S.Tramp Et Al. / an Architecture of a Distributed Semantic Social Network
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Semantic Web 1 (2012) 1–16 1 IOS Press An Architecture of a Distributed Semantic Social Network Sebastian Tramp a;∗, Philipp Frischmuth a, Timofey Ermilov a, Saeedeh Shekarpour a and Sören Auer a a Universität Leipzig, Institut für Informatik, AKSW, Postfach 100920, D-04009 Leipzig, Germany E-mail: {lastname}@informatik.uni-leipzig.de Abstract. Online social networking has become one of the most popular services on the Web. However, current social networks are like walled gardens in which users do not have full control over their data, are bound to specific usage terms of the social network operator and suffer from a lock-in effect due to the lack of interoperability and standards compliance between social networks. In this paper we propose an architecture for an open, distributed social network, which is built solely on Semantic Web standards and emerging best practices. Our architecture combines vocabularies and protocols such as WebID, FOAF, Se- mantic Pingback and PubSubHubbub into a coherent distributed semantic social network, which is capable to provide all crucial functionalities known from centralized social networks. We present our reference implementation, which utilizes the OntoWiki application framework and take this framework as the basis for an extensive evaluation. Our results show that a distributed social network is feasible, while it also avoids the limitations of centralized solutions. Keywords: Semantic Social Web, Architecture, Evaluation, Distributed Social Networks, WebID, Semantic Pingback 1. Introduction platform or information will diverge. Since there are only a few large social networking players, the Web Online social networking has become one of the also loses its distributed nature. According to a recent most popular service on the Web. Especially Facebook comScore study1, Facebook usage times already out- with its 600M+ million users creates a web inside the number traditional Web usage by factor two and this Web. Drawing on the metaphor of islands, Facebook divergence is continuing to increase. is becoming more like a continent. However, users are We argue that social networking must be distributed locked up on this continent with hardly any opportu- and users should be empowered to regain control over nity to communicate easily with users on other islands their data. The currently vast oceans between social and continents or even to relocate trans-continentally. networking continents and islands should be bridged Users are bound to a certain platform and hardly have by high-speed connections allowing data and users to the chance to migrate easily to another social network- travel easily and quickly between these places. In fact, ing platform if they want to preserve their connections. we envision the currently few social networking conti- Once users have published their personal information nents to be complemented by a large number of smaller within a social network, they often also lose control islands with a tight network of bridges and ferry con- over the data they own, since it is stored on a single nections between them. company’s servers. Interoperability between platforms In this paper we describe the main technological is very rudimentary and largely limited to proprietary ingredients for a distributed semantic social network APIs. In order to keep data up-to-date on multiple plat- (DSSN) as well as their interplay. The semantic rep- forms, users have to modify the data on every single resentation of personal information is facilitated by a *Corresponding author. 1http://allthingsd.com/20110623/ WebID: http://sebastian.tramp.name the-web-is-shrinking-now-what/ 1570-0844/12/$27.50 c 2012 – IOS Press and the authors. All rights reserved 2 S.Tramp et al. / An Architecture of a Distributed Semantic Social Network WebID profile. The WebID protocol allows the use of Service Decoupling. A second fundamental design a WebID profile for authentication and access control principle is the decoupling of user data from services purposes. Semantic Pingback facilitates the first con- as well as applications [8]. It ensures that users of the tact between users of the social network and provides network are able to choose between different services a method for communication about resources (such and applications. In addition, this principle helps users as images, status messages, comments, activities) on of the DSSN to distinguish between their own data, the social network. Finally, PubSubHubbub-based sub- which they share with and license to other people and scription services allow obtaining near-instant notifica- services, and foreign data, which they create by using these services and which they do not own. tions of specific information as WebID profile change sets and activity streams from people in one’s so- Protocol Minimalism. The main task for social net- cial network. Combined these standards, protocols and working protocols is to communicate RDF triples be- technologies provide all necessary ingredients to re- tween DSSN nodes, not to enforce a specific work flow alize a distributed social network having all the cru- nor an exact interpretation of the data. This constraint cial social networking features provided by centralized ensures the extensibility of the data model and keeps ones. the overall architecture clean. This article is structured as follows: We present our 2.2. Data Layer reference architecture of a distributed social semantic network in Section 2. Our implementation based on The data layer comprises two main data structures: the OntoWiki framework is demonstrated in Section 3, resources for the description of static entities and feeds while the evaluation results of our approach are dis- for the representation and publishing of events and ac- cussed in Section 4. Finally, we give an overview of re- tivities. lated work in Section 5 and conclude with an outlook 2.2.1. Resources on future work in Section 6. We distinguish between three main categories of DSSN resources: WebIDs for persons as well as appli- cations, data artefacts and media artefacts. The prop- 2. Architecture of a Distributed Semantic Social erties, conditions and roles in the network of these re- sources are described in the next paragraphs. Network WebID [17]2 is a best practice recently conceived in In this section we describe the DSSN reference ar- order to simplify the creation of a digital ID for end chitecture. After introducing a few design principles users. Since its focus lies on simplicity, the require- ments for a WebID profile are minimal. In essence, on which the architecture is based, we present its dif- a WebID profile is a de-referenceable RDF document ferent layers, i.e. the data, protocol, service and appli- (possibly even an RDFa-enriched HTML page) de- cation layers. The overall architecture is depicted in scribing its owner3. That is, a WebID profile con- Figure 1. tains RDF triples which have the IRI identifying the owner as subject. The description of the owner can be 2.1. Basic Design Principles performed in any (mix of) suitable vocabulary (-ies), but FOAF [6] has emerged as the ‘industry standard’ Our DSSN architecture is based on the following for that purpose. An example WebID profile compris- three design principles. ing some personal information (lines 8-12) and two rel:worksWith4 links to co-workers (lines 6-7) is Linked Data. The main protocol for data publishing, shown in Listing 1. retrieval and integration is based on the Linked Data principles [3]. All of the information contained in the 2The latest spec is available at http://webid.info/spec/. DSSN is represented according to the RDF paradigm, 3The usage of an IRI with a fragment identifier allows the indi- made de-referencable and interlinked with other re- rect identification of an owner by reference to the (FOAF) profile sources. This principle facilitates heterogeneity and document. 4Taken from RELATIONSHIP: A vocabulary for describing enables the distribution of data and services on the relationships between people at http://purl.org/vocab/ Web. relationship. S.Tramp et al. / An Architecture of a Distributed Semantic Social Network 3 Profile Bookmark Foto ''' Blog Manager Collection Sharing Application Layer + ping ) create update 5 search push subscribe - * delegeate access to Ping Update Search Push , Service Layer + announce ( create update 5 index ( announce - read access #ebI s Activity History Streams Feeds Data & Media announce Artefacts ( Resources Feeds Data Layer Fig. 1. Architecture of a Distributed Semantic Social Network (without protocol layer): (1) Resources announce services and feeds, feeds announce services – in particular a push service. (2) Applications initiate ping requests to spin the Linked Data Network. (3) Applications subscribe to feeds on push services and receive instant notifications on updates. (4) Update services are able to modify resources and feeds (e.g. on demand of an application). (5) Personal and global search services can index resources and are used by applications. (6) Access to resources and services can be delegated to applications by a WebIDs, i.e. the application can act in the name of the WebID owner. (7) The majority of all access operations is executed through standard web requests. Apart from the main focus on representing user pro- parts of the profile data and can add or change some files, our architecture extends the WebID concept by of the profile information, e.g. create activities or cre- facilitating two additional tasks: service discovery and ate and link images. Applications on the DSSN are access delegation. also identified by using WebID profiles, but are not Service discovery is used to equip a WebID with re- described as a person but as an application. They can lations to trusted services which have to be used with act on behalf of a person but rely on delegated access that WebID. The usage of the WebID itself ensures rights for such an activity. This process is described in that an agent can trust this service in the same way as Section 2.3.1.