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https://issues.apache.org/jira/browse/SOLR-5468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14039012#comment-14039012
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Timothy Potter commented on SOLR-5468:
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I've renamed this ticket as the previous name was misleading as to what the
actual solution is doing. If you're interested in this subject, I encourage you
to read my comments above. Basically, Solr's replication mechanism isn't well
suited for enforcing a majority quorum before accepting a write. In a nutshell,
if you can't rollback a write that succeeds on some nodes if the majority isn't
reached, then you can't enforce it ;-) The solution in this ticket is very
basic. It allows a client to request that Solr return the achieved replication
factor for a given update request (single or batch). All this allows is for the
client application to take some additional measures, such as resending the
updates, if so desired. Put simply, this moves the problem out to the client.
It's useful for environments that have RF>=3 and if 2 of the replicas are down,
then there is a risk of losing writes if the leader fails and doesn't come
back. This gives the client application the ability to know which writes were
accepted by a degraded shard and perhaps re-try them in the future if needed.
That's it!
> Option to notify client when desired replication factor not achieved for an
> update request.
> -------------------------------------------------------------------------------------------
>
> Key: SOLR-5468
> URL: https://issues.apache.org/jira/browse/SOLR-5468
> Project: Solr
> Issue Type: New Feature
> Components: SolrCloud
> Affects Versions: 4.5
> Environment: All
> Reporter: Timothy Potter
> Assignee: Timothy Potter
> Priority: Minor
> Fix For: 4.9
>
> Attachments: SOLR-5468.patch, SOLR-5468.patch, SOLR-5468.patch
>
>
> I've been thinking about how SolrCloud deals with write-availability using
> in-sync replica sets, in which writes will continue to be accepted so long as
> there is at least one healthy node per shard.
> For a little background (and to verify my understanding of the process is
> correct), SolrCloud only considers active/healthy replicas when acknowledging
> a write. Specifically, when a shard leader accepts an update request, it
> forwards the request to all active/healthy replicas and only considers the
> write successful if all active/healthy replicas ack the write. Any down /
> gone replicas are not considered and will sync up with the leader when they
> come back online using peer sync or snapshot replication. For instance, if a
> shard has 3 nodes, A, B, C with A being the current leader, then writes to
> the shard will continue to succeed even if B & C are down.
> The issue is that if a shard leader continues to accept updates even if it
> loses all of its replicas, then we have acknowledged updates on only 1 node.
> If that node, call it A, then fails and one of the previous replicas, call it
> B, comes back online before A does, then any writes that A accepted while the
> other replicas were offline are at risk to being lost.
> SolrCloud does provide a safe-guard mechanism for this problem with the
> leaderVoteWait setting, which puts any replicas that come back online before
> node A into a temporary wait state. If A comes back online within the wait
> period, then all is well as it will become the leader again and no writes
> will be lost. As a side note, sys admins definitely need to be made more
> aware of this situation as when I first encountered it in my cluster, I had
> no idea what it meant.
> My question is whether we want to consider an approach where SolrCloud will
> not accept writes unless there is a majority of replicas available to accept
> the write? For my example, under this approach, we wouldn't accept writes if
> both B&C failed, but would if only C did, leaving A & B online. Admittedly,
> this lowers the write-availability of the system, so may be something that
> should be tunable?
> From Mark M: Yeah, this is kind of like one of many little features that we
> have just not gotten to yet. I’ve always planned for a param that let’s you
> say how many replicas an update must be verified on before responding
> success. Seems to make sense to fail that type of request early if you notice
> there are not enough replicas up to satisfy the param to begin with.
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