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From "Mark Miller (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-5468) Option to enforce a majority quorum approach to accepting updates in SolrCloud
Date Tue, 06 May 2014 01:09:17 GMT

    [ https://issues.apache.org/jira/browse/SOLR-5468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13990164#comment-13990164
] 

Mark Miller commented on SOLR-5468:
-----------------------------------

No, hinted hand off is for when a shard is not being served. Less of a interest of mine because
auto replica failover with hdfs should be a much better solution for that. 

> 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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