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https://issues.apache.org/jira/browse/SLING-4627?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14512206#comment-14512206
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Timothee Maret edited comment on SLING-4627 at 4/25/15 2:47 AM:
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Yes, I believe we could provide better tooling for figuring out the diff in
topology views.
That would be backward compatible and would allow to process diffs outside of
the scope of receiving an events.
I have added a patch (SLING-4627.patch) which contains a potential version for
it.
It would be some sort of builder that can takes either a TopologyEvent or a set
of TopologyViews and allows to first partition the changes (added, removed,
retained) and then apply further filtering on the partition (the most likely
are by default, custom filtering leverage the InstanceFilter interface).
The patch has no test, and likely is buggy (I haven't tried it..) but it is
good enough for the approach. Assuming we add this, then I believe this issue
could be solved without SPI and leaving each topology listener to figure out
what to do with topology changes.
For instance they could do
{code}
TopologyEvent event = ...;
// Leader instances that have been removed and are in the local cluster
ClusterView localView =
event.getOldView().getLocalInstance().getClusterView();
Set<String> slingIds = new TopologyViewChange(event)
.removed()
.isLeader()
.isInClusterView(localView)
.get();
if (slingIds.size() > 0) {
// do something ...
}
// Non leader, non local instances, from any cluster view, that match
some custom properties and have been added
TopologyView v1 = ...;
TopologyView v2 = ...;
InstanceFilter customFilter = new InstanceFilter() {
public boolean accept(InstanceDescription instance) {
return instance.getProperties().containsKey("my-property");
}
};
Set<String> slingIds = new TopologyViewChange(v1, v2)
.added()
.isNotLeader()
.isNotLocal()
.filterWith(customFilter).get();
if (slingIds.size() > 0) {
// do something ...
}
// Non leader instances that have been retained in a specific cluster
view
ClusterView specificView = ...;
Set<String> slingIds = new TopologyViewChange(v1, v2)
.retained(true)
.isInClusterView(specificView)
.get();
if (slingIds.size() > 0) {
// do something ...
}
// Leader instances which properties have changed in any cluster view
Set<String> slingIds = new TopologyViewChange(v1, v2)
.retained(true, true)
.get();
if (slingIds.size() > 0) {
// do something ...
}
{code}
was (Author: marett):
Yes, I believe we could provide better tooling for figuring out the diff in
topology views. I have added a patch (SLING-4627.patch) which contains a
potential version for it.
It would be some sort of builder that can takes either a TopologyEvent or a set
of TopologyViews and allows to first partition the changes (added, removed,
retained) and then apply further filtering on the partition (the most likely
are by default, custom filtering leverage the InstanceFilter interface).
The patch has no test, and likely is buggy (I haven't tried it..) but it is
good enough for the approach. Assuming we add this, then I believe this issue
could be solved without SPI and leaving each topology listener to figure out
what to do with topology changes.
For instance they could do
{code}
TopologyEvent event = ...;
// Leader instances that have been removed and are in the local cluster
ClusterView localView =
event.getOldView().getLocalInstance().getClusterView();
Set<String> slingIds = new TopologyViewChange(event)
.removed()
.isLeader()
.isInClusterView(localView)
.get();
if (slingIds.size() > 0) {
// do something ...
}
// Non leader, non local instances, from any cluster view, that match
some custom properties and have been added
TopologyView v1 = ...;
TopologyView v2 = ...;
InstanceFilter customFilter = new InstanceFilter() {
public boolean accept(InstanceDescription instance) {
return instance.getProperties().containsKey("my-property");
}
};
Set<String> slingIds = new TopologyViewChange(v1, v2)
.added()
.isNotLeader()
.isNotLocal()
.filterWith(customFilter).get();
if (slingIds.size() > 0) {
// do something ...
}
// Non leader instances that have been retained in a specific cluster
view
ClusterView specificView = ...;
Set<String> slingIds = new TopologyViewChange(v1, v2)
.retained(true)
.isInClusterView(specificView)
.get();
if (slingIds.size() > 0) {
// do something ...
}
// Leader instances which properties have changed in any cluster view
Set<String> slingIds = new TopologyViewChange(v1, v2)
.retained(true, true)
.get();
if (slingIds.size() > 0) {
// do something ...
}
{code}
> 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
> Attachments: SLING-4627.patch
>
>
> 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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