Architectural Approaches for Social Networks. Presenter: Qian Li

Transcription

1 Architectural Approaches for Social Networks Presenter: Qian Li

2 The Gossple Anonymous Social Network Marin Bertier, Davide Frey, Rachid Guerraoui, AnneMarie Kermarrec and Vincent Leroy

3 Gossple overview Challenges: Approaches: Folksonomy -- freely chosen keywords are used to tag user generated items Associating users with similar interests together anonymously Achievements: Higher completeness and accuracy in terms of query matching Low bandwidth and storage costs Fast convergence

4 Gossple Protocol Overview Goals: to build and maintain dynamic communities of anonymous acquaintances -- GNets to determine a metric to identify which profiles exhibit similar interests to devise an effective communication procedure for cost-effective anonymous profile exchange

5 Metrics Cosine similarity: comparing two profiles at a time leading to only considering the dominant interests Item set cosine similarity: comparing a set of profiles at a time considering all interests and their distribution

6 Discovering Acquaintances Gossple relies on two sub protocols RPS -- Random Peer Sampling protocol GNet -- Multi-interest clustering protocol

7 Discovering Acquaintances -- RPS Random Peer Sampling protocol (RPS) maintains a view of a random changing subset of network nodes the view is used to bootstrap and maintain GNet a view entry contains IP Gossple ID based on Brahms Profile Digest (Bloom filter) # Items

8 Discovering Acquaintances -- GNet Multi-interest clustering protocol (GNet) A GNet entry contains IP Profile Digest GNet protocol: Gossple ID high computational complexity A heuristic protocol lower complexity -good approximation in use # Items Timestamp Profile

9 Discovering Acquaintances -- GNet Protocol

10 Gossiping Digests Bloom filters an array of bits when an item is added to the array, h hash functions are used on the item to obtain h positions which are set to 1 the false positive rate increases with the number of items being added a bloom filter never returns a false negative saves bandwidth significantly (20 fold) than gossiping full profiles

11 Evaluation -- Metric Hidden interests: the subset of items that are removed from each node s profile Recall: the proportion of hidden interests of a node n that are present in the profile of at least one node in n s GNet. b = 0: traditional clustering algorithms

12 Evaluation -- convergence time Experiment setup 50,000 nodes in simulation 446 nodes on 223 machines on Planet testbed During GNet construction, Gossple reaches 90% potential after 14 cycles in simulation 12 cycles on testbed Simulation setup: 1% of new nodes join an existing Gossple network at each cycle 9 cycles to achieve 90% potential

13 Evaluation -- bandwidth Bandwidth cost analysis the exchange of profile digests the download of full profiles the extra communication required to maintain anonymity replacing digest exchange with full profile exchange will increase bandwidth consumption by 20 times

14 Gossple at work: Query Expansion Query expansion: extending a query with additional keywords to improve search results

15 Query expansion -- tag map Tag map: a personalized view of the relations between all tags in a node s profile and in its GNet Item based cosine similarity

16 Query expansion -- GRank computes the relative importance of tags on a given node estimates the relevance of each tag in the TagMap with respect to the query and assigns a score to the tag

17 Evaluation of query expansion TagMap improves the recall through personalization GRank improves precision through tag centrality

18 Quasar: A Probabilistic PublishSubscribe System for Social Networks Bernard Wong and Saikat Guha

19 Overview Quasar: a scalable rendezvous-less publish-subscribe system Designed for social networks with many groups -- on the order of the number of users in the system Satisfying coverage: more than 95% of nodes are covered Modest delay: the number of overlay hops from source to the furthest destination is modest and grows logarithmically to the network size

20 System Architecture Proactive dissemination of highly aggregated routing vectors (attenuated bloom filters) to create gravity wells Reactive searching for paths between a publisher and its subscribers by using parallel random walks (to find gravity wells) and directed walks (to walk into the wells) Use of negative information (a set of node IDs) to avoid loops

21 Subscription Attenuated bloom filter with depth parameter K: is a collection of K identically-sized bloom filters the n-th bloom filter holds information about the groups and node IDs n-hops away false positive rate increases at each level

22 Publication A message is addressed to a group ID is tagged with negative information -- the visited node IDs contains a TTL to limit the number of hops it can traverse

23 Impact of false positives False positive in matching a target group results in a longer route False positive in matching a node ID results in a longer route or the prohibition of messages from being delivered to a member

24 Evaluation -- coverage large scale simulation MySpace data bloom filter depth = 3 bloom filter size = 512 B coverage: #group members receiving a published message / total number of members in the group 97% coverage independent of network size

25 Evaluation -- latency and false positives metric: latency between publishing a message and the last group member receiving that message result: the number of hops increases modestly as the network size increases false positive: 0.5% false positive all recovered at subsequent hops

26 Questions for discussion -- Paper one How to decide the size of an RPS view? Is it related to network size and GNet size? When to use heuristic algorithms? What should we do before we decide to use a less expensive but suboptimal algorithm? What are the state-of-the-art semantic search engines or recommending systems?

27 Questions for discussion -- Paper two In Quasar, nodes periodically exchange bloom filters. Does this cause significant overhead? Are there any alternative solutions? How should we set system parameters such as size of gravity well and TTL? Is there a way to improve coverage? Or reduce false positive rate?