# module Puppet::Util::Queue

Implements a message queue client type plugin registry for use by the indirector facility. Client modules for speaking a particular protocol (e.g. Stomp::Client for Stomp message brokers, Memcached for Starling and Sparrow, etc.) register themselves with this module.

Client classes are expected to live under the Puppet::Util::Queue namespace and corresponding directory; the attempted use of a client via its typename (see below) will cause Puppet::Util::Queue to attempt to load the corresponding plugin if it is not yet loaded. The client class registers itself with Puppet::Util::Queue and should use the same type name as the autloader expects for the plugin file.

    class Puppet::Util::Queue::SpecialMagicalClient < Messaging::SpecialMagic
        ...
        Puppet::Util::Queue.register_queue_type_class(self)
    end

This module reduces the rightmost segment of the class name into a pretty symbol that will serve as the queuing client’s name. Which means that the “SpecialMagicalClient” above will be named :special_magical_client within the registry.

Another class/module may mix-in this module, and may then make use of the registered clients.

    class Queue::Fue
        # mix it in at the class object level rather than instance level
        extend ::Puppet::Util::Queue
    end

Queue::Fue instances can get a message queue client through the registry through the mixed-in method `client`, which will return a class-wide singleton client instance, determined by `client_class`.

The client plugins are expected to implement an interface similar to that of Stomp::Client:

- `new` should return a connected, ready-to-go client instance. Note that no arguments are passed in.

- `publish_message(queue, message)` should publish the message to the specified queue.

- `subscribe(queue)` block subscribes to queue and executes block upon receiving a message.

- queue names are simple names independent of the message broker or client library. No "/queue/" prefixes like in Stomp::Client.

### Public Class Methods

queue_type_from_class(klass) click to toggle source

Given a class object klass, returns the programmatic default queue type name symbol for klass. The algorithm is as shown in earlier examples; the last namespace segment of klass.name is taken and converted from mixed case to underscore-separated lowercase, and interned.

    queue_type_from_class(Foo) -> :foo
    queue_type_from_class(Foo::Too) -> :too
    queue_type_from_class(Foo::ForYouTwo) -> :for_you_too

The implicit assumption here, consistent with Puppet’s approach to plugins in general, is that all your client modules live in the same namespace, such that reduction to a flat namespace of symbols is reasonably safe.

    # File lib/puppet/util/queue.rb, line 77
    def self.queue_type_from_class(klass)
      # convert last segment of classname from studly caps to lower case with underscores, and symbolize
      klass.name.split('::').pop.sub(/^[A-Z]/) {|c| c.downcase}.gsub(/[A-Z]/) {|c| '_' + c.downcase }.intern
    end

queue_type_to_class(type) click to toggle source

Given a queue type symbol, returns the associated `Class` object. If the queue type is unknown (meaning it hasn’t been registered with this module), an exception is thrown.

    # File lib/puppet/util/queue.rb, line 61
    def self.queue_type_to_class(type)
      c = loaded_instance :queue_clients, type
      raise Puppet::Error, "Queue type #{type} is unknown." unless c
      c
    end

register_queue_type(klass, type = nil) click to toggle source

Adds a new class/queue-type pair to the registry. The type argument is optional; if not provided, type defaults to a lowercased, underscored symbol programmatically derived from the rightmost namespace of klass.name.

    # register with default name +:you+
    register_queue_type(Foo::You)

    # register with explicit queue type name +:myself+
    register_queue_type(Foo::Me, :myself)

If the type is already registered, an exception is thrown. No checking is performed of klass, however; a given class could be registered any number of times, as long as the type differs with each registration.

    # File lib/puppet/util/queue.rb, line 53
    def self.register_queue_type(klass, type = nil)
      type ||= queue_type_from_class(klass)
      raise Puppet::Error, "Queue type #{type} is already registered" if instance_hash(:queue_clients).include?(type)
      instance_hash(:queue_clients)[type] = klass
    end

### Public Instance Methods

client() click to toggle source

Returns (instantiating as necessary) the singleton queue client instance, according to the client_class. No arguments go to the client class constructor, meaning its up to the client class to know how to determine its queue message source (presumably through Puppet configuration data).

    # File lib/puppet/util/queue.rb, line 93
    def client
      @client ||= client_class.new
    end

client_class() click to toggle source

The class object for the client to be used, determined by queue configuration settings. Looks to the `:queue_type` configuration entry in the running application for the default queue type to use.

    # File lib/puppet/util/queue.rb, line 86
    def client_class
      Puppet::Util::Queue.queue_type_to_class(Puppet[:queue_type])
    end