amqp-spec

by: Arvicco url: github.com/ruby-amqp/amqp-spec

Summary

Simple API for writing asynchronous EventMachine/AMQP specs. Supports RSpec and RSpec2. Should work under MRI 1.8.7, 1.9.2 (and JRuby 1.6.0 - work in progress!)

Description

EventMachine-based code, including synchronous AMQP library is notoriously difficult to test. To the point that many people recommend using either Mocks or synchronous libraries instead of EM-based libraries in unit tests. This is not always an option, however - sometimes your code just has to run inside the event loop, and you want to test a real thing, not just mocks.

EM-Spec gem made it easier to write evented specs, but it has several drawbacks. First, it is not easy to manage both EM.run and AMQP.start loops at the same time. Second, AMQP is not properly stopped and deactivated upon exceptions and timeouts, resulting in state leak between examples and multiple mystereous failures.

AMQP-Spec is based on EM-Spec code but extends it to support AMQP event loops (as well as RSpec2). API is very similar to EM-Spec’s, only a bit extended. The final goal is to make writing AMQP specs reasonably pleasant experience and dispel the notion that evented libs are impossible to unit-test.

Mind you, you still have to properly manage your AMQP broker in order to prevent broker state from leaking between examples. You can try to combine AMQP-Spec and Moqueue if you want to abstract away actual broker interactions, but still specify some event-based expectations.

Rspec

There are several ways to use amqp-spec. To use it as a helper, include AMQP::SpecHelper in your describe block. You then use either #amqp or #em methods to wrap your evented test code. Inside the amqp/em block, you must call #done after your expectations. Everything works normally otherwise. You can set default_timeout and default_options to avoid manually setting AMQP options for each example. However, if you DO supply options to #amqp method inside the example, they override the defaults.

Default options and default timeout are local for each example group and inherited by its nested groups, unconnected example groups DO NOT share defaults. Please note that this is different from EM-Spec where default_timeout is effectively a global setting.

In order to setup/teardown EM state before/after your examples, you’ll need to use em_before and em_after hooks. These hooks are similar to standard RSpec’s before/after hooks but run inside the EM event loop before/after your example block. If you are using #amqp method, em_before hook will run just BEFORE AMQP connection is attempted, and em_after is run after AMQP is stopped.

Sometimes, you may want to setup/teardown state inside AMQP connection (inside block given to AMQP.start): for example, to make sure that the connection is established before your examples run, or to pre-declare some queues and exchanges common for all examples. In this case, please use amqp_before and amqp_after hooks. These hooks run inside the AMQP.start block just before/after your example block.

require "amqp-spec/rspec"

describe AMQP do
  include AMQP::SpecHelper

  # Can be used to set default options for your amqp{} event loops
  default_options {:host => 'my.amqp.broker.org', :port => '21118'}

  # Can be used to set default :spec_timeout for your evented specs
  default_timeout 1

  # Runs inside the EM event loop before each block given to #amqp or #em
  em_before { @start = Time.now }

  # Runs inside the AMQP.start before each block given to #amqp
  amqp_before do
    @channel = AMQP::Channel.new
    @queue = @channel.queue('my_queue')
  end

  it "works normally when not using #amqp or #em" do
    1.should == 1
  end

  it "makes testing evented code easy with #em" do
    em do
      EM.add_timer(0.5){
        (Time.now - @start).should be_close( 0.5, 0.1 )
        done
      }
    end
  end

  it "runs AMQP.start loop with options given to #amqp" do
    amqp(:host => 'my.amqp.broker.org', :port => '21118')do
      AMQP.conn.should be_connected
      @queue.name.should == 'my_queue'
      done
    end
  end

  it "optionally raises timeout exception if your loop hangs for some reason" do
    proc {
      amqp(:spec_timeout => 0.5) do
      # no done!
      end
    }.should raise_error SpecTimeoutExceededError
  end

end

Another option is to include AMQP::Spec in your describe block. This will patch Rspec so that all of your examples run inside an amqp block automatically. A word of caution about before/after hooks in your example groups including AMQP::Spec. Each of these hooks will run in its separate EM loop that you’ll need to shut down either manually (#done) or via timeout. Essentially, this means that any EM-related state that you’d like to set up or tear down using these hooks will be lost as example itself will run in a different EM loop.

In short, you should avoid using before/after if you include AMQP::Spec - instead, use em_before/em_after or amqp_before/amqp_after hooks that run inside the EM event loop. You don’t need to call #done inside these evented hooks, otherwise you’ll shut down the EM reactor before your example runs.

require 'amqp-spec'

describe AMQP do
  include AMQP::Spec

  default_options {:host => 'my.amqp.broker.org', :port => '21118'}
  default_timeout 1

  em_before { @start = Time.now }

  it "requires a call to #done in every example" do
    1.should == 1
    done
  end

  it "runs test code in an amqp block automatically" do

    EM.add_timer(0.5){
      (Time.now - @start).should be_close( 0.5, 0.1 )
      done
    }
  end

  it "runs AMQP.start loop with default_options" do
    AMQP.conn.should be_connected
    done
  end

  it "raises timeout exception ONLY if default_timeout was set" do
    proc do
      # no done!
      end.should raise_error SpecTimeoutExceededError
  end
end

Finally, you can include AMQP::EMSpec in your describe block. This will run all the group examples inside em block instead of amqp. Non-evented before/after hooks should be finished with #done, same as when including AMQP::Spec, and same caution about using them applies - use em_before/em_after instead.

describe AMQP do
  include AMQP::EMSpec

  it "requires a call to #done in every example" do
    1.should == 1
    done
  end

  it "runs test code in an em block, instead of amqp block" do
    start = Time.now

    AMQP.conn.should be_nil

    EM.add_timer(0.5){
      (Time.now-start).should be_close( 0.5, 0.1 )
      done
    }
  end
end

General notes

For a developer new to evented specs, it is not easy to internalize that the blocks given to asynchronous methods are turned into real callbacks, intended to fire some time later. It is not easy to keep track of the actual execution path of your code, when your blocks are supposed to fire and in what sequence.

Take the following spec as an example:

it 'receives published message' do
  amqp do
    q = MQ.queue('queue_name')
    q.subscribe do |hdr, msg|
      msg.should_not == 'data'
    end
    MQ.queue('queue_name').publish 'data'
    q.unsubscribe
    q.delete
    done
  end
end

Seems like a straightforward spec: you subscribe to a message queue, you set expectations inside your subscribe block, then you publish into this queue, then you call done. What may be wrong with it? Well, if you happen to use this spec against live AMQP broker, everything may be wrong. First, communication delays. There is no guarantee that by the time you publish your message, the queue have been either created or subscribed to. There is also no guarantee that your subscriber received the message by the time you are unsubscribing and deleting your queue. Second, sequence of your blocks. Remember they are delayed callbacks! Don’t just assume your previous block is already executed when you start your new asynchronous action. In this spec, when done is called, it stops everything before your subscribe callback even has a chance to fire. As a result, you’ll get a PASSING spec even though your expectation was never executed!

How to improve this spec? Allow some time for async actions to finish: either use EM timers or pass :nowait=>false to your asynch calls to force them into synchronicity. Keep in mind the sequence in which your callbacks are expected to fire - so place your done call at the end of subscribe block in this example. If you want to be paranoid, you can set flags inside your callbacks and then check that they actually fired at all AFTER your amqp/em block. Something like this will do the trick:

it 'receives published message' do
  amqp do
    q = MQ.queue('queue_name')
    q.subscribe do |hdr, msg|
      @subscribe_fired == true
      msg.should == 'data'
      done {q.unsubscribe; q.delete}
    end
    EM.add_timer(0.2) do
      MQ.queue('queue_name').publish 'data'
    end
  end
  @subscribe_fired.should be_true
end

Legacy EM-Spec based specs support

In order to run your existing EM-Spec based specs unmodified, instead of require ‘em-spec’:

require 'amqp-spec'
require 'amqp-spec/em_spec_shim'

Please note that in amqp-spec default options and default timeout are local for each example group and inherited by its nested groups, unconnected example groups DO NOT share defaults. This is different from EM-Spec where default_timeout is effectively a global setting. So please make sure you’re setting default_timeout early on, in your outermost example group in order to get same behavior as in EM-Spec.

Extending library to spec other types of event loops

AMQP-Spec extends EM-Spec code to cover different types of event loops. Initially, vanilla EM and AMQP event loops are supported, but the lib can be extended quite easily to spec any other type of event loop, not necesarily EM-based (such as Cool.io, ZMQMachine and whatnot). All you need to do is just write appropriate EventedExample subclass (with minimal interface of just #run and #done to start and stop your loop) and RSpec wrapper for it similar to SpecHelper#em or SpecHelper#amqp. You can then call your wrapper in RSpec examples in a way similar to #em or #amqp.

Limitations

AMQP-Spec can be currently used with Rspec only. I suppose, it is not that difficult to extend EM-Spec’s Test::Unit and Bacon support, I just do not have experience doing it.

Any help improving this library is greatly appreciated…

LICENSE:

See LICENSE for details.