[rabbitmq-discuss] Work on plugins

Tue Sep 20 10:52:38 BST 2011

On 19/09/11 21:51, Alvaro Videla wrote:
> Something I noticed while playing with custom exchanges for RabbitMQ
> is that if you create them and later you remove the plugin without
> deleting the exchange then RabbitMQ won't start at all because the
> recovery process will fail since it can't find the custom exchange
> module.

Yeah, that's certainly a problem. As you might guess I bumped into this 
several times developing the federation plugin.

> Wouldn't it be nice to have an "uninstall plugin" kind of command?
> Perhaps also in the same way you do -rabbit_boot_step(). declarations
> on your modules, you could add something like a
> -rabbit_uninstall_plugin(). declaration that could be executed to
> remove the plugin. Then it's up to the plugin developer to take care
> at this point of removing Mnesia entries and other stuff related to
> his plugin.

But I'm not sure that this is the solution. What if a plugin gets 
uninstalled temporarily by mistake? That's far too easy to do at the 
moment during an upgrade, and while the changes to the plugin system 
should mitigate that *a lot* the idea that:

# rabbitmq-plugin uninstall rabbitmq_federation
# rabbitmq-plugin install rabbitmq_federation

could delete exchanges and bindings seems pretty alarming to me.

It seems that this is primarily an issue around exchange type plugins. I 
wonder if we could just not recover exchanges that have an unrecognised 
type? The trouble is, then you have a "hidden" exchange that will pop 
back as soon as you reinstall the plugin, but without the plugin you 
can't get rid of it, and attempts to redeclare it with a different type 
will always fail. Hmm.

Cheers, Simon

-- 
Simon MacMullen
RabbitMQ, VMware