Tuesday, December 06, 2011

Repondez s'il vous plait !

No, this isn't a post in French (my school French would be too rusty for this !); this is about a new protocol in JGroups, called RSVP :-)

As the name possibly suggests, this feature allows for messages to get ack'ed by receivers before a message send returns. In other words, when A broadcasts a message M to {A,B,C,D}, then JChannel.send() will only return once itself, B, C and D have acknowledged that they delivered M to the application.

This differs from the default behavior of JGroups which always sends messages asynchronously, and guarantees that all non-faulty members will eventually receive the message. If we tag a message as RSVP, then we basically have 2 properties:
  1. The message send will only return when we've received all acks from the current members. Members leaving or crashing during the wait are treated as if they sent an ack. The send() method can also throw a (runtime) TimeoutException if a timeout was defined (in RSVP) and encountered.
  2. If A sent (asynchronous) messages #1-10, and tagged #10 as RSVP, then - when send() returns successfully - A is guaranteed that all members received A's message #10 and all messages prior to #10, that's #1-9.
This can be used for example when completing a unit of work, and needing to know that all current cluster members received all of the messages sent up to now by a given cluster member.

This is similar to FLUSH, but less strict in that it is a per-sender flush, there is no reconciliation phase, and it doesn't stop the world.

An alternative is to use a blocking RPC. However, I wanted to add the capability of synchronous messages directly into the base channel.

Note that this also solves another problem: if A sends messages #1-5, but some members drop #5, and A doesn't send more messages for some time, then A#5 won't get delivered at some members for quite a while (until stability (STABLE) kicks in).

RSVP will be available in JGroups 3.1. If you want to try it out, get the code from master [2]. The documentation is at [1], section

For questions, I suggest one of the mailing lists.

[1] http://www.jgroups.org/manual-3.x/html/user-channel.html#SendingMessages

[2] https://github.com/belaban/JGroups

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