Existing Social Storage Systems

Next, we provide an overview of some important social storage systems. Apart from helping the reader to know the most relevant systems in this regard, we believe that this section will also give a sense on the potential applicability of our contributions in Part II of this thesis.

Friend-to-Friend (F2F) Storage Systems. F2F storage systems originally emerged as an alter-native to traditional P2P storage systems. Although many relevant systems and designs have been proposed in the literature (e.g., OceanStore [56], PAST [3], Farsite [16], etc), P2P storage systems suffer from inherent drawbacks that are hard to overcome. First, the instability of peers [18] makes difficult and costly to provide high data availability. Furthermore, despite important efforts [19], the existence of free riding and selfish behaviors complicates the effi-cient management of the existing resources. Moreover, many users are still reluctant to store their data in unknown hosts due to trust and security reasons.

Instead of interacting with random nodes, the main strength of the F2F paradigm lies on building storage interactions upon trust, social or real-world relationships. Thus, F2F systems assume that the existence of social connections among participants gives a node reasons to trust that these contracts will be respected, and behave accordingly [70]. This makes the whole system to operate in a more favorable scenario, reducing the overhead of dealing with high rates of free-riding and malicious behaviors. Next, we briefly describe several F2F systems:

• BlockParty is a distributed backup application, originally presented in the pioneering work of Li and Dabek [61], that provides an off-site backup service for home users. As other systems, BlockParty breaks the data to be backed up into chunks and distributes each chunk to one or more neighbor machines depending on the desired replication level. To ensure storage balance, the BlockParty software at a node dedicates at least as much space to storing other nodes’ backups as the node wishes to use on other nodes.

• Friendstore [20] is a cooperative backup systems that also relies on trust relationships among participants to ameliorate the impact of free-riding and malicious behavior that

may harm the operation of the system. Friendstore puts special emphasis on keeping track of the limited and heterogeneous resources of users (bandwidth, storage space) to optimize backup operations, which are potentially resource consuming. For instance, they reduce storage consumption by applying a simple coding scheme.

• Crashplan¹ is an offsite backup service that enable users to store data in the cloud or in other remote locations, such as other computers in a user’s network of friends and family. Although the technical details are not public, Crashplan can be seen as one of the first commercial products offering F2F storage.

Distributed On-line Social Networks (DOSN). Online social networks, such as Facebook, Google+ and LinkedIn, are becoming a predominant service today. Catering for people of all ages, gender and class, social networking services have become the primary means of commu-nication between friends, family and colleagues. However, major social networks are currently operated by private companies that control the data of users, which represents a potential threat for privacy and security [71, 72, 73].

As a reaction to the risk that a centralized social network architecture represents for users’

privacy, researchers started to devise decentralized social networking systems [74, 75, 76, 77, 78]. As any other decentralized system, DOSNs integrate the spare resources contributed by users (bandwidth, storage) to provide a social networking service. Moreover, in terms of stor-age, a DOSN must take care all the operational aspects of any decentralized storage system, including data placement, failure detection and redundancy, among other aspects.

• PeerSon [77] is a pioneering DOSN and its design is built upon three pillars: encryption, decentralization, and direct data exchange. In PeerSon, data is stored encrypted for keeping users’ data private, and decentralization —by means of leveraging an underlying P2P overlay— provides independence from OSN providers. Authors state that decentral-ization makes it easier to integrate direct data exchange between users’ devices into the system. This, in turn, allows users to use the system without constant Internet connec-tivity, leveraging real-life social networking and locality.

• Supernova [78] represents an evolution of the PeerSon design. Supernova introduces the concept of super-peers in the system in order to improve data availability and deal with heterogeneity in a more effective manner. Moreover, super-peer resources can be

1https://www.code42.com/crashplan/

2.2 Definition of a Social Storage System 31

shared across users. The share a user obtains from a super-peer depends on his behavior, which is a mechanism to incentivize user cooperation.

• SafeBook [76] is a decentralized and privacy-preserving online social networking site.

This system relies in two design principles, decentralization (P2P substrate) and exploit-ing real-life trust. In this settexploit-ing, SafeBook integrates various privacy and security mech-anisms to provide data storage and data management functions that preserve users’ pri-vacy, data integrity and availability.

• Vis-`a-Vis [79] is a decentralized framework for OSNs based on the privacy-preserving notion of a Virtual Individual Server (VIS). The main idea behind Vis-`a-Vis is to make use of existing cloud infrastructures to sustain a social networking site (running VIS in-stances), being owned by the users in the system instead of a single OSN company. Tech-nically, Vis-`a-Vis is self-organized into overlay networks corresponding to social groups and puts especial emphasis on preserving privacy of user location information.

The Social Cloud. The advent of social networks and digital relationships creates new op-portunities to spur the adoption of socially oriented computing. One representative example of this trend is the concept of “social cloud” as a means of facilitating resource sharing by utilizing the relationships established between members of a social network [22, 23, 66].

A social cloud leverages preexisting trust relationships between users to enable mutually beneficial sharing. This facilitates long term sharing with lower privacy and security require-ments than those that are present in traditional cloud environrequire-ments. For the time being, the cloud accrues massive amounts of private information to provide for instance highly targeted advertisements. Not surprisingly, security breaches, poor judgment, or even the lack of judi-cial oversight leaves users vulnerable. In this sense, the “sojudi-cial cloud” represents a new form for the users to retake control of the cloud service, avoiding to be tracked or give personal in-formation against their will, or in a way in which they feel uncomfortable. In fact, as pointed out by S. Pearson [80], one of the “top six” recommended privacy practices for cloud systems is to maximize user control, which is one of the outstanding feature of the “social cloud”.

Another distinguishing feature of the “social cloud” is that the network comes first. It is not a cloud or middleware extended with a social network; rather, it is a social network extended with cloud functionality. Users form the basic infrastructure and share resources around their social graphs. Such an organization brings out many benefits. For instance, one of those advantages is usability, since the interface and tools for resource sharing are already

familiar to users. Another one is that it allows users to maximize the control of the cloud service by letting users choose how their resources will be used. Giving users the control over their personal information and resources engenders trust, but this can be difficult in a cloud computing scenario. This feature is very interesting for the adoption of the “social cloud”, as it permits users to define a series of preferences for the management of their personal data, and take account for that, among other advantages.

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