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From "Stefania (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-14554) LifecycleTransaction encounters ConcurrentModificationException when used in multi-threaded context
Date Fri, 09 Nov 2018 03:18:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-14554?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16680787#comment-16680787
] 

Stefania commented on CASSANDRA-14554:
--------------------------------------

bq.  The other methods on a LifecycleTransaction could simply be marked synchronised as well.

If we are synchronizing the LifecycleTransaction methods anyway, I'm not sure I understand
why we need child transactions. Even in 4.0, where Netty threads call {{trackNew}}, I don't
think we're adding sstables so frequently to introduce contention on a shared, synchronized
txn. Considering {{trackNew}} performs a file sync as you correctly reminded me, surely this
blocks Netty threads more than a synchronized {{trackNew}}. Maybe if many sstables are created
concurrently during streaming, child transactions would make sense. I'm still not totally
sure.

It's fine with me if we prefer to try a different alternative, the patch is available at any
time. This code is not changing much so there is little risk of the patch getting stale. For
info, internally [~snazy] already reviewed the patch.


> LifecycleTransaction encounters ConcurrentModificationException when used in multi-threaded
context
> ---------------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-14554
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-14554
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Dinesh Joshi
>            Assignee: Dinesh Joshi
>            Priority: Major
>
> When LifecycleTransaction is used in a multi-threaded context, we encounter this exception
-
> {quote}java.util.ConcurrentModificationException: null
>  at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:719)
>  at java.util.LinkedHashMap$LinkedKeyIterator.next(LinkedHashMap.java:742)
>  at java.lang.Iterable.forEach(Iterable.java:74)
>  at org.apache.cassandra.db.lifecycle.LogReplicaSet.maybeCreateReplica(LogReplicaSet.java:78)
>  at org.apache.cassandra.db.lifecycle.LogFile.makeRecord(LogFile.java:320)
>  at org.apache.cassandra.db.lifecycle.LogFile.add(LogFile.java:285)
>  at org.apache.cassandra.db.lifecycle.LogTransaction.trackNew(LogTransaction.java:136)
>  at org.apache.cassandra.db.lifecycle.LifecycleTransaction.trackNew(LifecycleTransaction.java:529)
> {quote}
> During streaming we create a reference to a {{LifeCycleTransaction}} and share it between
threads -
> [https://github.com/apache/cassandra/blob/5cc68a87359dd02412bdb70a52dfcd718d44a5ba/src/java/org/apache/cassandra/db/streaming/CassandraStreamReader.java#L156]
> This is used in a multi-threaded context insideĀ {{CassandraIncomingFile}} which is anĀ {{IncomingStreamMessage}}.
This is being deserialized in parallel.
> {{LifecycleTransaction}} is not meant to be used in a multi-threaded context and this
leads to streaming failures due to object sharing. On trunk, this object is shared across
all threads that transfer sstables in parallel for the given {{TableId}} in a {{StreamSession}}.
There are two options to solve this - make {{LifecycleTransaction}} and the associated objects
thread safe, scope the transaction to a single {{CassandraIncomingFile}}. The consequences
of the latter option is that if we experience streaming failure we may have redundant SSTables
on disk. This is ok as compaction should clean this up. A third option is we synchronize access
in the streaming infrastructure.



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