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https://issues.apache.org/jira/browse/SOLR-9504?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Shalin Shekhar Mangar updated SOLR-9504:
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Attachment: SOLR-9504.patch
Patch with a test that fails without the fix.
Basically, when we need to bail out because we have no versions, we peek at the
other replicas. If even one has versions, then we return this bit of
information to the ShardLeaderElectionContext.runLeaderProcess and rejoin the
election, else we proceed as before. The hacky bit is that there is now a
PeerSyncResult class which has a success flag as well as an optional
otherHasVersions flag.
I'm going to run some tests in a loop to ensure I haven't broken anything.
> A replica with an empty index becomes the leader even when other more
> qualified replicas are in line
> ----------------------------------------------------------------------------------------------------
>
> Key: SOLR-9504
> URL: https://issues.apache.org/jira/browse/SOLR-9504
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Components: SolrCloud
> Affects Versions: master (7.0)
> Reporter: Shalin Shekhar Mangar
> Priority: Critical
> Labels: impact-high
> Fix For: 6.3, master (7.0)
>
> Attachments: SOLR-9504.patch
>
>
> I haven't tried branch_6x or any release yet. But this is trivially
> reproducible on master with the following steps:
> # Start two solr nodes
> # Create a collection with 1 shard, 1 replica so that one node is empty.
> # Index some documents
> # Shutdown the leader node
> # Use addreplica API to create a replica of the collection on the
> still-running node. For some reason this API hangs until you restart the
> other node (possibly a bug itself) but do not wait for the API to complete.
> # Restart the former leader node
> You'll find that the replica with 0 docs has become the leader. The former
> leader recovers from the leader without replicating any index files. It still
> has the old index which has some docs.
> This is from the logs of the 0 doc replica:
> {code}
> 713102 INFO (zkCallback-4-thread-5-processing-n:127.0.1.1:7574_solr) [ ]
> o.a.s.c.c.ZkStateReader Updating data for [gettingstarted] from [9] to [10]
> 714377 INFO (qtp110456297-15) [c:gettingstarted s:shard1 r:core_node2
> x:gettingstarted_shard1_replica2] o.a.s.c.ShardLeaderElectionContext Enough
> replicas found to continue.
> 714377 INFO (qtp110456297-15) [c:gettingstarted s:shard1 r:core_node2
> x:gettingstarted_shard1_replica2] o.a.s.c.ShardLeaderElectionContext I may be
> the new leader - try and sync
> 714377 INFO (qtp110456297-15) [c:gettingstarted s:shard1 r:core_node2
> x:gettingstarted_shard1_replica2] o.a.s.c.SyncStrategy Sync replicas to
> http://127.0.1.1:7574/solr/gettingstarted_shard1_replica2/
> 714380 INFO (qtp110456297-15) [c:gettingstarted s:shard1 r:core_node2
> x:gettingstarted_shard1_replica2] o.a.s.u.PeerSync PeerSync:
> core=gettingstarted_shard1_replica2 url=http://127.0.1.1:7574/solr START
> replicas=[http://127.0.1.1:8983/solr/gettingstarted_shard1_replica1/]
> nUpdates=100
> 714381 INFO (qtp110456297-15) [c:gettingstarted s:shard1 r:core_node2
> x:gettingstarted_shard1_replica2] o.a.s.u.PeerSync PeerSync:
> core=gettingstarted_shard1_replica2 url=http://127.0.1.1:7574/solr DONE. We
> have no versions. sync failed.
> 714382 INFO (qtp110456297-15) [c:gettingstarted s:shard1 r:core_node2
> x:gettingstarted_shard1_replica2] o.a.s.c.SyncStrategy Leader's attempt to
> sync with shard failed, moving to the next candidate
> 714382 INFO (qtp110456297-15) [c:gettingstarted s:shard1 r:core_node2
> x:gettingstarted_shard1_replica2] o.a.s.c.ShardLeaderElectionContext We
> failed sync, but we have no versions - we can't sync in that case - we were
> active before, so become leader anyway
> 714387 INFO (qtp110456297-15) [c:gettingstarted s:shard1 r:core_node2
> x:gettingstarted_shard1_replica2] o.a.s.c.ShardLeaderElectionContextBase
> Creating leader registration node
> /collections/gettingstarted/leaders/shard1/leader after winning as
> /collections/gettingstarted/leader_elect/shard1/election/96579592334475268-core_node2-n_0000000001
> 714398 INFO (qtp110456297-15) [c:gettingstarted s:shard1 r:core_node2
> x:gettingstarted_shard1_replica2] o.a.s.c.ShardLeaderElectionContext I am the
> new leader: http://127.0.1.1:7574/solr/gettingstarted_shard1_replica2/ shard1
> {code}
> It basically tries to sync but has no versions and because it was active
> before (it is a new core starting up for the first time), it becomes the
> leader and publishes itself as active.
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