Wonky Bunny Issue “Fixed”

For those who don’t know what the headline means:

  1. Bunny is an open source command line utility written in Python that provides a shell for talking to and testing AMQP brokers (tested on RabbitMQ).
  2. AMQP is a queuing protocol. It’s defined as a binary wire-level protocol as well as a command set. The spec also defines a good portion of the server semantics, so by that logic Bunny should work against other AMQP brokers besides RabbitMQ
  3. RabbitMQ is written in Erlang atop OTP, so clustering is ‘free and easy’. My experience with RabbitMQ so far has been fantastic, though I’d like to see client libraries in general mature a bit further.

So, Bunny had this really odd quirk upon its first release. If you did something to cause an error that resulted in a connection being dropped, bunny wouldn’t trap the error. It would patiently wait for you to enter the next command, and fail miserably. The kicker is that I actually defined a ‘check_conn’ method to make sure that the connection was alive before doing anything else, and that really wasn’t working.

The reason is because py-amqplib (or, perhaps, its interpretation of the AMQP spec, which defines a Connection class), implements a high-level Connection class, along with a Channel class (also defined in the spec), which is what seems to actually map to what you and I as users actually care about: some “thing” that lets us communicate with the server, and without which we can’t talk to the server.

With py-amqplib, a Connection is actually defined as a channel 0, and always channel 0. I gather that channel 0 gets some special treatment in other sections of the library code, and the object that lives at index ‘0’ in Connection.channels is actually defined as a Connection object, whereas others are Channel objects.

The result of all of this is that creating a channel in my code and then checking my own object’s ‘chan’ attribute is useless because channels can be dropped on the floor in py-amqplib, and the only way I can tell to figure that out is to check the connection object’s ‘channels’ dictionary. So that’s what I do now. It seems to be working well.

Not only does bunny now figure out that your connection is gone, but it’ll also attempt a reconnect using the credentials you gave it in the last ‘connect’ command. You see, bunny extends the Python built-in cmd.Cmd object, which lets me define my whole program as a single class. That means that whatever you type in, like the credentials to the ‘connect’ command, can be kept handy, since the lifetime of the instance of this class is the same as the lifetime of a bunny session.

So, in summary, bunny is more useful now, but it’s still not “done”. I made this fix over the weekend during an hour I unexpectedly found for myself. It’s “a” solution, but it’s not “the” solution. The real solution is to map out all of the errors that actually cause a connection to drop and give the user a bit more feedback about what happened. I also want to add more features (like support for getting some stats back from Alice to replace bunny’s really weak ‘qlist’ command).