Ability to specify queue arguments

[etyrrill]

Is it be possible to specify queue arguments for a REQ socket? I need to send a message to a queue that has a dead letter exchange. When I try to connect the socket I get the following error:

Error: Channel closed by server: 406 (PRECONDITION-FAILED) with message "PRECONDITION_FAILED - inequivalent arg 'x-dead-letter-exchange' for queue 'myqueue' in vhost '/': received none but current is the value 'DeadLetterExchange' of type 'longstr'"
at Channel.C.accept (/Users/etyrrill/mgmt-code/ui/build/node_modules/rabbit.js/node_modules/amqplib/lib/channel.js:398:24)

amqplib supports dead letter exchanges, but I couldn't figure out how to pass those parameters into rabbit.js so it can pass the parameters down to amqplib.

[squaremo]

Ah I see, you created the queue with a dead-letter exchange already (through the management UI perhaps?), then because rabbit.js doesn't know this and can't be told, it falls over. What a pain!

I would really like to keep the interface to rabbit.js as simple as possible, which is why I don't expose those kinds of options. But I can see this is a major pain for you. Maybe I could check for the queue first, and leave it alone if it exists already.

As a workaround for now, you could use a policy to set the dead letter exchange for the queue; I've tried this, and it seems that queue attributes that come from policies don't count when it comes to checking for inequivalent args (as the error above has it).

[etyrrill]

Checking for the queue first would be a great solution. I considered making that change instead of exposing the deadLetterExchange option (see pull request #98), but being lazy, I made the easy change instead. The queue is created by the consumer, and will always be there before our node server starts.

I appreciate you taking a look at this.

Thanks,
Ed

[squaremo]

I have pushed a change to master which lets you tell a socket not to try and create objects when connecting; this means you can use exchanges and queues created elsewhere with properties different to those rabbit.js would give them.

var ctx = require('rabbit.js').createContext();
var push = ctx.socket('PUSH', {noCreate: true});
push.connect(existingQueue);

I figure this is a solution to the general problem of using existing objects. Would you let me know if it works for you? If so I will tidy it up and make it a lasting change to the API. Thanks!

[corpulent]

@squaremo how would you subscribe and consume an existing queue?