SIENA. Presentation of a pub/sub implementation. Ruben Wangberg Jørgen Frøysadal Håvard Tegelsrud

Transcription

1 SIENA Presentation of a pub/sub implementation Ruben Wangberg Jørgen Frøysadal Håvard Tegelsrud

2 What is pub/sub? Introduction to SIENA SIENAs pub/sub API Filters and patterns Architectures Routing strategies SIENA evaluated Questions Topics

3 What is pub/sub? Publish/subscribe: An asynchronous messaging paradigm Publishers of messages are not programmed to send their messages to specific receivers. Rather, published messages are characterized into classes, without knowledge of what (if any) subscribers there may be. Subscribers express interest in one or more classes, and only receive messages that are of interest, without knowledge of what (if any) publishers there are.

4 What is pub/sub? This model ensures decoupling of publisher and subscriber, which in turn allows for greater scalability and a more dynamic network topology. The disadvantages is that it is hard to specify stronger properties that the application might need on an end-to-end basis (e.g. message loss) and the greater challenges concering security, as opposed to a more traditional client/server model Commonly used technology in news tickers, delivery of financial data, military application, online games etc.

5 What is pub/sub? Subscribers receive only a sub-set of the total messages published, selected through filtering Can be topic-, type- or content-based filtering If topic- or type-based, the publisher defines classes which the subscriber can subscribe to If content-based, the subscriber defines constraints, and messages are only delivered to the subscriber if they match these constraints.

6 What is pub/sub? The simplest general pub/sub implementation would require an interface consisting of two functions: Publish and subscribe We will take a closer look at a pub/sub implementation that has a slightly more advanced approach: SIENA

7 SIENA "SIENA (Scalable Internet Event Notification Architectures) is a research project aimed at designing and constructing a generic scalable publish/subscribe event-notification service. The technical basis of SIENA is an innovative type of network service called content-based networking"

8 SIENA The SIENA API exists as a prototype for Java (and C++), developed mainly by Antonio Carzaniga and Alexander L. Wolf The main concepts of the SIENA implementation: A network of servers acts as access-points for clients to publish and subscribe for notifications. The servers will deliver notifications of interest to the clients, and receive and forward publications We will now take a somewhat closer look into the details of this

9 SIENAs pub/sub API As mentioned the pub/sub paradigm states a minimum of functionality. SIENA seeks to optimize and extend this approach and make it more suitable for wide area networks. pub/sub: - subscribe() - publish() SIENA additions: - advertise() - unadvertise() - unsubscribe()

10 SIENAs API Publish: a publisher publish a notification. Subscribe: a subscriber describe types of notifications it is interested in. Unsubscribe: a subscriber describe types of notifications it is no longer interested in. Advertise: a publisher describe types of notifications it publishes. Unadvertise: a publisher describe types of notifications it no longer publishes.

11 Notifications Here you see an example of a notification in SIENA. It consists of a set of typed attributes, where the attributes simply are character strings. The types are predefined and are all common types in programming environments in general.

12 Advertise The introduction of advertisements is meant to ease the routing and reduce overall information flow. While a subscription defines a set of interesting notifications for the subscriber, an advertisement is meant to define a set of notifications that a publisher can produce. When a server receives a subscription, it will know where to forward it. This will in many cases not be every connected server and thus it's not longer a broadcast. The number of advertisements will likely be a lot lower than the number of subscriptions and the number of broadcasts have been reduced.

13 Filters The event filter cheques if it has any instance that match the types and values in the notification. An instance in the filter is a set of attributes consisting of a type, a name, a predicate and a value. Its considered a match when all the features in all the attributes in the filter set are found in the notification. This is what a filter could look like in SIENA.

14 Patterns Patterns in SIENA are constructed simply by using more than one filter in order. Incoming notifications are filtered through the filters in order, meaning that if the pattern specifies "notification A must come before notification B", all notifications that come before any A- notification will be discarded. When an A has arrived, the pattern discards everything but notification matching B, as well as matching B-notifications with an earlier timestamp than A. Now, the whole pattern has been matched, and it will start looking for an A again, though with an even later timestamp.

15 Patterns The described pattern illustrated: The top number describes in what order the notification is received, while the bottom number illustrates the ordering in time The first match of the pattern will be the first A combined with the third B (second and third notification)

16 Hierarchical client/server architecture

17 Acyclic peer-to-peer architecture

18 General peer-to-peer architecture

19 Routing strategies in SIENA The simple solution would be to broadcast, but this creates a lot of overhead. How can we improve this? - two principles: - downstream replication - upstream evaluation

20 Replication & assembling

21 Evaluating SIENA difficult to test scalability in real environment three approaches - qualitative reasoning - simulations - prototype

22 Simulations - framework

23 objects = publishers, interested parties = subscribers, hs = hierarchical architecture with subscription forwarding as = acyclic architecture with subscription forwarding aa = acyclic architecture with advertisement forwarding Results - total cost

24 Results - cost per service request

25 Cost per subscription & per notification total cost analysis + cost per service analysis shows promising results for both: hierarchical architecture with subscription forwarding acyclic architecture with subscription forwarding differs in density of publishers and subscribers ha = best in low density

26 Worst case per site cost not able to simulate congestion calculate which architecture reach congestion first how? - 10 simulations over each scenario - average cost at each site - find maximum of these averages

27 Results - worst case per site cost

28 Simulation - summary hierarchical architecture has a lower per subscription cost than the acyclic architecture both architectures: cost differences reduces for increased densities of publishers and subscribers. cost of delivering a notification to subscribers is about the same for both architectures

29 ? Questions