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https://issues.apache.org/jira/browse/SLING-4640?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14974545#comment-14974545
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Stefan Egli edited comment on SLING-4640 at 10/26/15 5:02 PM:
--------------------------------------------------------------
This would require help from the underlying repository. Without knowing how
large the read/write-delay of the local instance is, there's no way of dealing
with this.
However, if such a read/write-delay would be known and periodically checked
for, then the instance could put itself into a {{TOPOLOGY_CHANGING}} state as
soon as the read-delay is larger than the configured heartbeat timeout (or
actually a little bit before that to account for glitches).
Removing the fix-version here for now as this requires non-standard repository
support
was (Author: egli):
This would require help from the underlying repository. Without knowing how
large the read-delay of the local instance is, there's no way of dealing with
this.
However, if such a read-delay would be known and periodically checked for, then
the instance could put itself into a {{TOPOLOGY_CHANGING}} state as soon as the
read-delay is larger than the configured heartbeat timeout (or actually a
little bit before that to account for glitches).
Removing the fix-version here for now as this requires non-standard repository
support
> Possibility of duplicate leaders w/discovery.impl on eventually consistent
> repo
> -------------------------------------------------------------------------------
>
> Key: SLING-4640
> URL: https://issues.apache.org/jira/browse/SLING-4640
> Project: Sling
> Issue Type: Bug
> Components: Extensions
> Affects Versions: Discovery Impl 1.1.0
> Reporter: Stefan Egli
> Assignee: Stefan Egli
>
> Note: This is a fork of SLING-3432 based on a
> [comment|https://issues.apache.org/jira/browse/SLING-3432?focusedCommentId=14495936&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14495936].
> So here is that comment again:
> Note that [the
> above|https://issues.apache.org/jira/browse/SLING-3432?focusedCommentId=14492494&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14492494]
> does not solve the problem where the underlying repository is eventually
> consistent and the heartbeat configured is too low to catch all possible
> delays (that such an eventually consistent repository might produce under
> load). Consider the following:
> # a cluster consisting of 3 nodes: A, B and C, A is the leader
> # writes from B and C are fast - and can be read by all 3 nodes fast
> # writes from A though are slow (ie A behaves asymmetric: slow writes but
> fast reads)
> # at some point writes from A are slower than the configured heartbeat
> timeout: at this point B and C find out about this and vote on a new
> clusterView consisting only of B and C and (let's say) B becomes leader.
> #* meanwhile at A however: A is still happy: it sees the heartbeats of B and
> C in time and would not start a new voting.
> # at some later point (with a *certain read delay*) A sees that B and C have
> declared a new {{/establishedViews}} - at this point it would (according to
> the new rule above) immediately send a TOPOLOGY_CHANGING and things would be
> 'ok' again.
> #* *but* until it does send this event - *between 4. and 5. - we have two
> leaders: A and B*! -> thus could see issues reported here in SLING-3432 still
> during that small timeframe (which is basically the amount of time it takes
> for the new established view declared by B and C to be read by A).
> #* at a later time, when eg the delays in the repository have come down, A
> would rejoin the cluster - but would have to *not become leader* again, as
> the leader is B and must stay stable.
> This IMHO highlights the problem that using an eventually consistent
> repository (that has no max guaranteed delay) is *not*
> pseudo-network-partition/duplicate-leader free under load.
> Note that what is described here is not fixed by SLING-4627.
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