gocept.amqprun 0.5

Basic concepts and terms

• A message handler is a function which is bound with a routing key to exactly one queue. It is called for each message on that queue, and may return a list of messages as a result.

• The result messages of one handled message are sent in one transaction together with the ACK of the handled message.

• When an exception is raised during message processing, the transaction is aborted. (The received message would be NACKed if RabbitMQ was supporting it.)

• A message handler handles exactly one message at a time. Multiple messages can be processed at the same time using threads. Those threads are called workers.

Things you don’t need to take care of

• Threading of multiple workers

• Socket handling and locking for communicating with the AMQP broker

• Transaction handling

• Message Ids

  • Each outgoing message gets a email-like message id.

  • The corrlation id of outoing message is set to the message id of the incomming message.

  • Each outgoing message gets a custom references header which is set to the incoming message’s reference header plus the incoming message’s message id.

Getting started: receiving messages

To get started define a function which does the work. In this case, we log the message body and send a message. The declare decorator takes two arguments, the queue name and the routing key (you can also pass in a list if you want to bind the function to multiple routing keys). The declare decorator also supports an optional arguments argument as a dictionary that is passed to the AMQP queue_declare call e.g to support mirrored queues on RabbitMQ:

import logging
import gocept.amqprun.handler
import gocept.amqprun.message

log = logging.getLogger(__name__)

@gocept.amqprun.handler.declare('test.queue', 'test.routing')
def log_message_body(message):
    log.info(message.body)
    msg = gocept.amqprun.message.Message(
        header=dict(content_type='text/plain'),
        body=u'Thank you for your message.',
        routing_key='test.thank.messages')
    return [msg]

The handler function needs to be registered as a named utility. With ZCML this looks like this[1]_:

<configure xmlns="http://namespaces.zope.org/zope">
  <include package="gocept.amqprun" />
  <utility component="path.to.package.log_message_body" name="basic" />
</configure>

To set up a server, it’s recommended to create a buildout. The following buildout creates a config for gocept.amqprun, a ZCML file for the component configuration, and uses ZDaemon to daemonize the process:

[buildout]
parts =
        config
        zcml
        app
        server

[deployment]
name = queue
recipe = gocept.recipe.deploymentsandbox
root = ${buildout:directory}

[config]
recipe = lovely.recipe:mkfile
path = ${deployment:etc-directory}/queue.conf

amqp-hostname = localhost
amqp-username = guest
amqp-password = guest
amqp-virtualhost = /

eventlog =
    <eventlog>
      level DEBUG
      <logfile>
        formatter zope.exceptions.log.Formatter
        path STDOUT
      </logfile>
    </eventlog>
amqp-server =
    <amqp-server>
      hostname ${:amqp-hostname}
      username ${:amqp-username}
      password ${:amqp-password}
      virtual_host ${:amqp-virtualhost}
    </amqp-server>

content =
    ${:eventlog}
    ${:amqp-server}
    <worker>
      amount 10
      component-configuration ${zcml:path}
    </worker>
    <settings>
      your.custom.settings here
    </settings>

[zcml]
recipe = lovely.recipe:mkfile
path = ${deployment:etc-directory}/queue.zcml
content =
    <configure xmlns="http://namespaces.zope.org/zope">
      <include package="gocept.amqprun" />
      <include package="your.package" />
    </configure>

[app]
recipe = zc.recipe.egg:script
eggs =
   gocept.amqprun
   your.package
   zope.exceptions
arguments = '${config:path}'
scripts = server=app

[server]
recipe = zc.zdaemonrecipe
deployment = deployment
program = ${buildout:bin-directory}/app

Sending messages

If all you want to do is send messages, you don’t have to register any handlers, but can use gocept.amqprun.server.Server.send() directly. While the handlers usually run in their own process, started by the server entrypoint (as described above), if you’re just sending messages, you can also skip the extra process and run the gocept.amqprun.server.Server in your original process, in its own thread. Here is some example code to do that:

def start_server(**kw):
    parameters = gocept.amqprun.connection.Parameters(**kw)
    server = gocept.amqprun.server.Server(parameters)
    server_thread = threading.Thread(target=server.start)
    server_thread.daemon = True
    server_thread.start()
    import time
    time.sleep(0.1)
    return server

(When you’re using the ZCA, you’ll probably want to register the Server as a utility at that point, too, so clients can get to it and send messages easily).

Settings

For application specific settings gocept.amqprun makes the <settings> section from the configuration available via an ISettings utility:

settings = zope.component.getUtility(
    gocept.amqprun.interfaces.ISettings)
settings.get('your.settings.key')

Limitations

• Currently all messages are send and received through the amq.topic exchange. Other exchanges are not supported at the moment.

Interfacing with the filesystem

gocept.amqprun provides a quick way to set up a handler that writes incoming messages as individual files to a given directory, using the <amqp:writefiles> ZCML directive. You need the writefiles extra to enable this directive:

<configure xmlns="http://namespaces.zope.org/zope"
           xmlns:amqp="http://namespaces.gocept.com/amqp">

  <include package="gocept.amqprun" file="meta.zcml" />

  <amqp:writefiles
    routing_key="test.data"
    queue_name="test.queue"
    directory="/path/to/output-directory"
    />
</configure>

All messages with routing key ‘test.data’ would then be written to ‘output-directory’, two files per message, one containing the body and the other containing the headers (in zope.xmlpickle format). (Note that in the buildout example above, you would need to put the writefiles directive into the [zcml] section, not the [config] section.)

You can specify multiple routing keys separated by spaces:

<amqp:writefiles
  routing_key="test.foo test.bar"
  queue_name="test.queue"
  directory="/path/to/output-directory"
  />

You can configure the way files are named with the pattern parameter, for example:

<amqp:writefiles
  routing_key="test.data"
  queue_name="test.queue"
  directory="/path/to/output-directory"
  pattern="${routing_key}/${date}/${msgid}-${unique}.xml"
  />

pattern performs as string.Template substitution. The following variables are available:

date:

The date the message arrived, formatted %Y-%m-%d

msgid:

The value of the message-id header

routing_key:

The routing key of the message

unique:

A token that guarantees the filename will be unique in its directory

The default value for pattern is ${routing_key}-${unique}.

To support zc.buildout, {variable} is accepted as an alternative syntax to ${variable}. (zc.buildout uses ${} for its own substitutions, but unfortunately does not support escaping them.)

If pattern contains slashes, intermediate directories will be created below directory, so in the example, messages would be stored like this:

/path/to/output-directory/example.route/2011-04-07/asdf998-1234098791.xml

The <amqp:writefiles> ZCML directive also supports an optional arguments parameter just like the declare decorator that is passed to the AMQP queue_declare call e.g to support RabbitMQ mirrored queues:

<amqp:writefiles
  routing_key="test.foo test.bar"
  queue_name="test.queue"
  directory="/path/to/output-directory"
  arguments="
  x-ha-policy = all
  "
  />

Development

The source code is available in the mercurial repository at https://code.gocept.com/hg/public/gocept.amqprun

Please report any bugs you find at https://projects.gocept.com/projects/projects/gocept-amqprun/issues

CHANGES