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https://issues.apache.org/jira/browse/SLING-4627?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14502789#comment-14502789
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Carsten Ziegeler commented on SLING-4627:
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I think the application itself can't handle this - it has not enough knowledge
of the implementation details to do so (which is good). This also includes that
the application does not know whether the data store has strong consistency or
is eventual consistent.
Let's assume for a second, we don't solve it within the discovery impl. What
would the app code exactly do?
It will receive a topology event, but does not really know what changed - so
the app needs to do a diff to find out which of the above cases it is.
If it's none of the critical cases, fine - if it is a critical case, the app
calls a wait-for-sync utility. Where is this defined? How does the api look
like?
I have the feeling that we can't abstract this in a nice way, and then every
app code using a topology listener needs to do this stuff.
So what about this: the discovery impl can be configured to use such a sync
service. If it's configured to do so, it looks it up and uses it which means
the discovery impl waits for sending out the changed event until the sync
service notified so.
With this, we just create such a SPI interface in the discovery - but leave the
implementation to someone else.
> TOPOLOGY_CHANGED in an eventually consistent repository
> -------------------------------------------------------
>
> Key: SLING-4627
> URL: https://issues.apache.org/jira/browse/SLING-4627
> Project: Sling
> Issue Type: Improvement
> Components: Extensions
> Reporter: Stefan Egli
> Priority: Critical
>
> This is a parent ticket describing the +coordination effort needed between
> properly sending TOPOLOGY_CHANGED when running ontop of an eventually
> consistent repository+. These findings are independent of the implementation
> details used inside the discovery implementation, so apply to discovery.impl,
> discovery.etcd/.zookeeper/.oak etc. Tickets to implement this for specific
> implementation are best created separately (eg sub-task or related..). Also
> note that this assumes immediately sending TOPOLOGY_CHANGING as described [in
> SLING-3432|https://issues.apache.org/jira/browse/SLING-3432?focusedCommentId=14492494&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14492494]
> h5. The spectrum of possible TOPOLOGY_CHANGED events include the following
> scenarios:
> || scenario || classification || action ||
> | A. change is completely outside of local cluster | (/) uncritical | changes
> outside the cluster are considered uncritical for this exercise. |
> | B. a new instance joins the local cluster, this new instance is by contract
> not the leader (leader must be stable \[0\]) | (/) uncritical | a join of an
> instance is uncritical due to the fact that it merely joins the cluster and
> has thus no 'backlog' of changes that might be propagating through the
> (eventually consistent) repository. |
> | C. a non-leader *leaves* the local cluster | (x) *critical* | changes that
> were written by the leaving instance might still not be *seen* by all
> surviving (ie it can be that discovery is faster than the repository) and
> this must be assured before sending out TOPOLOGY_CHANGED. This is because the
> leaving instance could have written changes that are *topology dependent* and
> thus those changes must first be settled in the repository before continuing
> with a *new topology*. |
> | D. the leader *leaves* the local cluster (and thus a new leader is elected)
> | (x)(x) *very critical* | same as C except that this is more critical due to
> the fact that the leader left |
> | E. -the leader of the local cluster changes (without leaving)- this is not
> supported by contract (leader must be stable \[0\]) | (/) -irrelevant- | |
> So both C and D are about an instance leaving. And as mentioned above the
> survivors must assure they have read all changes of the leavers. There are
> two parts to this:
> * the leaver could have pending writes that are not yet in mongoD: I don't
> think this is the case. The only thing that can remain could be an
> uncommitted branch and that would be rolled back afaik.
> ** Exception to this is a partition: where the leaver didn't actually crash
> but is still hooked to the repository. *For this I'm not sure how it can be
> solved* yet.
> * the survivers could however not yet have read all changes (pending in the
> background read) and one way to make sure they did is to have each surviving
> instance write a (pseudo-) sync token to the repository. Once all survivors
> have seen this sync token of all other survivors, the assumption is that all
> pending changes are "flushed" through the eventually consistent repository
> and that it is safe to send out a TOPOLOGY_CHANGED event.
> * this sync token must be *conflict free* and could be eg:
> {{/var/discovery/oak/clusterInstances/<slingId>/syncTokens/<newViewId>}} -
> where {{newViewId}} is defined by whatever discovery mechanism is used
> * a special case is when only one instance is remaining. It can then not wait
> for any other survivor to send a sync token. In that case sync tokens would
> not work. All it could then possibly do is to wait for a certain time (which
> should be larger than any expected background-read duration)
> [~mreutegg], [~chetanm] can you pls confirm/comment on the above "flush/sync
> token" approach? Thx!
> /cc [~marett]
> \[0\] - see [getLeader() in
> ClusterView|https://github.com/apache/sling/blob/trunk/bundles/extensions/discovery/api/src/main/java/org/apache/sling/discovery/ClusterView.java]
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