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https://issues.apache.org/jira/browse/SOLR-9835?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15879721#comment-15879721
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Cao Manh Dat commented on SOLR-9835:
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Thanks [~shalinmangar]!
bq. LeaderInitiatedRecoveryThread – What is the reason behind adding
SocketTimeoutException in the list of communication errors on which no more
retries are made?
This change come from a jepsen test. This bug is also affect current mode. I
created another issue for this bug SOLR-9913. We can skip this change for this
ticket.
bq.ZkController.register method – The condition for !isLeader &&
onlyLeaderIndexes can be replaced by the isReplicaInOnlyLeaderIndexes variable.
Yeah, that's right
bq. Since there is no log replay on startup on replicas anymore, what if the
replica is killed (which keeps its state as 'active' in ZK) and then the
cluster is restarted and the replica becomes leader candidate? If we do not
replay the discarded log then it could lead to data loss?
Very good catch, I try to resolve this problem.
bq. UpdateLog – Can you please add javadocs outlining the motivation/purpose of
the new methods such as copyOverBufferingUpdates and switchToNewTlog e.g. why
does switchToNewTlog require copying over some updates from the old tlog?
Sure!
bq.It seems that any commits that might be triggered explicitly by the user can
interfere with the index replication. Suppose that a replication is in progress
and a user explicitly calls commit which is distributed to all replicas, in
such a case the tlogs will be rolled over and then when the ReplicateFromLeader
calls switchToNewTlog(), the previous tlog may not have all the updates that
should have been copied over. We should have a way to either disable explicit
commits or protect against them on the replicas.
I don't think so, switchToNewTlog() is based on commit version at lucene index
level ({{commit.getUserData().get(SolrIndexWriter.COMMIT_COMMAND_VERSION)}}),
so we will always roll over updates in right way.
bq.UpdateLog – why does copyOverBufferUpdates block updates while calling
switchToNewTlog but ReplicateFromLeader doesn't? How are they both safe?
Good catch I think we should blockUpdates in switchToNewTlog as well.
bq.Can we add tests for testing CDCR and backup/restore with this new
replication scheme?
CDCR is very complex, I don't think we should support CDCR in this new
replication mode now.
bq. ZkController.startReplicationFromLeader – Using a ConcurrentHashMap is not
enough to prevent two simultaneous replications from happening concurrently.
You should use the atomic putIfAbsent to put a core to the map before starting
replication.
Yeah, that's sounds a good idea.
bq.Aren't some of the guarantees of real-time-get are relaxed in this new mode
especially around delete-by-queries which no longer apply on replicas? Can you
please document them as a comment on the issue that we can transfer to the ref
guide in future?
I will update the ticket description now. Basically RTG is not consistency for
DBQs
> Create another replication mode for SolrCloud
> ---------------------------------------------
>
> Key: SOLR-9835
> URL: https://issues.apache.org/jira/browse/SOLR-9835
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Cao Manh Dat
> Assignee: Shalin Shekhar Mangar
> Attachments: SOLR-9835.patch, SOLR-9835.patch, SOLR-9835.patch,
> SOLR-9835.patch, SOLR-9835.patch, SOLR-9835.patch, SOLR-9835.patch,
> SOLR-9835.patch, SOLR-9835.patch, SOLR-9835.patch, SOLR-9835.patch
>
>
> The current replication mechanism of SolrCloud is called state machine, which
> replicas start in same initial state and for each input, the input is
> distributed across replicas so all replicas will end up with same next state.
> But this type of replication have some drawbacks
> - The commit (which costly) have to run on all replicas
> - Slow recovery, because if replica miss more than N updates on its down
> time, the replica have to download entire index from its leader.
> So we create create another replication mode for SolrCloud called state
> transfer, which acts like master/slave replication. In basically
> - Leader distribute the update to other replicas, but the leader only apply
> the update to IW, other replicas just store the update to UpdateLog (act like
> replication).
> - Replicas frequently polling the latest segments from leader.
> Pros:
> - Lightweight for indexing, because only leader are running the commit,
> updates.
> - Very fast recovery, replicas just have to download the missing segments.
> On CAP point of view, this ticket will trying to promise to end users a
> distributed systems :
> - Partition tolerance
> - Weak Consistency for normal query : clusters can serve stale data. This
> happen when leader finish a commit and slave is fetching for latest segment.
> This period can at most {{pollInterval + time to fetch latest segment}}.
> - Consistency for RTG : just like original SolrCloud mode
> - Weak Availability : just like original SolrCloud mode. If a leader down,
> client must wait until new leader being elected.
> To use this new replication mode, a new collection must be created with an
> additional parameter {{liveReplicas=1}}
> {code}
> http://localhost:8983/solr/admin/collections?action=CREATE&name=newCollection&numShards=2&replicationFactor=1&liveReplicas=1
> {code}
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