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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-2870) dynamic snitch + read repair off can cause LOCAL_QUORUM reads to return spurious UnavailableException
Date Thu, 07 Jul 2011 19:36:17 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-2870?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Jonathan Ellis updated CASSANDRA-2870:
--------------------------------------

    Attachment: 2870.txt

patch extracts the "give me the minimum set of endpoints necessary" code into preferredEndpoints(),
and makes it DC-aware for LOCAL_QUORUM reads.

> dynamic snitch + read repair off can cause LOCAL_QUORUM reads to return spurious UnavailableException
> -----------------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-2870
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-2870
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 0.7.0
>            Reporter: Jonathan Ellis
>            Assignee: Jonathan Ellis
>            Priority: Minor
>             Fix For: 0.7.8, 0.8.2
>
>         Attachments: 2870.txt
>
>
> When Read Repair is off, we want to avoid doing requests to more nodes than necessary
to satisfy the ConsistencyLevel.  ReadCallback does this here:
> {code}
>         this.endpoints = repair || resolver instanceof RowRepairResolver
>                        ? endpoints
>                        : endpoints.subList(0, Math.min(endpoints.size(), blockfor));
// min so as to not throw exception until assureSufficient is called
> {code}
> You can see that it is assuming that the "endpoints" list is sorted in order of preferred-ness
for the read.
> Then the LOCAL_QUORUM code in DatacenterReadCallback checks to see if we have enough
nodes to do the read:
> {code}
>         int localEndpoints = 0;
>         for (InetAddress endpoint : endpoints)
>         {
>             if (localdc.equals(snitch.getDatacenter(endpoint)))
>                 localEndpoints++;
>         }
>         if (localEndpoints < blockfor)
>             throw new UnavailableException();
> {code}
> So if repair is off (so we truncate our endpoints list) AND dynamic snitch has decided
that nodes in another DC are to be preferred over local ones, we'll throw UE even if all the
replicas are healthy.

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