Stefan Egli created SLING-4627:
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Summary: 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
GRANITE-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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