Content-based Publish-Subscribe Over Structured P2P Networks

1. Content-based Publish-Subscribe Over Structured P2P Networks Peter Triantafillou and Ioannis Aekaterinidis Presented by Jesse Chen 4/10/07

2. Outline of Talk • What is a publish/subscribe system? • Why do we build publish/subscribe systems over p2p networks? • What is an event/subscription? • How to store subscriptions? • How to update subscriptions? • The matching algorithm • Delivering of events to the subscribers

3. What is a publish/subscribe system? • A communication (information dissemination) system that delivers published notifications from every producer (publishers) to all interested consumers (subscribers). • Two types of publish/subscribe systems: • Topic based • Users express interest by joining a group (topic) • Content based • Users express interest by specifying subscriptions (a collection of predicates over the values of some attributes).

4. Why Build Publish/Subscribe Systems Over P2P Networks? • To avoid the lack of scalability and fault-tolerance of centralized approaches. • To take advantages of Chord: decentralized, scalable, self-organizing. • Chord’s structured network: using DHT for easy resource lookup/routing.

5. What is a publish-subscribe system? Publisher 1 Event Subscription Subscriber 1 Publisher 2 Subscriber 2 Matching Algorithm Event Subscription … … - Publishers and subscribers are decoupled. - Publishers are the information producers and the subscribers are the information consumers. - Subscriptions act as filters that filter out unrelated events and the system only delivers matched events to the subscribers.

6. What is a publish-subscribe system? • An example of a publish/subscribe system: • Online stock quote • Publishers: stock exchanges - NYSE, NASDAQ • Events might contain the following information: • Opening price, closing price, high/low price, volume… • Subscribers: stock traders • Subscription: A stock’s price at a particular moment.

7. What is an event? • A set of typed attributes • Each attribute consists of a type, a name and a value

8. What is a subscription? Subscription 1 Subscription 2 An event matches a subscription if and only if all the subscription’s attribute predicate/constraints are satisfied.

9. The subscription identifier • Each subscription is identified by an id (subID) • C1: The id of the node receiving the subscription • m bits for a m-bit Chord identifier address space • C2: The id of the subscription itself • Size is the rounded-up log of the maximum number of outstanding subscriptions a node can have. • C3: The number of attributes declared

10. How to store subscriptions?

11. How to store subscriptions? • For an attribute a with value v(a): • Generate the key k: k = h(v(a)) • Place subID at the node = successor(k) • Equality constraint attribute • Place subID at node = successor(h(“NYSE”)) • Range value attribute • Place subID at the following nodes: Successor(h(8.31)), successor(h(3.32)), …, successor(h(8.69)).

12. How to store subscriptions? Successor(h(NYSE)) SubID list: SubID 1 Successor(h(OTE)) SubID list: SubID1 Subscription generator Successor(h(8.69)) SubID list: SubID1 … The average number of hops needed to store a subID: Equality constraint: Range attribute: Successor(h(8.31)) SubID list: SubID 1

13. Updating subscriptions • The old node is forced to delete the SubID for the attribute that belongs to the subscription. • The new node is going to store the subID Successor(h(NYSE)) SubID list: SubID 1 Subscription generator Successor(h(NASDAQ)) SubID list: SubID 1 An update is performed by contacting:

14. Updating subscriptions • Range attribute • Updating depends on the new values of the range bounds(V low-new(a),V high-new(a)) compare to the old range bounds. • V low-new(a) < V low(a) • Store SubID at the following nodes: [V low-new(a), V low(a)) • V high-new(a) > V high(a) • Store SubID at the following nodes: (V high(a), V high-new(a)] • V low-new(a) > V low(a) or V high-new(a) < V high(a) • Delete SubID at the following nodes: [V low(a), V low-new(a)) or (V high-new(a), Vhigh(a)] • The average number of nodes need to be contacted: K(logN). • K depends on whether new range is smaller or wider than old one.

15. The Matching Algorithm – Matching Events with Subscriptions

16. The Matching Algorithm – Matching Events with Subscriptions Successor(h(NYSE)) SubID list: SubID 1 Matching algorithm will retrieve the SubID lists for each attribute in the event by going to the successor nodes. Count the number of lists containing SubID1: 3 Number of attributes declared in C3 field of SubID1: 3 Successor(h(OTE)) SubID list: SubID1, SubID 2 Successor(h(8.40)) SubID list: SubID1 Successor(h(8.80)) SubID list:

17. How to deliver the event to the subscriber? • Informing the node which generated the subscription with the matched event. • The node stores the subscription and it also holds metadata information about the subscription (the IP address of the subscription generator). • Finally, the event is delivered to the subscriber.

18. Thank You!