[jboss-jira] [JBoss JIRA] Created: (JGRP-335) Hangs with FLUSH

[Bela Ban (JIRA)]

Hangs with FLUSH
----------------

                 Key: JGRP-335
                 URL: http://jira.jboss.com/jira/browse/JGRP-335
             Project: JGroups
          Issue Type: Bug
    Affects Versions: 2.3 SP1
            Reporter: Bela Ban
         Assigned To: Bela Ban
             Fix For: 2.4




2 use cases where we can run into the problem (members A and B).

#1 View change

    * A is running, B joins
    * B is *not* blocking in FLUSH, A is blocking after START_FLUSH
    * A starts the flush
    * A returns the new view to B in a JOIN_RSP. This causes B's Channel.connect() to return
    * B sends a unicast message to A, to which A sends a response *in the same thread* (service STATE_REQ)
    * A competes the flush, multicasting a STOP_FLUSH message
    * The STATE_REQ at A hangs on FLUSH.down()
    * The STOP_FLUSH at A can never unblock FLUSH.down() because it was received *after* the STATE_REQ from B !

SOLUTION:

   1. Make B block in FLUSH.down() as soon as the client sends the JOIN_REQ to A
   2. Make STOP_FLUSH *synchronous*. This means we only return from Channel.connect() (for example) once every member has ack'ed the STOP_FLUSH. See next issue (state transfer) for a description of what happens if we don't do this.


#2 State transfer

    * A and B are members of the group
    * B calls Channel.getState()
    * A and B receive a START_FLUSH, start the block in FLUSH
    * State is transferred from A to B
    * B multicasts a STOP_FLUSH and *immediately afterwards* sends a *unicast* message (which can 'pass' multicast messages, as they're unrelated)
    * A happens to receive the unicast message *before* the STOP_FLUSH. The unicast blocks and the STOP_FLUSH, which would unblock it, cannot be delivered

SOLUTION:

   1. Same as solution 2 above. If we make the STOP_FLUSH phase synchronous, connect() or getState() only return once everyone has been unblocked


LONG TERM SOLUTION:

    * The much better solution of course is to make the STOP_FLUSH message *out-of-band*, so it can be delivered in parallel to other messages, and is not blocked (e.g.) by the unicast in the queue. So even if the unicast message was blocked waiting for STOP_FLUSH, once STOP_FLUSH has been received, it will be delivered, causing the unicast to unblock
    * Once we have this solution in place (2.5, threadless stack and out-of-band messages), we can revert the STOP_FLUSH to only use 1 phase rather than 2



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