[SLING-4627] TOPOLOGY_CHANGED in an eventually consistent repository

• Type: Improvement
• Status: Closed
• Priority: Critical
• Resolution: Fixed
• Fix Version/s: Discovery Commons 1.0.0, Discovery Oak 1.0.0
• Component/s: Extensions
• Assignee: Stefan Egli
• Reporter: Stefan Egli
• Created: 15/Apr/15 14:39
• Updated: 02/Nov/15 16:49
• Resolved: 26/Oct/15 14:49

Description

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

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	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)	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

Issue Links

• is related to: SLING-4665 Provide a Helper class to compare topology views
• is superceded by: SLING-5131 Introduce ConsistencyService to discovery.commons
• relates to: SLING-3432 pseudo network partition causes job deserialization issue in a cluster (when reading while job is being reassigned)
• relates to: SLING-4640 Possibility of duplicate leaders w/discovery.impl on eventually consistent repo