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https://issues.apache.org/jira/browse/SOLR-14942?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17216948#comment-17216948
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Chris M. Hostetter commented on SOLR-14942:
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spot check comments on skimming the diff...
* every method should have javadocs, especially public ones
* ZkController.exlectionContexts is a syncrhonized map – but the new code that
streams over it's `values()` doesn't synchronize on it which smells like a bug?
* if canceling & closing elections is a "slow" enough operation that it makes
sense to parallelize them, then does it make sense to also check
zkClient.isClosed() inside the loop, in case the client gets closed out from
under us? (it's a cheap call, so i don't see any advantage to only checking
once)
* 'handler instanceof UpdateRequestHandler' feels somewhat brittle?
** are there other "inflight" cases where it's risky to shutdown in the middle
of? replication? peersync? core admin?
** instead of hardcoding this instanceof check in HttpSolrCall would it make
more sense to add a new 'default' method to SolrRequestHandler that
UpdateRequestHandler (and potentially other handlers) could override to let
them inspect the request and return true/false if it's "important" enough that
it must be allowed to block shutdown until complete?
*** this would also make it easier to bring back the "only block updates if
i'm the leader" type logic (living inside thee UpdateRequestHandler impl of
the new method) at a later date if viable – w/o more API changes
> Reduce leader election time on node shutdown
> --------------------------------------------
>
> Key: SOLR-14942
> URL: https://issues.apache.org/jira/browse/SOLR-14942
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Components: SolrCloud
> Affects Versions: 7.7.3, 8.6.3
> Reporter: Shalin Shekhar Mangar
> Assignee: Shalin Shekhar Mangar
> Priority: Major
> Time Spent: 0.5h
> Remaining Estimate: 0h
>
> The credit for this issue and investigation belongs to [~caomanhdat]. I am
> merely reporting the issue and creating PRs based on his work.
> The shutdown process waits for all replicas/cores to be closed before
> removing the election node of the leader. This can take some time due to
> index flush or merge activities on the leader cores and delays new leaders
> from being elected.
> This process happens at CoreContainer.shutdown():
> # zkController.preClose(): remove current node from live_node and change
> states of all cores in this node to DOWN state. Assuming that the current
> node hosting a leader of a shard, the shard becomes leaderless after calling
> this method, since the state of the leader is DOWN now. The leader election
> process is not triggered for the shard since the election node is still
> on-hold by the current node.
> # Waiting for all cores to be loaded (if there are any).
> # SolrCores.close(): close all cores.
> # zkController.close(): this is where all ephemeral nodes are removed from ZK
> which include election nodes created by this node. Therefore other replicas
> in the shard can take part in the leader election from now.
> Note that CoreContainer.shutdown() is invoked when Jetty/Solr nodes receive
> SIGTERM signal.
> On receiving SIGTERM, Jetty will also stop accepting new connections and new
> requests. This is a very important factor, since even if the leader replica
> is ACTIVE and its node in live_nodes, the shard will be considered as
> leaderless if no-one can index to that shard. Therefore shards become
> leaderless as soon as the node (which contains shard’s leader) receives
> SIGTERM.
> Therefore the longer time step 1, 2 and 3 needed to finish, the longer shards
> remain leaderless. The time needed for step 3 scales with the number of cores
> so the more cores a node has, the worse. This time is spent in
> IndexWriter.close() where the system will
> # Flush all pending updates to disk
> # Waiting for all merge finish (this most likely is the meaty part)
> The shutdown process is proposed to changed to:
> # Wait for all in-flight indexing requests and replication requests to
> complete
> # Remove election nodes
> # Close all replicas/cores
> This ensures that index flush or merges do not block new leader elections
> anymore.
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