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https://issues.apache.org/jira/browse/SOLR-5468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13989664#comment-13989664
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Timothy Potter commented on SOLR-5468:
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So I've thought about this some more and am not sure it's even worth pursuing
any further?
Enabling this feature incurs a cost because it layers a synchronous blocking
action (CountDownLatch.await) on top of an asynchronous process (replication
from leader to N replicas). So given there's a cost, what's the benefit? On the
surface, it seems like a reasonable idea to tell a client application the level
of replication that was achieved for an update request, but the best a client
application can do is retry the update once the problem that caused degraded
replication is resolved (only if it's an idempotent update at that). However,
that's exactly what happens automatically when a previously partitioned replica
is healed. Specifically, the replica will be marked DOWN (see SOLR-5495) and
then it must recover before becoming active, which gives the same result as the
client application re-trying a request.
Lastly, I thought more about Mark's point on failing fast when we know that a
request cannot meeting the desired RF requested by the client. In this one
case, we won't have to worry about any backing out because we'd catch the
problem before the local add/update on the leader. However, this sort of lulls
the client into a false security in that we really can't "fail fast" if a
replica goes down after the local add/update on the leader. So that would mean
two different behaviors depending on timing which I don't think we want. Again,
I think we should focus on hardening the out-of-sync replica recovery and
leader failover scenarios.
> 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
> Attachments: 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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