimized the processing delay still they are almost unable
to cope up with the demands of latency-sensitive queries.
The geo-distributed query processing has introduced
WAN-aware query optimization to deal with resource het-
erogeneity and WAN dynamics. But geo-distributed query
processing schemes are paying in terms of WAN data trans-
mission cost. Edge-cloud-based query processing has
reduced networking delay and WAN usage by performing
partial computations on edge, but it suers from the
resource-constrained and heterogeneous nature of edge
nodes. Likewise, edge-based query processing succeeded in
cutting down the networking delay by performing complete
query computations in the edge. However, it lacks support
for complex and resource-intensive queries due to the lim-
ited network-compute resources of the edge environment.
6. Future Work
Analysis of the reviewed literature shows that there exist two
major execution models for stream query processing. Each
execution model has its benets and challenges, as discussed
in Section 5. One execution model cannot be weighed to
another because each comprises dierent capabilities and
challenges. These execution models vary regarding query
response time, resource provision, and the number of com-
puting layers. Therefore, we can utilize them according to
the query type by introducing the idea of query categoriza-
tion. As it is already discussed in Section 5 that queries
belonging to dierent applications vary in their resource,
latency, and data ingestion demands, we can utilize these
metrics to determine the execution locality of queries. The
idea is to categorize and schedule the queries based on their
execution locality as edge-executable, edge-cloud executable,
and cloud-executable by employing multilayer query pro-
cessing (MLQP) architecture. The MLQP architecture will
cover both centralized and distributed query execution styles
by including all computing layers; edge, fog, and cloud layer.
The main aim of this categorization scheme is to process the
heterogeneous queries according to their resource-latency
7. Conclusion
Streaming data is increasing at a greater rate, and its timely
processing is crucial for real-time analytics. The streaming
data is analyzed by querying it over short time windows.
This survey paper reviews stream query processing literature
by mainly focusing on latency optimization techniques. It
also categorizes the query processing techniques within the
context of dierent query execution modelscloud-based
query processing, geo-distributed query processing, edge-
cloud-based query processing, and edge-based query process-
ing. The comparative analysis of query processing techniques
shows that the geo-distributed schemes have performed
latency optimization by incorporating the resource heteroge-
neity and WAN dynamics in their QEPs. The edge-cloud-
based approaches have reduced the transmit data size over
WAN links to cut down the networking delay. Likewise, the
edge-based schemes have performed complete computations
on edge for fast query processing. In contrast, the cloud-
based schemes have focused on the reduction of processing
delay by reducing the queryingdata size. The query processing
techniques reviewed in this survey have shown varying
Table 4: Query execution models within the context of their strengths and weaknesses.
Query execution models Strengths Weaknesses
Cloud-based query processing (i) Abundant compute and storage resources
(ii) Optimized processing delay
(i) Longer networking delays
(ii) In-aordable for latency-sensitive queries
Geo-distributed query processing
(i) WAN-aware query computation
(ii) Optimized networking delay
(iii) Network-awareness incorporation in QEPs
(i) WAN-data transmission cost
Edge-cloud-based query processing
(i) Partial computations on edge
(ii) Reduced WAN usage cost
(iii) Reduced networking delay
(i) Limited and resource-constrained nature
of edge devices
(ii) Network-compute resource heterogeneity
Edge-based query processing
(i) Complete query execution on edge
(ii) Hop count reduction between data sources
and query execution node
(iii) Reduced networking delay
(i) Unsuitable for the complex type of queries
(ii) Resource heterogeneity
14 Wireless Communications and Mobile Computing
degrees of eectiveness in overcoming the challenges of their
execution models. In short, each query execution style is
unique within the context of its benets and limitations.
Therefore, we cannot rule out any execution style, but we
can exploit their positive features to handle heterogeneous
queries. We propose the ideaof query categorization to handle
the processing of heterogeneous queries. This categorization
scheme categorizes the queries as edge-executable, edge-
cloud executable, and edge-executable based on their resource,
latency, and data ingestion demand.
Conflicts of Interest
The authors declare that they have no conicts of interest.
This study was supported by the BK21 FOUR project (AI-
driven Convergence Software Education Research Program)
funded by the Ministry of Education, School of Computer
Science and Engineering, Kyungpook National University,
Korea (4199990214394). It was also supported in part by
the National Research Foundation of Korea(NRF) grant
funded by the Korea government(MSIT) (No. NRF-
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