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MDEV-4506: Parallel replication

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MDEV-5217: Incorrect MyISAM event execution order causing incorrect parallel replication

In parallel replication, if transactions A,B group-commit together on the
master, we can execute them in parallel on a replication slave. But then, if
transaction C follows on the master, on the slave, we need to be sure that
both A and B have completed before starting on C to be sure to avoid
conflicts.

The necessary wait is implemented such that B waits for A to commit before it
commits itself (thus preserving commit order). And C waits for B to commit
before it itself can start executing. This way C does not start until both A
and B have completed.

The wait for B's commit on A happens inside the commit processing. However, in
the case of MyISAM with no binlog enabled on the slave, it appears that no
commit processing takes place (since MyISAM is non-transactional), and thus
the wait of B for A was not done. This allowed C to start before A, which can
lead to conflicts and incorrect replication.

Fixed by doing an extra wait for A at the end of B before signalling C.
This commit is contained in:
unknown 2013-11-06 14:51:06 +01:00
parent c90f4f0212
commit dcb3650d63
1 changed files with 21 additions and 3 deletions
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24
sql/rpl_parallel.cc
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@ -127,10 +127,28 @@ finish_event_group(THD *thd, int err, uint64 sub_id,
/*
Remove any left-over registration to wait for a prior commit to
complete. Normally, such wait would already have been removed at
this point by wait_for_prior_commit(), but eg. in error case we
might have skipped waiting, so we would need to remove it explicitly.
this point by wait_for_prior_commit() called from within COMMIT
processing. However, in case of MyISAM and no binlog, we might not
have any commit processing, and so we need to do the wait here,
before waking up any subsequent commits, to preserve correct
order of event execution. Also, in the error case we might have
skipped waiting and thus need to remove it explicitly.
It is important in the non-error case to do a wait, not just an
unregister. Because we might be last in a group-commit that is
replicated in parallel, and the following event will then wait
for us to complete and rely on this also ensuring that any other
event in the group has completed.
But in the error case, we have to abort anyway, and it seems best
to just complete as quickly as possible with unregister. Anyone
waiting for us will in any case receive the error back from their
wait_for_prior_commit() call.
*/
wfc->unregister_wait_for_prior_commit();
if (err)
wfc->unregister_wait_for_prior_commit();
else
wfc->wait_for_prior_commit();
thd->wait_for_commit_ptr= NULL;
/*