[Zope3-dev] Re: rdb: Disappearing Connection

[Jim Washington]

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.

And, I've thought about a cron-like utility to ping the connection, but 
now that I mention threads, that might be a non-solution if a broken 
connection is in a different thread than the one contacted.

-Jim Washington