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From "Elias Levy (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-4502) Cassandra connector documentation has misleading consistency guarantees
Date Fri, 26 Aug 2016 17:37:20 GMT

    [ https://issues.apache.org/jira/browse/FLINK-4502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15439445#comment-15439445
] 

Elias Levy commented on FLINK-4502:
-----------------------------------

Chesnay, thanks for correcting me about the overwriting issue.  I missed the implication of
setting the timestamp on the queries.

I still think the documentation is not clear that idempotent queries are a prerequisite of
exactly-once semantics when the WAL is enabled, as shown by the portion of the documentation
I quoted.  The connector documentation page mentions idempotent once, in the paragraph previous
to the one describing the WAL.

The WAL functionality description could also be clearer.  For instance, is says "The write-ahead
log guarantees that the replayed checkpoint is identical to the first attempt."  But that
does not seem accurate.  The WAL doesn't appear to guarantee that a replayed checkpoint is
identical.  Rather, it guarantees that no output associated with a checkpoint is written to
the sink until the checkpoint is complete, which avoids possibly incompatible duplicate outputs
when data from non-completed checkpoints are replayed on recovery.


> Cassandra connector documentation has misleading consistency guarantees
> -----------------------------------------------------------------------
>
>                 Key: FLINK-4502
>                 URL: https://issues.apache.org/jira/browse/FLINK-4502
>             Project: Flink
>          Issue Type: Bug
>          Components: Cassandra Connector, Documentation
>    Affects Versions: 1.1.0
>            Reporter: Elias Levy
>
> The Cassandra connector documentation states that  "enableWriteAheadLog() is an optional
method, that allows exactly-once processing for non-deterministic algorithms."  This claim
appears to be false.
> From what I gather, the write ahead log feature of the connector works as follows:
> - The sink is replaced with a stateful operator that writes incoming messages to the
state backend based on checkpoint they belong in.
> - When the operator is notified that a Flink checkpoint has been completed it, for each
set of checkpoints older than and including the committed one:
>   * reads its messages from the state backend
>   * writes them to Cassandra
>   * records that it has committed them to Cassandra for the specific checkpoint and operator
instance
>    * and erases them from the state backend.
> This process attempts to avoid resubmitting queries to Cassandra that would otherwise
occur when recovering a job from a checkpoint and having messages replayed.
> Alas, this does not guarantee exactly once semantics at the sink.  The writes to Cassandra
that occur when the operator is notified that checkpoint is completed are not atomic and they
are potentially non-idempotent.  If the job dies while writing to Cassandra or before committing
the checkpoint via committer, queries will be replayed when the job recovers.  Thus the documentation
appear to be incorrect in stating this provides exactly-once semantics.
> There also seems to be an issue in GenericWriteAheadSink's notifyOfCompletedCheckpoint
which may result in incorrect output.  If sendValues returns false because a write failed,
instead of bailing, it simply moves on to the next checkpoint to commit if there is one, keeping
the previous one around to try again later.  But that can result in newer data being overwritten
with older data when the previous checkpoint is retried.  Although given that CassandraCommitter
implements isCheckpointCommitted as checkpointID <= this.lastCommittedCheckpointID, it
actually means that when it goes back to try the uncommitted older checkpoint it will consider
it committed, even though some of its data may not have been written out, and the data will
be discarded.



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