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From "Alex Petrov (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-10786) Include hash of result set metadata in prepared statement id
Date Tue, 17 May 2016 10:37:12 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-10786?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15286345#comment-15286345
] 

Alex Petrov edited comment on CASSANDRA-10786 at 5/17/16 10:37 AM:
-------------------------------------------------------------------

I have a patch to fix the described behaviour. In order to verify whether or not the patch
works, I had to also come up with at some patch for the client that would work.

After adding result metadata to md5, as the message metadata on the client is never updated
and new client ID of the re-prepared message is not saved anywhere, client would fall into
the infinite re-prepare loop: 
  * (1) try running the query
  * fail with {{UNPREPARED}}
  * re-prepare (although without saving the new ID)
  * go to (1)

I first tried to transparently swap the request when getting {{UNPREPARED}}, although that
would mean that the original "outdated" prepared query would re-prepare no matter how many
times it runs (even though results it returns would be correct). On the other hand, making
changes in the {{BoundStatement}} itself (or in it's {{PreparedStatement}}) might lead to
undesired behaviour on the client-side or even be prone to races (when request is sent assuming
one id and metadata and came back after it got swapped).

I put the failing test on ignore for now until we decide what could should be done on the
driver side in this case.

I've created a corresponding [java driver issue|https://datastax-oss.atlassian.net/browse/JAVA-1196]


was (Author: ifesdjeen):
I have a patch to fix the described behaviour. In order to verify whether or not the patch
works, I had to also come up with at some patch for the client that would work.

After adding result metadata to md5, as the message metadata on the client is never updated
and new client ID of the re-prepared message is not saved anywhere, client would fall into
the infinite re-prepare loop: 
  * (1) try running the query
  * fail with {{UNPREPARED}}
  * re-prepare (although without saving the new ID)
  * go to (1)

I first tried to transparently swap the request when getting {{UNPREPARED}}, although that
would mean that the original "outdated" prepared query would re-prepare no matter how many
times it runs (even though results it returns would be correct). On the other hand, making
changes in the {{BoundStatement}} itself (or in it's {{PreparedStatement}}) might lead to
undesired behaviour on the client-side or even be prone to races (when request is sent assuming
one id and metadata and came back after it got swapped).

I put the failing test on ignore for now until we decide what could should be done on the
driver side in this case.

> Include hash of result set metadata in prepared statement id
> ------------------------------------------------------------
>
>                 Key: CASSANDRA-10786
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-10786
>             Project: Cassandra
>          Issue Type: Bug
>          Components: CQL
>            Reporter: Olivier Michallat
>            Assignee: Alex Petrov
>            Priority: Minor
>              Labels: client-impacting
>             Fix For: 3.x
>
>
> This is a follow-up to CASSANDRA-7910, which was about invalidating a prepared statement
when the table is altered, to force clients to update their local copy of the metadata.
> There's still an issue if multiple clients are connected to the same host. The first
client to execute the query after the cache was invalidated will receive an UNPREPARED response,
re-prepare, and update its local metadata. But other clients might miss it entirely (the MD5
hasn't changed), and they will keep using their old metadata. For example:
> # {{SELECT * ...}} statement is prepared in Cassandra with md5 abc123, clientA and clientB
both have a cache of the metadata (columns b and c) locally
> # column a gets added to the table, C* invalidates its cache entry
> # clientA sends an EXECUTE request for md5 abc123, gets UNPREPARED response, re-prepares
on the fly and updates its local metadata to (a, b, c)
> # prepared statement is now in C*’s cache again, with the same md5 abc123
> # clientB sends an EXECUTE request for id abc123. Because the cache has been populated
again, the query succeeds. But clientB still has not updated its metadata, it’s still (b,c)
> One solution that was suggested is to include a hash of the result set metadata in the
md5. This way the md5 would change at step 3, and any client using the old md5 would get an
UNPREPARED, regardless of whether another client already reprepared.



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