[jboss-jira] [JBoss JIRA] Commented: (JGRP-952) MERGE: UNICAST can lose messages on merging

[Bela Ban (JIRA)]

    [ https://jira.jboss.org/jira/browse/JGRP-952?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12461532#action_12461532 ] 

Bela Ban commented on JGRP-952:
-------------------------------

This is the revival of the ack based scheme ! Because the following use case is a disadvantage to the 'trash the sender window on SEND_GET_FIRST message' scheme:
- A has #25, B trashed its connection window
- After a partition heals, A becomes the merge leader and sends a unicast MERGE-REQ to B
- This is #26 and in A's sender window
- B sends a SEND_FIRST_SEQNO message to A
- A trashes its sender window and, along with it, its seqno #26 (the MERGE-REQ)
- Unless A copies all element from the old sender-win into the new one and assigns them new seqnos, MERGE-REQ will not be received by B !
--> Copying existing elements of A's sender window might be dangerous: we could have had #11 #13 #14 (#12 has already been ack'ed), and so the original #12 would never be delivered at B !

> MERGE: UNICAST can lose messages on merging
> -------------------------------------------
>
>                 Key: JGRP-952
>                 URL: https://jira.jboss.org/jira/browse/JGRP-952
>             Project: JGroups
>          Issue Type: Bug
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>             Fix For: 2.6.10, 2.8
>
>
> The following use case loses messages:
> - A sends #5 to B
> - B expects #4 from A, but adds #5 and acks it
> - A receives the ack(#5) and removes #5 from its sender window
> - Now there is a partition such that B trashes its connection window form A, but A keeps its window for B (A: {A,B}, B: {B})
> - The partition heals and A sends #6 to B
> - B asks A for its lowest seqno, A resends #4 (with a conn_id)
> - B creates a receiver window for A with seqno=#4
> - A resends #6
> - B adds #6 to its window, but doesn't deliver it because it is missing #5
> --> However, A will NEVER resend #5 because the ack(#5) from B removed #5 from A's sender window !
> ==> Possible SOLUTION: when A gets the SEND_FIRST_SEQNO and there are (unacked) messages in A's sender window, and they are not in order, then A will trash its connection window and copy the pending messages into the new window (with new seqnos !) before sending #1 (with conn_id)
> ==> This solution might lose the original message #5, but that's better than B never being able to deliver any messages anymore !

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