On Sep 23, 2005, at 3:19 PM, Tim Bunce wrote:
I'm interested to hear stories of people who have subclassed the DBI,
or used modules that subclass the DBI.
Was subclassing the DBI a good approach?
Where there any problems?
Could it have been done well/better some other way?
Thanks!
Tim.
I subclassed DBI for DBIx::HA, a high-availability module.
I subclassed connect(), prepare() and execute().
The new connect() takes as input a stack of DSNs with its associated
meta info (timeouts, type of failover, etc...) and determines which
DSN to connect to based on availability and inter-process communication.
The prepare() and execute() are subclassed to trap errors and kick
back to reconnect, then reprepare and reexecute into another DSN.
Who is using it:
We're internally using it very heavily. I know of one other external
person/group using the module.
What went well:
It solved many problems, one of which was to be able to use Sybase
OpenSwitch (a load balancer/failover proxy for Sybase ASE) with DBI.
Other problems solved were the ability to automatically failover in
case of hard database failure, and an easy failback mechanism that
doesn't necessitate an Apache restart under mod_perl.
Subclassing the DBI looked like an easy approach at first, and I was
able to get most needed functionality quickly and easily.
Problems:
There were many painful issues to deal with: signal handling,
trapping of errors, making sure the database behaves properly,
remembering the state of the query, etc...
At this point I can't guarantee even that I've nailed all possible
race conditions. It's extremely difficult to debug all the use cases.
Could it have been done a better way?
Absolutely. I think that this whole high availability layer should be
at the driver level as a proxy driver, ultimately part of
DBD::Multiplex.
In fact DBIx::HA couldn't have worked at all without the added
functionality in DBI of swap_inner_handle(), but even with that I ran
into a wall where I didn't have enough control over the row-level
info that a proper DBD driver would have.
