[Zope3-dev] Re: rdb: Disappearing Connection

[Jim Washington]

Jim Washington wrote:

> Florent Guillaume wrote:
>
>> Jim Washington wrote:
>>
>>> Spelunking a bit in the code, 
>>> zope.app.rdb.ZopeDatabaseAdapter.isConnected() looks at whether 
>>> _v_connection is present, not whether the connection is actually 
>>> alive.  If we fix this here, isConnected() perhaps should handle the 
>>> case where the connection might be reaped by the back end db.
>>
>>
>>
>> Frankly anything that uses _v_ attributes is asking for trouble 
>> because it relies on low level behaviour of the ZODB and persistence 
>> mechanisms. They can disappear at unpredictable times. Avoid them.
>>
> Hi, Florent
>
> In this case, it is not particulary a problem if the _v_ attribute is 
> not there.  It gets recreated if it does not exist.  It is designed 
> this way so that different zope threads get separate rdb connections, 
> which is good.  The problem here is false positives, where the _v_ 
> attribute exists, but the connection it promises has been silently 
> expired and disconnected by the MySQL server.
>
> I was thinking of a check in isConnected() by making a cursor then 
> "try"ing executing "select 1" and return True if it does not raise the 
> exception.  But that would be bad because it ~doubles the number of 
> requests to the SQL server. Perhaps not smart.
>
Now, looking closer at the code, a ping like this might be not too bad, 
because isConnected() is only called when a connection is requested, not 
for every SQL statement executed.  So, it might not be so onerous as 
originally thought. Still not ideal, though.

-Jim Washington