Unnecessary ReadRepair request during RangeScan
------------------------------------------------
Key: CASSANDRA-3843
URL: https://issues.apache.org/jira/browse/CASSANDRA-3843
Project: Cassandra
Issue Type: Bug
Components: Core
Affects Versions: 1.0.0
Reporter: Philip Andronov
Priority: Critical
During reading with Quorum level and replication factor greater then 2,
Cassandra sends at least one ReadRepair, even if there is no need to do that.
With the fact that read requests await until ReadRepair will finish it slows
down requsts a lot, up to the Timeout :(
It seems that the problem has been introduced by the CASSANDRA-2494,
unfortunately I have no enought knowledge of Cassandra internals to fix the
problem and do not broke CASSANDRA-2494 functionality, so my report without a
patch.
Code explanations:
class RangeSliceResponseResolver {
// ....
private class Reducer extends MergeIterator.Reducer<Pair<Row,InetAddress>,
Row>
{
// ....
protected Row getReduced()
{
ColumnFamily resolved = versions.size() > 1
? RowRepairResolver.resolveSuperset(versions)
: versions.get(0);
if (versions.size() < sources.size())
{
for (InetAddress source : sources)
{
if (!versionSources.contains(source))
{
// [PA] Here we are adding null ColumnFamily.
// later it will be compared with the "desired"
// version and will give us "fake" difference which
// forces Cassandra to send ReadRepair to a given source
versions.add(null);
versionSources.add(source);
}
}
}
// ....
if (resolved != null)
repairResults.addAll(RowRepairResolver.scheduleRepairs(resolved, table, key,
versions, versionSources));
// ....
}
}
}
2. public class RowRepairResolver extends AbstractRowResolver {
// ....
public static List<IAsyncResult> scheduleRepairs(ColumnFamily resolved,
String table, DecoratedKey<?> key, List<ColumnFamily> versions,
List<InetAddress> endpoints)
{
List<IAsyncResult> results = new
ArrayList<IAsyncResult>(versions.size());
for (int i = 0; i < versions.size(); i++)
{
// Sooner or later we have to compare null and resolved which are
obviously
// not equals, so it will fire a ReadRequest, however it is not
needed here
ColumnFamily diffCf = ColumnFamily.diff(versions.get(i), resolved);
if (diffCf == null)
continue;
// ....
Imagine the following situation:
NodeA has X.1 // row X with the version 1
NodeB has X.2
NodeC has X.? // Unknown version, but because write was with Quorum it is 1 or 2
During the Quorum read from nodes A and B, Cassandra creates version 12 and
send ReadRepair, so now nodes has the following content:
NodeA has X.12
NodeB has X.12
which is correct, however Cassandra also will fire ReadRepair to NodeC. There
is no need to do that, the next consistent read have a chance to be served by
nodes A/B (no ReadRepair) or by any pair with node C, but in that case
ReadRepair will be fire which will brings nodeC to the consistent state
If you are reading from the Index then sooner or later you will get
TimeOutException because cluster is overloaded by the ReadRepairRequests *even*
if all nodes has the same data :(
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