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https://issues.apache.org/jira/browse/SOLR-5468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13983035#comment-13983035
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Mark Miller commented on SOLR-5468:
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bq. for now it seems sufficient to let users decide how many replicas a write
must succeed on to be considered successful.
I agree that that is the low hanging fruit. We just have to let the user know
exactly what we are trying to promise.
bq. there would need to be some "backing out" work to remove an update that
succeeded on the leader but failed on the replicas.
Yup - that will be the hardest part of doing this how we would really like and
a large reason it was punted on in all the initial work. Even if the leader
didn't process the doc first (which is likely a doable optimization at some
point), I still think it's really hard.
bq. Lastly, batches! What happens if half of a batch (sent by a client)
succeeds and the other half fails (due to losing a replica in the middle of
processing the batch)?
Batches and streaming really don't make sense yet in SolrCloud other than for
batch loading. We need to implement better, fine grained responses first. When
that happens, it should all operate the same as single update per request.
> Option to enforce a majority quorum approach to accepting updates in SolrCloud
> ------------------------------------------------------------------------------
>
> 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
>
> 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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