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

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