[
https://issues.apache.org/jira/browse/SOLR-5468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13989724#comment-13989724
]
Mark Miller commented on SOLR-5468:
-----------------------------------
I won't be able to catch up to this in a competent for a week or two but a
quick comment:
First, this is def a hard problem. It's something I have thought a bit about,
but have not cracked yet.
bq. However, that's exactly what happens automatically
I'm not sure 'exactly' fits. I think it's probably important to let the client
know that an update only made it to one replica for example. That mean's you
are in a position to possibly lose the update. One possibility is to define the
fail the same way as a leader fail in the middle of your update. You don't know
what happened in that case - the doc may be in or not. If we do the same here,
the client will know, hey, this didn't make it to 2 or 3 replicas - it may be
in the cluster, but we can't count on it's durability to the level we
requested. The client can then choose how to handle this - accept what happened
or take another action.
Just spit balling at a conference, but I think there is a way to define the
semantics here so that it's easier on us, but still gives the client the info
they need to understand how durable that update was. This would not be the only
case a fail does not mean the update is for sure not in the cluster - you can't
get around that on fails of the leader in the middle of an update anyway.
> 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.
--
This message was sent by Atlassian JIRA
(v6.2#6252)
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
