[jboss-jira] [JBoss JIRA] (JGRP-1821) SEQUENCER2: new impl of total order protocol

[Bela Ban (JIRA)]

     [ https://issues.jboss.org/browse/JGRP-1821?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Bela Ban resolved JGRP-1821.
----------------------------

    Resolution: Done

    
> SEQUENCER2: new impl of total order protocol
> --------------------------------------------
>
>                 Key: JGRP-1821
>                 URL: https://issues.jboss.org/browse/JGRP-1821
>             Project: JGroups
>          Issue Type: Feature Request
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>             Fix For: 3.5
>
>
> When in {{SEQUENCER}} a member P wants to send a multicast message M, it unicasts it to the coordinator, who multicasts it on behalf of P.
> The new impl {{SEQUENCER2}} is different:
> * P asks the coord for a seqno
> * The coord responds with a (monotonically increasing) seqno
> * P multicasts M with that seqno
> * Everyone uses one global {{Table}} to deliver messages and weed out duplicates
> Advantages:
> # A sender sends messages itself, so the sequencer doesn't need to do sending (and potential retransmissions)
> # Compared to {{SEQUENCER}}, the data is only sent and marshalled once (better for large messages)
> # A sender grabs entire ranges of seqnos, so this should be efficient
> The edge case handling though requires some work, e.g.
> * A member B crashes after having received a seqno (e.g. 4)
> ** The sequencer will give out 5 next, but since nobody received 4, all subsequent messages will get stuck, waiting for 4
> * The sequencer (coord) dies or leaves
> ** The next-in-line probably needs to run some reconciliation protocol, asking all members for their highest received seqnos
> ** Messages like 4 would get marked as dummy, removed from table and dropped

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