Towards a Three-tiered Social Graph in
Decentralized Online Social Networks
Technische Universität Berlin
Technische Universität Berlin
Technische Universität Berlin
Online Social Networks have become one of the main tools
for interpersonal online communication. In the age of the
smartphone, mobile user scenarios become more and more
important for Online Social Networks. Smartphones enable
location-based and context-aware services, but bring the in-
creased risk of privacy violations - at least in centralized
OSN architectures. Decentralized Online Social Network
architectures are promising as they inherently oﬀer better
privacy and less dependence on a single service provider, but
they bring new challenges regarding core features of Online
Social Networks. In this paper, we introduce a three-tiered
view of the social graph and propose a new architecture for
decentralized Online Social Networking applications, sup-
porting the three-tiered view and focusing on location-based
and context-aware user scenarios.
Categories and Subject Descriptors
C.2.4 [Computer-Communication Networks]: Dis-
tributed Systems; H.3.5 [Information Storage and Re-
trieval]: Online Information Services
online social networks; mobile social networks; social graph;
location-based services; context-aware services
Current Online Social Network s (OSN) utilize centralized
architectures and store any and all information about their
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users. The resulting loss of control over one’s personal infor-
mation and the resulting lack of privacy is a major concern
for many users. In centralized OSN architectures, lack of
privacy is often an issue, because the operator of the OSN
might base their business model on the users’ private data,
while the users might not have intended to share their data
to that extent [8, 6, 4]. Furthermore, the usage of propri-
etary interfaces prohibits communication between diﬀerent
OSN service providers, creating a well calculated lock-in ef-
fect for their users. Moreover, with the steep rise in the
processing power and enhanced features of smartphones as
well as their fast adoption by users, OSNs have moved mas-
sively to the mobile world (inspiring the term Mobile Social
Networks (MSN) for OSNs focused on mobile scenarios [11,
3]). This has opened new opportunities for OSNs and their
users, but also brings increased privacy risks. For instance,
many location-based and context-aware user scenarios are
only possible in MSNs, but the amount of sensitive data col-
lected, stored, and shared typically increases in these sce-
In this paper, we address these aspects of OSNs. In par-
ticular, we argue that distributing the social graph, which is
one of the core pieces of information of any social network, is
a vital step towards better privacy. We also argue that, ad-
ditionally, the capabilities of smartphones can be leveraged
to achieve both more ﬂexible use of context information and
higher privacy of sensitive data for users of OSNs.
We investigate the relevant core features of OSNs and re-
view existing architectural approaches for implementing on-
line social networks. Based on this, we propose a three-tiered
view of the social graph and a new architectural concept,
called T3 architecture, supporting this three-tiered view. For
oﬀering location-based and context-aware services and tak-
ing care of better privacy protection at the same time, our
approach is to use a global directory for retrieving informa-
tion on how to contact other users, and to use additional
decentralized directories that can oﬀer search and recom-
mendations for speciﬁc contexts or locations. Thus, in sum-
mary, the three main contributions of this paper are (1)
the elaboration of core features of OSNs, (2) the proposal
to view the social graph as a three-tiered concept, and (3)
an architectural approach for distributed online social net-
work applications supporting the three-tiered social graph
and location-based and context-aware user scenarios.
The rest of this paper is structured as follows: After elab-
orating the core features of OSNs (Section 2) and investigat-
ing existing architectures for OSNs (Section 3), we develop
our three-tier view of the social graph in Section 4 and in-
troduce the T3 architecture in Section 5. In Section 6, we
conclude and point out future work.
2. FROM ONLINE TO MOBILE SOCIAL
After analyzing user scenarios for OSNs for smartphone
users and existing deﬁnitions for OSNs and MSNs, we deﬁne
and discuss the core features of OSNs.
2.1 Mobile OSN User Scenarios
Before detailing the scenarios, we ﬁrst explain why
the smartphone is the essential social networking device:
McLuhan imagined each technology as an extension of man
: if the car is the extension of feet, glasses an exten-
sion of the eyes, then the smartphone is most certainly an
extension of all parts that constitute a person’s social inter-
actions. We believe that while the smartphone is capable of
being such an extension of man, current OSN architectures
or applications do not reﬂect this sentiment. Carrying the
smartphone at all times - with its sensors, processing power,
and connectivity like Bluetooth and WiFi-Direct - especially
local services do not need to rely on cloud services or tradi-
Imagine Alice, a smartphone user living in a larger city.
Going through her daily life, Alice is collecting data on her
smartphone, e.g., visited locations or artists she listened to.
By collecting this data, Alice’s interests are expressed im-
plicitly rather than explicitly as it is done by clicking a ”like”-
button. Going shopping, she can share selected parts of her
collected context and location data to receive personalized
recommendations. After shopping, imagine Alice receiving a
notiﬁcation from Bob, who wants to play baseball. She does
not play baseball herself, but her smartphone can calculate
and suggest friends of hers that like baseball, and are not
friends with Bob. Alice can decide to forward recommended
Studying at a university, Alice can talk to the people that
are in the same class. Going to the library to study, she can
look for people who are both at the library and in her classes.
Going downtown in the evening, when visiting a crowded
place, she can chat with strangers that are in her proximity,
e.g., to ﬁgure out what is at the center of the crowd. At
night, Alice goes to a party and quickly realizes that she
does not know many people there. She can retrieve infor-
mation about the people around her and ﬁnd out mutual
contacts. Traveling to another city, Alice should be able to
query a public computer at her travel destination for recom-
mendations of local attractions or restaurants. Furthermore,
current contact information, like the postal address, could
be maintained locally with the users, making an up-to-date
version available for contacts.
2.2 Deﬁnitions of OSNs and MSNs
Some related work distinguishes between OSN and MSN.
In this section, we analyze the diﬀerent deﬁnitions of the
In their paper about decentralized OSNs, Paul et al. de-
ﬁne OSNs as providing a form of proﬁle management,re-
lationship management between users, interaction between
users, as well having an API (for interaction with third party
applications), search,recommendations, and social network
connector s (for connections between diﬀerent OSNs) . In
contrast to , we argue that an API is not essential for the
user of an OSN and is thus not necessarily a deﬁning fea-
ture. Social network connectors contradict the lock-in eﬀect
that centralized OSN platforms desire; for an approach to-
wards interconnection between OSNs, see . Furthermore,
the authors of  do not consider the importance of mobile
user scenarios as described in Section 2.1.
In , Kayastha et al. deﬁne Mobile Social Networks as a
combination of OSNs with location-based services and con-
nections via Bluetooth and Wi-Fi. In , Bellavista et al.
deﬁne MSNs as OSNs that oﬀer the possibility for oppor-
tunistic communities based on location information, with
the possibility of not needing a connection to a server. On
a less technical level, they deﬁne an MSN as the combina-
tion of virtual communities (”virtual social spaces”) of OSNs
with ”physical social spaces.”
Aside from architectural implications regarding the con-
nections between mobile devices, we consider context and
location information to be the essential new element that
mobile usage and smartphones bring into the world of OSNs.
We argue that with the pervasiveness of smartphones, the
mobile aspect of an OSN is indispensable, even for tradi-
tional OSNs like Facebook. In , Facebook, typically re-
garded as an OSN, is deﬁned as an MSN, as it oﬀers location-
based services, and is reachable via a mobile website or mo-
bile application. We therefore use the terms OSN and MSN
2.3 Core Features
Motivated by the analysis and observations above, we dis-
tinguish between the following six core features for OSNs:
Personal Proﬁle. Each user has a personal proﬁle in
which he/she can describe oneself, typically with a pro-
Social Graph. The social graph consists of the connections
between a user of the OSN with other users or prod-
Interaction. Messages, updates, posts, or likes by friends
are typically displayed to linked users.
Context Information. The user’s context, e.g., current
location, availability for instant messaging, etc., is of-
ten displayed for linked users or used for group cre-
Search. Search for speciﬁc users or users with a similar
Recommendations. Recommendations of friends or prod-
ucts, based on social graph and context information, is
the most common feature with which users can explore
the network around them.
There are diﬀerent OSNs with diﬀerent focuses.
LinkedIn1, for example, focuses on work relationships and
career opportunities, Twitter2is about - typically publicly
- broadcasting short messages. For each OSN, the six listed
features might be of diﬀering importance, and might have
With the rise of smartphone devices in recent years, In-
stant Messenger applications seem to fulﬁll the role of OSNs,
with messaging apps like WeChat3or WhatsApp oﬀering
some OSN features like a short proﬁle. Looking closer, the
contact list or address book of each of those applications -
or the phone’s contact list with PTSN telephone numbers -
is actually a part of a social graph: Each user sees just di-
rect connections, he does not see people more than one hop
away. A social graph that is stored in a distributed way of-
fers inherently more privacy to the users of the OSN. At the
same time, recommendations that are based on knowledge
about the graph, pose a new challenge.
Today, each OSN or messaging application is vertically in-
tegrated: They have their own identity management, their
own social graph, and - most often - proprietary communi-
cation protocols. Thus, each OSN acts like a silo, with users
being bound to restrictions their OSN of choice comes with.
The modern smartphone is an optimal online social net-
working device. It contains a camera, sensors that register
the user’s context, it can connect to the Internet, and it can
display websites, videos, or images that other users posted.
The generated context information especially deserves pro-
tection, as it can contain the most private information like
health data or exact user locations at any time. Given the
processing power of current smartphones, they can be used
to locally process data, e.g., lots of the context data a phone
generates. For instance, a phone could track and rank vis-
ited places or favorite artists, independent of speciﬁc OSN
applications, leaving the user in control of his data, letting
him decide when to share what data. Additionally, collected
context and location data oﬀers the possibility to create dy-
namic social graphs, connecting users without them having
to explicitly befriend each other. This way, people who are
in the same city and are interested in a speciﬁc style of music
genre could automatically be connected and share informa-
tion about local shows and concerts.
3. ARCHITECTURAL APPROACHES
In this section, we motivate the utilization of decentralized
system architectures, before we summarize current research
about OSNs and MSNs.
3.1 On Decentralized Architectures
In the beginning of the 1990s, AOL (formerly America On-
line) provided its costumers access to their catalog of sites
and services. With the advent of the World Wide Web and
email, users were no longer bound to the web of a single com-
pany, but could explore the Internet freely and write emails
to any other user using a diﬀerent ISP. The decentralized
concept of the World Wide Web allowed the development of
open communication platforms.
A more recent example for a decentralized system is
XMPP4(Extensible Messaging and Presence Protocol). The
messaging works similar to email, with users having an iden-
tiﬁer with a server, and servers sending messages to each
other. Additionally, the idea of presence allows users to reg-
ister with their server with diﬀerent devices. This presence
can be published to the own address book (buddies ).
At the same time, the recent past also showed a trend
towards new centralized systems, most prominently Face-
book5. Every few weeks, especially in the EU, Facebook is
in the news because of privacy issues and many users are
concerned [6, 4].
Overall, one major advantage of decentralized systems
is that they oﬀer users the possibility to change service
providers. Interconnection between diﬀerent providers can
be guaranteed through standardized interfaces.
3.2 Online and Mobile Social Networks
In , decentralized OSNs are surveyed. There is a cate-
gorization of architectural approaches that distinguishes be-
tween peer-to-peer (P2P), federated OSNs (F-OSN), and hy-
brid approaches. Going with these deﬁnitions, P2P OSNs
leverage peer-to-peer-technology without a central server.
F-OSNs use client/server infrastructure with multiple inter-
connected servers maintained by diﬀerent providers or peo-
ple. Hybrid approaches combine the two other approaches.
PeerSoN is a P2P OSN that focuses on privacy issues .
The project considers oﬄine scenarios by proposing to store
messages for oﬄine users in a distributed hash table (DHT).
Prometheus is another OSN project that utilizes a DHT .
A public-key infrastructure is employed, and users can deﬁne
access control lists for access to their data. By collecting
data from existing OSNs, emails, etc., weighted and named
edges are constructed in the social graph. Data is replicated
with trusted nodes in the DHT. The nodes should always
be online, and mobile devices are not considered suitable
nodes for the project. Overall, the focus here is both the
preservation of privacy as well as an approximation of the
real social graph.
Diaspora6is an example for a federated OSN. Here, users
can choose between existing servers or set up their own
server. The users beneﬁt from this architectural approach
with the ability to change providers, similar to how it is
possible with email. Privacy concerns are reduced because
no single service provider has all the information about all
users. The user can be in control of his own data, but for the
system as a whole, search and recommendations are more
complicated because of the distributed nature of the data.
According to , one advantage of F-OSNs in comparison
with P2P OSNs is the typically better usability.
The main categories that are further analyzed in  are
storage, access control, and interaction methods (i.e., com-
munication protocols). According to Paul et al. , the
main motivation for research in the ﬁeld of decentralized
OSNs is privacy concerns with centralized OSNs. Location-
based scenarios and mobile devices are not considered.
Regarding MSNs, in , a distinction between three ar-
chitectural approaches is made: centralized,distributed, and
hybrid architectures. A centralized architecture is an archi-
tecture, in which a server delivers data to users on mobile
devices. One example given is Facebook. For such a cen-
tralized architecture, one main advantage is that search and
recommendations are better supported through knowledge
about all users and the whole social graph. For the users
of such a system, there are the disadvantages of the lock-in-
eﬀect and privacy concerns.
With ”distributed” architectures, the authors describe ap-
proaches that do not rely on existing infrastructures, but
opportunistic connections between mobile devices via Blue-
tooth or Wi-Fi . One example is MobiClique . Blue-
tooth is used for the transferal of data between users. Mes-
sages are forwarded between peers, instant communication
as well as the arrival of a message cannot be guaranteed.
While especially the uncertain delivery of a message is a
major disadvantage, there are scenarios where such an ap-
proach might be useful, e.g., after a natural catastrophe that
leaves no intact communication infrastructure behind. A hy-
brid architecture combines the two models, centralized and
distributed, by allowing communication between users on
mobile devices utilizing opportunistic connections, while al-
lowing the retrieval of content via central servers.
Kayastha et al. further investigate community detection
for location-based scenarios . The presented approaches
seem to assume knowledge about the whole social graph.
We argue that when utilizing a distributed social graph, the
whole social graph is not known to any one entity and that
new mechanisms based on such a distributed social graph
are needed for community detection and formation.
Bellavista et al. give a classiﬁcation of MSNs on a diﬀerent
level in . Here, a distinction is made by the parameters by
which communities can be formed. The classiﬁcation system
spans pairs of mutually exclusive properties: user-centric
(friends of someone) versus place-centric (e.g., fans of a spe-
ciﬁc museum), location-dependent (people at the location)
versus location-independent (people anywhere). Addition-
ally, regarding the storage and analysis of user metadata,
Bellavista et al. distinguish between a spatial scope - by
deﬁning a global (data from anywhere) versus a local (data
stored at the museum only) scope - and a temporal scope
(if user metadata is stored for a longer time or only used
instantaneously). In Section 4, we propose an new approach
to categorize the creation of groups in OSNs.
Regarding the collection of context data on smartphones,
some previous work has been done. With P3 Systems, Jones
and Grandhi describe people-to-people-to-geographical-places
. The authors describe location-based services and what
potential lies in them, especially with respect to online so-
cial networking and recommendations. Ideas of using users’
proximity or location histories for recommender systems are
In WhozThat?, Bluetooth is used for the broadcasting of
social IDs, e.g., usernames from existing OSNs . The
users’ phones can then retrieve information about other
users in proximity by requesting public proﬁle information
from OSNs. The authors report about an implementation
that leverages this approach to realize a music player that
automatically plays music according to the tastes of the peo-
ple in proximity. Security and privacy issues are mentioned,
but not discussed further in .
In SocialFusion, the idea of collection context data is ex-
tended to not only data from existing OSNs, but also from
mobile phones and nearby sensors . In both WhozThat?
and SocialFusion, OSN aspects, like the interaction between
users, is not intended.
4. THREE-TIER VIEW OF THE
In contrast to , we propose a new, easier, and in our
eyes more useful approach to categorize group formation in
OSNs. By detailing this approach, we will identify architec-
tural requirements for an OSN architecture. The following
three-tier view of the social graph will be motivated by an
advanced user scenario:
•Location-based groups: short-term groups which can
form spontaneously when people are at the same loca-
•Context-based groups: medium-term groups which are
formed based on context, e.g., being in the same class
•Friends: long-term relations which are independent
from context and location.
Using the terminology from , the friend-tier connections
represent the self-reported social network, while context- and
location-tier relations represent the inferred or detected so-
To further illustrate these three tiers we will look at an-
other user scenario. Imagine a chain of gyms Alice is regis-
tered with and regularly goes to. The machines she works
out with transmit data about her workout to her smart-
phone. Furthermore, her wearable ﬁtness tracker collects
data. While she is working out, she listens to music on her
smartphone. Her top ten artists are always automatically
maintained by her mobile device. From the workout data
on her phone, a ﬁtness index is calculated. As all the data
is on her phone, Alice is in control of her data. Should she
decide to change to another gym chain, she has all her data
with her and can carry settings for workout machines etc.
To share her ﬁtness index or her top ten favorite music
artists with her friends, Alice ﬁrst needs to connect to her
friends. What is needed here is a global directory in which
Alice can look up other users by some form of ID or user-
name. Such a DNS-like directory should further contain in-
formation about the presence of the friend, and information
about how to contact this friend, e.g., IP address and proto-
col. Such a global directory can be used independently from
actual OSN applications and would be used for looking up
how to contact people, not storing social graph information.
Connecting to her friends, Alice has formed her part of the
distributed social graph in the friend-tier.
On the context-tier, Alice can be added to a group with
the other people that work out at the same gym chain, inde-
pendent of the branch location. As the global directory does
only store information about how to contact other users, an-
other form of directory is needed for creating a group based
on context. The gym chain could oﬀer a Wi-Fi hotspot that
oﬀers its customers to share their ID or username.
Additionally to just sharing IDs to form dynamic context-
tier groups, a gym could have a TV in the lobby where the
users that are currently working out are displayed. Addi-
tionally, the top music artists listened to by the people work-
ing out could be displayed. The users would stay in control
of their data and could decide if and what data they share
when entering the gym. Furthermore, the gym could use the
provided context data to give personalized recommendations
for products that ﬁt a customer’s workout style.
contacts and interests
Figure 1: Three-tier Social Graph
During the summertime, some gym customers might pre-
fer to go jogging outside in a park instead of working out
in a gym. Here, location-tier groups can be formed. Some
trusted online directory, e.g., provided by the gym chain or
the city, is needed for the collection of users that are cur-
rently at the park. When such a directory oﬀers its users the
ability to publish more data than just their IDs, e.g., their
ﬁtness index, the directory can be used for searching, for
example, to ﬁnd new jogging partners with a similar ﬁtness
5. T3 ARCHITECTURE
The three-tier view developed in the previous section in-
duces requirements for a supporting OSN architecture which
we will summarize in the following. We introduce an archi-
tecture called T3 satisfying these requirements.
To enable users to look up how to contact other users in
decentralized OSN architectures, we propose a global direc-
tory, independent of the concrete OSN application or used
communication protocol. Furthermore, a communication ar-
chitecture that allows for direct communications between
two smartphones over the Internet eliminates the need for a
Additional decentralized directories can be implemented
that allow users to publish their context data, enabling
search and recommendations, as well as enabling support for
the introduced context- and location-tier of the three-tiered
view of the social graph. Leveraging the smartphone’s capa-
bilities, context data can be collected and processed locally.
Standardized data formats are needed for the exchange be-
tween diﬀerent users.
In Figure 1, we give an example of a social graph according
to our three-tier approach. The smartphone in the center of
the image is in a location-tier group with other users that
are at the same location, as indicated by the dark gray circle.
The light gray ellipse indicates membership in a context-tier
group based on a common context, for example, the people
that are enrolled in the same class at a university. The friend
tier connections are indicated by the light gray triangle.
1. Send Context and
Based on Similar Users
3. Give Recommendations
Figure 2: Recommendation Functionality of a Decentralized
The black lines indicate direct interactions. The black
line on the right hand side in Figure 1 indicates that the
center smartphone is calculating its common contacts and
interests with another smartphone in the area. The black
line on the left hand side in Figure 1 indicates that from
another smartphone, the center smartphone retrieves proﬁle
Figure 2 shows the general concept of a decentralized di-
rectory. Imagine a travel scenario in which user Alice is
querying a computer she encounters at a central train sta-
tion of the city she is visiting. Alice sends location (history)
and context data to the computer and receives personalized
recommendations based on the data she sent. Additionally
to recommendations, a decentralized directory can be uti-
lized for the search for nearby restaurants or attractions.
In Figure 3, we outline our proposed T3 architecture for
OSNs. In the following, we will detail the role the com-
ponents play by indicating what of the six core features of
OSNs (Section 2.3), they realize. The core components are:
The global directory is used to look up how to contact
other users for interaction, e.g., calling or messaging. Fur-
thermore, information about where and how to retrieve a
user’s personal proﬁle could be stored in the global direc-
tory as well.
Decentralized directories support search and recommen-
dations by allowing users registration and by oﬀering the
possibility for users to publish context and location data.
Compared to a single central directory service, one advan-
tage of the approach of having multiple decentralized direc-
tories is trust: users might put higher trust in their gym
chain that they visit regularly and where they see and know
the people working there. Furthermore, users can choose
which data to share with each directory, there is no need for
a central directory that has all the data.
On the users’ smartphones, the social graph is stored in a
distributed manner. Additionally, each user is in control of
the locally stored context information. The personal proﬁle
could be hosted on the mobile devices, or on an external
server for better availability. Some form of access control
will be necessary to ensure proper privacy settings for users.
For this, the three tiers could be utilized, or some additional
role-based model - with roles like co-workers or classmates
- could be used for access control automatization.
Global Dire ctory
Look Up Users
Recommend Create Context-Tier or
Look Up Users Look Up Users
Figure 3: T3 Architecture for OSNs
The presented T3 architecture supports the three-tier con-
cept of the social graph: Users can connect with friends by
looking them up in the global directory; distributed direc-
tories help form context-tier and location-tier groups. The
users’ privacy is protected by utilizing a distributed social
graph and by giving the users the control over their context
6. CONCLUSION AND OUTLOOK
In this paper, we argued that mobile user scenarios are
essential for OSNs. We deﬁned the core features of OSNs,
introduced a three-tier view of the social graph, and pro-
posed the decentralized OSN architecture T3 supporting this
three-tier view, that utilizes a global directory, additional
decentralized directories, and leverages the capabilities of
Future work includes the question how user IDs are cre-
ated for the global directory, what authentication and autho-
rization mechanisms are feasible, especially regarding reg-
istration, search, and recommendation with decentralized
directories. Additionally, a feasible publish/subscribe mech-
anism should be investigated to enable users to automat-
ically retrieve status updates from their friends. Further-
more, standardized interfaces for the communication be-
tween smartphones, as well as between smartphones and
directories, are to be researched.
This work has received funding from the European
Union’s Horizon 2020 research and innovation program un-
der grant agreement No 645342, project reTHINK; and from
project SONIC (grant number 01IS12056), which is funded
as part of the SoftwareCampus initiative by the German
Federal Ministry of Education and Research (BMBF) in co-
operation with EIT ICT Labs Germany GmbH. The authors
would like to thank Beg¨
Ilke Zilci for valuable discussions
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