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[ecf-dev] ECF Publish/Subscribe mechanism

Hi all,

I'd like to solicit everyone's feedback on something I've been working on recently. I think ECF is a good fit for group scenarios where one member has a service (e.g., a live model) that others may want to "subscribe" to; in other words, receive some domain-specific events that originate at the remote source. As an example, consider the scenario in which one group member publishes a blog (a series of entries) and that some other members want to view it and be notified as soon as any changes are made. (There are numerous other examples one can imagine -- monitoring someone's task list as it's being completed, error log as new entries are logged, etc.)

Continuing with the blog example, one way to implement this would be for the author to create a shared object that would replicate itself to all containers (and any new ones as they join), and whenever changes are made to the primary instance, events would be forwarded to remote replicas. Other members would monitor these events and react appropriately.

While this might work in the simple case, it wouldn't scale well -- even containers that are not explicitly interested in these events would have to bear the traffic. A more efficient approach might be to only create replicas in containers that request it -- the client would request that a certain service (published in a specific container) be replicated locally. When that occurs, the client would be notified and could consume the service in a domain-specific way.

To make this approach even more generic, a utility service (local to each container) would track publication of services in all remote containers, so that interested clients may be notified whenever a service is published at a remote site (so that they may then subscribe to it if they want to).

The more concrete class of replicated service that I'm interested in is the "shared model" (but no editing this time :-) ) -- the source container publishes some model with an "updater" that knows how to apply changes to it. Remote containers then subscribe to it and receive the model's current state, followed by any incremental updates to it (as they happen).

I created an example plugin (org.eclipse.ecf.example.pubsub) that contains the experimental API as well as the implementation, along with an example of a simple model that can be published and subscribed to (an appendable list of strings). The example consists of a view that shows remote published services and allows the user to publish shared lists. Remote services (if they're also the sample appendable lists) may be subscribed to and their contents are presented in a separate view. I'll put the details on how to run it on the wiki.

Some details about the protocol:
1. The client may use the IPublishedServiceDirectory to obtain/listen to remote services being published/unpublished. This instance is obtained as the local container's adapter. It is local to the container (i.e., does not replicate itself) and has a fixed ID.
2. IPublishedServiceDirectory uses a helper shared object (the Discovery Agent) to learn about what's published remotely. Upon activation, it creates the agent (with a GUID), which then replicates itself to all other containers (and any new ones, as they join). The remote agent replicas then collect information about published services (i.e., their activation/deactivation) and forward it to their IPublishedServiceDirectory (in their home container). With this strategy, no "master directory" of published services needs to be maintained, thus greatly simplifying the solution.
3. The client may also obtain an IPublishedServiceRequestor (again as the container's adapter), which allows it to subscribe to a specific published service on a specific container (and receive an ISubscription upon completion, which is disposed when done).
4. Upon obtaining the subscription, the client checks if it is a shared model (of the right kind -- and AppendableList, in this example), obtains its initial state and listens to any model changes.
5. Whenever the master instance is updated (i.e., an item is added), the change is sent out to all remote replicas and applied to their local states. The clients receive the appropriate model change notification.


There's a lot more to talk about, but this should be enough to start the conversation. I'll continue to harden the implementation and revise the API. I'll move the API to a dedicated plugin once it's matured a bit. Please let me know what you think. Thanks!

--Peter