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https://issues.apache.org/jira/browse/SOLR-7932?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14698683#comment-14698683
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Yonik Seeley commented on SOLR-7932:
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There's a ton of history behind the replication stuff - and it's had to change
over time due to changes in Lucene's versions / generations.
bq. It appears as if we are checking wall times across machines to check if we
are in sync,
At least in traditional master-slave replication, that's not the case... the
timestamps being compared are all generated on the master (even if they are
compared on the slaves?)
Anyway, using index generation is certainly not enough since one can blow away
an index and re-create it, reseting the generation to a low number.
> Solr replication relies on timestamps to sync across machines
> -------------------------------------------------------------
>
> Key: SOLR-7932
> URL: https://issues.apache.org/jira/browse/SOLR-7932
> Project: Solr
> Issue Type: Bug
> Components: replication (java)
> Reporter: Ramkumar Aiyengar
> Attachments: SOLR-7932.patch
>
>
> Spinning off SOLR-7859, noticed there that wall time recorded as commit data
> on a commit to check if replication needs to be done. In IndexFetcher, there
> is this code:
> {code}
> if (!forceReplication &&
> IndexDeletionPolicyWrapper.getCommitTimestamp(commit) == latestVersion) {
> //master and slave are already in sync just return
> LOG.info("Slave in sync with master.");
> successfulInstall = true;
> return true;
> }
> {code}
> It appears as if we are checking wall times across machines to check if we
> are in sync, this could go wrong.
> Once a decision is made to replicate, we do seem to use generations instead,
> except for this place below checks both generations and timestamps to see if
> a full copy is needed..
> {code}
> // if the generation of master is older than that of the slave , it
> means they are not compatible to be copied
> // then a new index directory to be created and all the files need to
> be copied
> boolean isFullCopyNeeded = IndexDeletionPolicyWrapper
> .getCommitTimestamp(commit) >= latestVersion
> || commit.getGeneration() >= latestGeneration || forceReplication;
> {code}
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