[Dbix-class] lost connection during query

[Brandon Black]

On 10/30/07, J. G. Konrad <konradjg at gmail.com> wrote:
> I was wondering if anyone had experience with this type of error, "DBI
> Exception: DBD::mysql::st execute failed: Lost connection to MySQL server
> during query" while using DBIx::Class ?
>
> our MySQL server was getting swamped with too many connections so I changed
> the wait_timeout to 30 seconds rather then the default of 28800 seconds.  In
> previous post that I read there is reconnecting behavior and this should
> work.  That is, if the connection is gone ( timed  out ) , DBIx::Class will
> reconnect. Am I hitting some kind of race condition with the reconnection
> behavior?
>
> I am using  version 0.08003  DBIx::Class.
>
>
> any thoughts / comments are appreciated.
>

The reconnect behavior of DBIx::Class was intended for when you get
disconnected in-between statements.  I'm not familiar with this issue
of MySQL disconnecting a query that's in-progress.  I would hope that
means the query has no side-effects (in the case of a txn, or even an
isolated update/insert/delete via AutoCommit).

That being said, the way the reconnect code currently operates is that
after a DBI exception it checks to see whether the connection is still
alive.  If it isn't, it assumes that's the cause of the exception and
then reconnects and retries the statement or transaction.  However, it
will only do this exactly once per statement/txn attempt on your part.
 If it's happening twice in a row (quite possible in the scenario
you're describing), it won't save you on the second failure in a row
of the same statement/txn.

The logic behind the "only reconnect+retry once per statement/txn"
idea is that we want to protect from things like disconnects due to
idle sessions or rebooted servers, but we don't want to retry
indefinitely if we keep getting disconnected (because that probably
means we're triggering a bug and killing the connect (or server) with
our statement, or something even worse, that needs to be addressed
manually).

You definitely should upgrade DBIx::Class though, there were related
bugfixes (in general - perhaps not specific enough to your case) since
0.08003 IIRC.

-- Brandon