dynamic snitch + read repair off can cause LOCAL_QUORUM reads to return
spurious UnavailableException
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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
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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