[jboss-jira] [JBoss JIRA] (JGRP-1904) NAKACK2: retransmit the last-message-missing sooner

[Bela Ban (JIRA)]

Bela Ban created JGRP-1904:
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             Summary: NAKACK2: retransmit the last-message-missing sooner
                 Key: JGRP-1904
                 URL: https://issues.jboss.org/browse/JGRP-1904
             Project: JGroups
          Issue Type: Enhancement
            Reporter: Bela Ban
            Assignee: Bela Ban
             Fix For: 3.6.1


When we multicast messages (with {{NAKACK2}}), if the last message M is dropped (e.g. because the receiver has a full thread pool), then it will take a while for M to be retransmitted.
Currently, detection of missing messages is done in {{STABLE}}: this protocol periodically (or based on receiving a certain number of bytes) broadcasts its digest. When the coordinator has received all digests, it computes the min and broadcasts a {{STABILITY}} message.
When members receive this message, they can purge all messages below the minimum vector. Receivers also detect if the digest contains messages higher than the ones they actually received and can thus perform retransmission, if needed.
So the main purpose of {{STABLE}} is purging of messages seen by everyone, *not* detecting missing messages. It also takes a while (consensus from all members) to generate a {{STABILITY}} message, so detecting a missing message takes long.
We therefore need a quicker way to detect and retransmit missing messages.

h5. Solution
* Add a flag to {{NAKACK2}} (*not* {{STABLE}}) which - if set - periodically (or based on the number of bytes sent), broadcasts the seqno of the highest message it sent.
* When a receiver detects that it didn't receive this message, it can ask the sender for retransmission
* The task should acquiesce when no messages are missing (i.e. the highest delivered message == the highest received message)



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