### Home Classes Methods

### In Files

- lib/amqp/queue.rb

### Parent

AMQ::Client::Queue

### Methods

- ::add_default_options
- ::consumer_class
- ::new
- #auto_recover
- #bind
- #callback
- #consumer_tag
- #default_consumer
- #delete
- #handle_connection_interruption
- #handle_declare_ok
- #once_declared
- #once_name_is_available
- #pop
- #publish
- #purge
- #rebind
- #reset
- #server_named?
- #status
- #subscribe
- #subscribed?
- #unbind
- #unsubscribe

### Class/Module Index

Quicksearch

- AMQP
- AMQP::Broker
- AMQP::Channel
- AMQP::ChannelClosedError
- AMQP::Client
- AMQP::Consumer
- AMQP::Error
- AMQP::Exchange
- AMQP::Header
- AMQP::IncompatibleOptionsError
- AMQP::Integration
- AMQP::Integration::Rails
- AMQP::PossibleAuthenticationFailureError
- AMQP::Queue
- AMQP::Session
- AMQP::TCPConnectionFailed
- AMQP::Utilities
- AMQP::Utilities::EventLoopHelper
- AMQP::Utilities::ServerType

No matching classes.

# AMQP::Queue

h2. What are AMQP queues?

Queues store and forward messages to consumers. They are similar to mailboxes in SMTP. Messages flow from producing applications to {Exchange exchanges} that route them to queues and finally queues deliver them to consumer applications (or consumer applications fetch messages as needed).

Note that unlike some other messaging protocols/systems, messages are not delivered directly to queues. They are delivered to exchanges that route messages to queues using rules knows as bindings.

h2. Concept of bindings

Binding is an association between a queue and an exchange. Queues must be bound to at least one exchange in order to receive messages from publishers. Learn more about bindings in {Exchange Exchange class documentation}.

h2. Key methods

Key methods of Queue class are

- {Queue#bind}

- {Queue#subscribe}

- {Queue#pop}

- {Queue#delete}

- {Queue#purge}

- {Queue#unbind}

h2. Queue names. Server-named queues. Predefined queues.

Every queue has a name that identifies it. Queue names often contain several segments separated by a dot (.), similarly to how URI path segments are separated by a slash (/), although it may be almost any string, with some limitations (see below). Applications may pick queue names or ask broker to generate a name for them. To do so, pass *empty string* as queue name argument.

Here is an example:

<script src="</script">gist.github.com/939596.js?file=gistfile1.rb"></script</a>>

If you want to declare a queue with a particular name, for example, "images.resize", pass it to Queue class constructor:

<script src="</script">gist.github.com/939600.js?file=gistfile1.rb"></script</a>>

Queue names starting with 'amq.' are reserved for internal use by the broker. Attempts to declare queue with a name that violates this rule will result in AMQP::IncompatibleOptionsError to be thrown (when queue is re-declared on the same channel object) or channel-level exception (when originally queue was declared on one channel and re-declaration with different attributes happens on another channel). Learn more in {file:docs/Queues.textile Queues guide} and {file:docs/ErrorHandling.textile Error Handling guide}.

h2. Queue life-cycles. When use of server-named queues is optimal and when it isn't.

To quote AMQP 0.9.1 spec, there are two common message queue life-cycles:

    * Durable message queues that are shared by many consumers and have an independent existence: i.e. they
      will continue to exist and collect messages whether or not there are consumers to receive them.
    * Temporary message queues that are private to one consumer and are tied to that consumer. When the
      consumer disconnects, the message queue is deleted.

There are some variations on these, such as shared message queues that are deleted when the last of many consumers disconnects.

One example of durable message queues is well-known services like event collectors (event loggers). They are usually up whether there are services to log anything or not. Other applications know what queues they use and can rely on those queues being around all the time, survive broker restarts and in general be available should an application in the network need to use them. In this case, explicitly named durable queues are optimal and coupling it creates between applications is not an issue. Another scenario of a well-known long-lived service is distributed metadata/directory/locking server like Apache Zookeeper, Google's Chubby or DNS. Services like this benefit from using well-known, not generated queue names, and so do other applications that use them.

Different scenario is in "a cloud settings" when some kind of workers/instances may come online and go down basically any time and other applications cannot rely on them being available. Using well-known queue names in this case is possible but server-generated, short-lived queues that are bound to topic or fanout exchanges to receive relevant messages is a better idea.

Imagine a service that processes an endless stream of events (Twitter is one example). When traffic goes up, development operations may spin up additional applications instances in the cloud to handle the load. Those new instances want to subscribe to receive messages to process but the rest of the system doesn't know anything about them, rely on them being online or try to address them directly: they process events from a shared stream and are not different from their peers. In a case like this, there is no reason for message consumers to not use queue names generated by the broker.

In general, use of explicitly named or server-named queues depends on messaging pattern your application needs. {www.eaipatterns.com/ Enterprise Integration Patters} discusses many messaging patterns in depth. RabbitMQ FAQ also has a section on {www.rabbitmq.com/faq.html#scenarios use cases}.

h2. Queue durability and persistence of messages.

Learn more in our {rubyamqp.info/articles/durability/}.

h2. Message ordering

RabbitMQ FAQ explains {www.rabbitmq.com/faq.html#message-ordering ordering of messages in AMQP queues}

h2. Error handling

When channel-level error occurs, queues associated with that channel are reset: internal state and callbacks are cleared. Recommended strategy is to open a new channel and re-declare all the entities you need. Learn more in {file:docs/ErrorHandling.textile Error Handling guide}.

@note Please make sure you read {rubyamqp.info/articles/durability/} that covers exchanges durability vs. messages

    persistence.

@see files.travis-ci.org/docs/amqp/0.9.1/AMQP091Specification.pdf AMQP 0.9.1 specification (Section 2.1.1) @see AMQP::Exchange

### Attributes

name[R]

Name of this queue

opts[RW]

Options this queue object was instantiated with

### Public Class Methods

consumer_class() click to toggle source

@return [Class] @private

    # File lib/amqp/queue.rb, line 763
    def self.consumer_class
      AMQP::Consumer
    end

new(channel, name = AMQ::Protocol::EMPTY_STRING, opts = {}, &block) click to toggle source

@option opts [Boolean] :passive (false) If set, the server will not create the queue if it does not

    already exist. The client can use this to check whether the queue
    exists without modifying  the server state.

@option opts [Boolean] :durable (false) If set when creating a new queue, the queue will be marked as

    durable.  Durable queues remain active when a server restarts.
    Non-durable queues (transient queues) are purged if/when a
    server restarts.  Note that durable queues do not necessarily
    hold persistent messages, although it does not make sense to
    send persistent messages to a transient queue (though it is
    allowed).

@option opts [Boolean] :exclusive (false) Exclusive queues may only be consumed from by the current connection.

    Setting the 'exclusive' flag always implies 'auto-delete'. Only a
    single consumer is allowed to remove messages from this queue.
    The default is a shared queue. Multiple clients may consume messages
    from this queue.

@option opts [Boolean] :auto_delete (false) If set, the queue is deleted when all consumers have finished

    using it. Last consumer can be cancelled either explicitly or because
    its channel is closed. If there was no consumer ever on the queue, it
    won't be deleted.

@option opts [Boolean] :nowait (true) If set, the server will not respond to the method. The client should

    not wait for a reply method.  If the server could not complete the
    method it will raise a channel or connection exception.

@option opts [Hash] :arguments (nil) A hash of optional arguments with the declaration. Some brokers implement

    AMQP extensions using x-prefixed declaration arguments. For example, RabbitMQ
    recognizes x-message-ttl declaration arguments that defines TTL of messages in
    the queue.

@yield [queue, declare_ok] Yields successfully declared queue instance and AMQP method (queue.declare-ok) instance. The latter is optional. @yieldparam [Queue] queue Queue that is successfully declared and is ready to be used. @yieldparam [AMQP::Protocol::Queue::DeclareOk] declare_ok AMQP queue.declare-ok) instance.

@api public

    # File lib/amqp/queue.rb, line 172
    def initialize(channel, name = AMQ::Protocol::EMPTY_STRING, opts = {}, &block)
      raise ArgumentError.new("queue name must not be nil; if you want broker to generate queue name for you, pass an empty string") if name.nil?

      @channel  = channel
      @name     = name unless name.empty?
      @server_named = name.empty?
      @opts         = self.class.add_default_options(name, opts, block)

      raise ArgumentError.new("server-named queues (name = '') declaration with :nowait => true makes no sense. If you are not sure what that means, simply drop :nowait => true from opts.") if @server_named && @opts[:nowait]

      # a deferrable that we use to delay operations until this queue is actually declared.
      # one reason for this is to support a case when a server-named queue is immediately bound.
      # it's crazy, but 0.7.x supports it, so... MK.
      @declaration_deferrable = AMQ::Client::EventMachineClient::Deferrable.new

      super(channel.connection, channel, name)

      shim = Proc.new do |q, declare_ok|
        case block.arity
        when 1 then block.call(q)
        else
          block.call(q, declare_ok)
        end
      end

      @channel.once_open do
        if @opts[:nowait]
          @declaration_deferrable.succeed
          block.call(self) if block
        end

        if block
          self.declare(@opts[:passive], @opts[:durable], @opts[:exclusive], @opts[:auto_delete], @opts[:nowait], @opts[:arguments], &shim)
        else
          # we cannot pass :nowait as true here, AMQ::Client::Queue will (rightfully) raise an exception because
          # it has no idea about crazy edge cases we are trying to support for sake of backwards compatibility. MK.
          self.declare(@opts[:passive], @opts[:durable], @opts[:exclusive], @opts[:auto_delete], false, @opts[:arguments])
        end
      end
    end

### Protected Class Methods

add_default_options(name, opts, block) click to toggle source

@private

    # File lib/amqp/queue.rb, line 896
    def self.add_default_options(name, opts, block)
      { :queue => name, :nowait => (block.nil? && !name.empty?) }.merge(opts)
    end

### Public Instance Methods

auto_recover() click to toggle source

Called by associated connection object when AMQP connection has been re-established (for example, after a network failure).

@api plugin

    # File lib/amqp/queue.rb, line 305
    def auto_recover
      self.exec_callback_yielding_self(:before_recovery)

      if self.server_named?
        old_name = @name.dup
        @name    = AMQ::Protocol::EMPTY_STRING

        @channel.queues.delete(old_name)
      end

      self.redeclare do
        self.rebind

        @consumers.each { |tag, consumer| consumer.auto_recover }

        self.exec_callback_yielding_self(:after_recovery)
      end
    end

bind(exchange, opts = {}, &block) click to toggle source

This method binds a queue to an exchange. Until a queue is bound it will not receive any messages. In a classic messaging model, store-and-forward queues are bound to a dest exchange and subscription queues are bound to a dest_wild exchange.

A valid exchange name (or reference) must be passed as the first parameter. @example Binding a queue to exchange using AMQP::Exchange instance

    ch       = AMQP::Channel.new(connection)
    exchange = ch.direct('backlog.events')
    queue    = ch.queue('', :exclusive => true)
    queue.bind(exchange)

@example Binding a queue to exchange using exchange name

    ch       = AMQP::Channel.new(connection)
    queue    = ch.queue('', :exclusive => true)
    queue.bind('backlog.events')

Note that if your producer application knows consumer queue name and wants to deliver a message there, direct exchange may be sufficient (in other words, if your code declares an exchange with the same name as a queue and binds it to that queue, consider using the default exchange and routing key on publishing).

@param [Exchange] Exchange to bind to. May also be a string or any object that responds to name.

@option opts [String] :routing_key Specifies the routing key for the binding. The routing key is

    used for routing messages depending on the exchange configuration.
    Not all exchanges use a routing key! Refer to the specific
    exchange documentation.  If the routing key is empty and the queue
    name is empty, the routing key will be the current queue for the
    channel, which is the last declared queue.

@option opts [Hash] :arguments (nil) A hash of optional arguments with the declaration. Headers exchange type uses these metadata

    attributes for routing matching.
    In addition, brokers may implement AMQP extensions using x-prefixed declaration arguments.

@option opts [Boolean] :nowait (true) If set, the server will not respond to the method. The client should

    not wait for a reply method.  If the server could not complete the
    method it will raise a channel or connection exception.

@return [Queue] Self

@yield [] Since queue.bind-ok carries no attributes, no parameters are yielded to the block.

@api public @see Queue#unbind

    # File lib/amqp/queue.rb, line 281
    def bind(exchange, opts = {}, &block)
      @channel.once_open do
        self.once_name_is_available do
          super(exchange, (opts[:key] || opts[:routing_key] || AMQ::Protocol::EMPTY_STRING), (opts[:nowait] || block.nil?), opts[:arguments], &block)
        end
      end

      self
    end

callback() click to toggle source

Compatibility alias for on_declare.

@api public @deprecated

    # File lib/amqp/queue.rb, line 853
    def callback
      return nil if !subscribed?

      @default_consumer.callback
    end

consumer_tag() click to toggle source

@return [String] Consumer tag of the default consumer associated with this queue (if any), or nil @note Default consumer is the one registered with the convenience {AMQP::Queue#subscribe} method. It has no special properties of any kind. @see Queue#subscribe @see AMQP::Consumer @api public

    # File lib/amqp/queue.rb, line 743
    def consumer_tag
      if @default_consumer
        @default_consumer.consumer_tag
      else
        nil
      end
    end

default_consumer() click to toggle source

@return [AMQP::Consumer] Default consumer associated with this queue (if any), or nil @note Default consumer is the one registered with the convenience {AMQP::Queue#subscribe} method. It has no special properties of any kind. @see Queue#subscribe @see AMQP::Consumer @api public

    # File lib/amqp/queue.rb, line 756
    def default_consumer
      @default_consumer
    end

delete(opts = {}, &block) click to toggle source

This method deletes a queue. When a queue is deleted any pending messages are sent to a dead-letter queue if this is defined in the server configuration, and all consumers on the queue are cancelled.

@return [NilClass] nil (for v0.7 compatibility)

@option opts [Boolean] :if_unused (false) If set, the server will only delete the queue if it has no

    consumers. If the queue has consumers the server does does not
    delete it but raises a channel exception instead.

@option opts [Boolean] :if_empty (false) If set, the server will only delete the queue if it has no

    messages. If the queue is not empty the server raises a channel
    exception.

@option opts [Boolean] :nowait (false) If set, the server will not respond to the method. The client should

    not wait for a reply method.  If the server could not complete the
    method it will raise a channel or connection exception.

@return [NilClass] nil (for v0.7 compatibility)

@yield [delete_ok] Yields AMQP method (queue.delete-ok) instance. @yieldparam [AMQP::Protocol::Queue::DeleteOk] delete_ok AMQP queue.delete-ok) instance. Carries number of messages that were in the queue.

@api public @see Queue#purge @see Queue#unbind

    # File lib/amqp/queue.rb, line 386
    def delete(opts = {}, &block)
      @channel.once_open do
        self.once_name_is_available do
          super(opts.fetch(:if_unused, false), opts.fetch(:if_empty, false), opts.fetch(:nowait, false), &block)
        end
      end

      # backwards compatibility
      nil
    end

handle_connection_interruption(method = nil) click to toggle source

@private @api plugin

    # File lib/amqp/queue.rb, line 881
    def handle_connection_interruption(method = nil)
      super(method)

      @declaration_deferrable = EventMachine::DefaultDeferrable.new
    end

handle_declare_ok(method) click to toggle source

    # File lib/amqp/queue.rb, line 887
    def handle_declare_ok(method)
      super(method)
      @declaration_deferrable.succeed
    end

once_declared(&block) click to toggle source

Defines a callback that will be executed once queue is declared. More than one callback can be defined. if queue is already declared, given callback is executed immediately.

@api public

    # File lib/amqp/queue.rb, line 217
    def once_declared(&block)
      @declaration_deferrable.callback do
        # guards against cases when deferred operations
        # don't complete before the channel is closed
        block.call if @channel.open?
      end
    end

pop(opts = {}, &block) click to toggle source

This method provides a direct access to the messages in a queue using a synchronous dialogue that is designed for specific types of application where synchronous functionality is more important than performance.

If queue is empty, `payload` callback argument will be nil, otherwise arguments are identical to those of {AMQP::Queue#subscribe} callback.

@example Fetching messages off AMQP queue on demand

    queue.pop do |metadata, payload|
      if payload
        puts "Fetched a message: #{payload.inspect}, content_type: #{metadata.content_type}. Shutting down..."
      else
        puts "No messages in the queue"
      end
    end

@option opts [Boolean] :ack (false) If this field is set to false the server does not expect acknowledgments

    for messages.  That is, when a message is delivered to the client
    the server automatically and silently acknowledges it on behalf
    of the client.  This functionality increases performance but at
    the cost of reliability.  Messages can get lost if a client dies
    before it can deliver them to the application.

@return [Qeueue] Self

@yield [headers, payload] When block only takes one argument, yields payload to it. In case of two arguments, yields headers and payload. @yieldparam [AMQP::Header] headers Headers (metadata) associated with this message (for example, routing key). @yieldparam [String] payload Message body (content). On Ruby 1.9, you may want to check or enforce content encoding.

@api public

    # File lib/amqp/queue.rb, line 459
    def pop(opts = {}, &block)
      if block
        # We have to maintain this multiple arities jazz
        # because older versions this gem are used in examples in at least 3
        # books published by O'Reilly :(. MK.
        shim = Proc.new { |method, headers, payload|
          case block.arity
          when 1 then
            block.call(payload)
          when 2 then
            h = Header.new(@channel, method, headers ? headers.decode_payload : nil)
            block.call(h, payload)
          else
            h = Header.new(@channel, method, headers ? headers.decode_payload : nil)
            block.call(h, payload, method.delivery_tag, method.redelivered, method.exchange, method.routing_key)
          end
        }

        @channel.once_open do
          self.once_name_is_available do
            # see AMQ::Client::Queue#get in amq-client
            self.get(!opts.fetch(:ack, false), &shim)
          end
        end
      else
        @channel.once_open do
          self.once_name_is_available do
            self.get(!opts.fetch(:ack, false))
          end
        end
      end
    end

publish(data, opts = {}) click to toggle source

Don't use this method. It is a leftover from very early days and it ruins the whole point of exchanges/queue separation.

@note This method will be removed before 1.0 release @deprecated @api public

    # File lib/amqp/queue.rb, line 868
    def publish(data, opts = {})
      exchange.publish(data, opts.merge(:routing_key => self.name))
    end

purge(opts = {}, &block) click to toggle source

This method removes all messages from a queue which are not awaiting acknowledgment.

@option opts [Boolean] :nowait (false) If set, the server will not respond to the method. The client should

    not wait for a reply method.  If the server could not complete the
    method it will raise a channel or connection exception.

@return [NilClass] nil (for v0.7 compatibility)

@yield [purge_ok] Yields AMQP method (queue.purge-ok) instance. @yieldparam [AMQP::Protocol::Queue::PurgeOk] purge_ok AMQP queue.purge-ok) instance. Carries number of messages that were purged.

@api public @see Queue#delete @see Queue#unbind

    # File lib/amqp/queue.rb, line 413
    def purge(opts = {}, &block)
      @channel.once_open do
        self.once_declared do
          super(opts.fetch(:nowait, false), &block)
        end
      end

      # backwards compatibility
      nil
    end

rebind(&block) click to toggle source

Used by automatic recovery machinery. @private @api plugin

    # File lib/amqp/queue.rb, line 297
    def rebind(&block)
      @bindings.each { |b| self.bind(b[:exchange], b) }
    end

reset() click to toggle source

Resets queue state. Useful for error handling. @api plugin

    # File lib/amqp/queue.rb, line 874
    def reset
      initialize(@channel, @name, @opts)
    end

server_named?() click to toggle source

@return [Boolean] true if this queue is server-named

    # File lib/amqp/queue.rb, line 227
    def server_named?
      @server_named
    end

status(opts = {}, &block) click to toggle source

Get the number of messages and active consumers (with active channel flow) on a queue.

@example Getting number of messages and active consumers for a queue

    AMQP::Channel.queue('name').status { |number_of_messages, number_of_active_consumers|
      puts number_of_messages
    }

@yield [number_of_messages, number_of_active_consumers] @yieldparam [Fixnum] number_of_messages Number of messages in the queue @yieldparam [Fixnum] number_of_active_consumers Number of active consumers for the queue. Note that consumers can suspend activity (Channel.Flow) in which case they do not appear in this count.

@api public

    # File lib/amqp/queue.rb, line 815
    def status(opts = {}, &block)
      raise ArgumentError, "AMQP::Queue#status does not make any sense without a block" unless block

      shim = Proc.new { |q, declare_ok| block.call(declare_ok.message_count, declare_ok.consumer_count) }

      @channel.once_open do
        self.once_name_is_available do
          # we do not use self.declare here to avoid caching of @passive since that will cause unexpected side-effects during automatic
          # recovery process. MK.
          @connection.send_frame(AMQ::Protocol::Queue::Declare.encode(@channel.id, @name, true, @opts[:durable], @opts[:exclusive], @opts[:auto_delete], false, @opts[:arguments]))

          self.append_callback(:declare, &shim)
          @channel.queues_awaiting_declare_ok.push(self)
        end
      end

      self
    end

subscribe(opts = {}, &block) click to toggle source

Subscribes to asynchronous message delivery.

The provided block is passed a single message each time the exchange matches a message to this queue.

Attempts to {Queue#subscribe} multiple times to the same exchange will raise an Exception. If you need more than one consumer per queue, use {AMQP::Consumer} instead. {file:docs/Queues.textile Documentation guide on queues} explains this and other topics in great detail.

@example Use of callback with a single argument

    EventMachine.run do
      exchange = AMQP::Channel.direct("foo queue")
      EM.add_periodic_timer(1) do
        exchange.publish("random number #{rand(1000)}")
      end

      queue = AMQP::Channel.queue('foo queue')
      queue.subscribe { |body| puts "received payload [#{body}]" }
    end

If the block takes 2 parameters, both the header and the body will be passed in for processing.

@example Use of callback with two arguments

    EventMachine.run do
      connection = AMQP.connect(:host => '127.0.0.1')
      puts "Connected to AMQP broker. Running #{AMQP::VERSION} version of the gem..."

      channel  = AMQP::Channel.new(connection)
      queue    = channel.queue("amqpgem.examples.hello_world", :auto_delete => true)
      exchange = channel.direct("amq.direct")

      queue.bind(exchange)

      channel.on_error do |ch, channel_close|
        puts channel_close.reply_text
        connection.close { EventMachine.stop }
      end

      queue.subscribe do |metadata, payload|
        puts "metadata.routing_key : #{metadata.routing_key}"
        puts "metadata.content_type: #{metadata.content_type}"
        puts "metadata.priority    : #{metadata.priority}"
        puts "metadata.headers     : #{metadata.headers.inspect}"
        puts "metadata.timestamp   : #{metadata.timestamp.inspect}"
        puts "metadata.type        : #{metadata.type}"
        puts "metadata.delivery_tag: #{metadata.delivery_tag}"
        puts "metadata.redelivered : #{metadata.redelivered}"

        puts "metadata.app_id      : #{metadata.app_id}"
        puts "metadata.exchange    : #{metadata.exchange}"
        puts
        puts "Received a message: #{payload}. Disconnecting..."

        connection.close {
          EventMachine.stop { exit }
        }
      end

      exchange.publish("Hello, world!",
                       :app_id      => "amqpgem.example",
                       :priority    => 8,
                       :type        => "kinda.checkin",
                       # headers table keys can be anything
                       :headers     => {
                         :coordinates => {
                           :latitude  => 59.35,
                           :longitude => 18.066667
                         },
                         :participants => 11,
                         :venue        => "Stockholm"
                       },
                       :timestamp   => Time.now.to_i)
    end

@example Using object as consumer (message handler), take one

    class Consumer

      #
      # API
      #

      def initialize(channel, queue_name = AMQ::Protocol::EMPTY_STRING)
        @queue_name = queue_name

        @channel    = channel
        # Consumer#handle_channel_exception will handle channel
        # exceptions. Keep in mind that you can only register one error handler,
        # so the last one registered "wins".
        @channel.on_error(&method(:handle_channel_exception))
      end # initialize

      def start
        @queue = @channel.queue(@queue_name, :exclusive => true)
        # #handle_message method will be handling messages routed to @queue
        @queue.subscribe(&method(:handle_message))
      end # start

      #
      # Implementation
      #

      def handle_message(metadata, payload)
        puts "Received a message: #{payload}, content_type = #{metadata.content_type}"
      end # handle_message(metadata, payload)

      def handle_channel_exception(channel, channel_close)
        puts "Oops... a channel-level exception: code = #{channel_close.reply_code}, message = #{channel_close.reply_text}"
      end # handle_channel_exception(channel, channel_close)
    end

@example Using object as consumer (message handler), take two: aggregatied handler

    class Consumer

      #
      # API
      #

      def handle_message(metadata, payload)
        puts "Received a message: #{payload}, content_type = #{metadata.content_type}"
      end # handle_message(metadata, payload)
    end

    class Worker

      #
      # API
      #

      def initialize(channel, queue_name = AMQ::Protocol::EMPTY_STRING, consumer = Consumer.new)
        @queue_name = queue_name

        @channel    = channel
        @channel.on_error(&method(:handle_channel_exception))

        @consumer   = consumer
      end # initialize

      def start
        @queue = @channel.queue(@queue_name, :exclusive => true)
        @queue.subscribe(&@consumer.method(:handle_message))
      end # start

      #
      # Implementation
      #

      def handle_channel_exception(channel, channel_close)
        puts "Oops... a channel-level exception: code = #{channel_close.reply_code}, message = #{channel_close.reply_text}"
      end # handle_channel_exception(channel, channel_close)
    end

@example Unit-testing objects that are used as consumers, RSpec style

    require "ostruct"
    require "json"

    # RSpec example
    describe Consumer do
      describe "when a new message arrives" do
        subject { described_class.new }

        let(:metadata) do
          o = OpenStruct.new

          o.content_type = "application/json"
          o
        end
        let(:payload)  { JSON.encode({ :command => "reload_config" }) }

        it "does some useful work" do
          # check preconditions here if necessary

          subject.handle_message(metadata, payload)

          # add your code expectations here
        end
      end
    end

@option opts [Boolean ]:ack (false) If this field is set to false the server does not expect acknowledgments

    for messages.  That is, when a message is delivered to the client
    the server automatically and silently acknowledges it on behalf
    of the client.  This functionality increases performance but at
    the cost of reliability.  Messages can get lost if a client dies
    before it can deliver them to the application.

@option opts [Boolean] :nowait (false) If set, the server will not respond to the method. The client should

    not wait for a reply method.  If the server could not complete the
    method it will raise a channel or connection exception.

@option opts [call] :confirm (nil) If set, this proc will be called when the server confirms subscription

    to the queue with a basic.consume-ok message. Setting this option will
    automatically set :nowait => false. This is required for the server
    to send a confirmation.

@option opts [Boolean] :exclusive (false) Request exclusive consumer access, meaning only this consumer can access the queue.

    This is useful when you want a long-lived shared queue to be temporarily accessible by just
    one application (or thread, or process). If application exclusive consumer is part of crashes
    or loses network connection to the broker, channel is closed and exclusive consumer is thus cancelled.

@yield [headers, payload] When block only takes one argument, yields payload to it. In case of two arguments, yields headers and payload. @yieldparam [AMQP::Header] headers Headers (metadata) associated with this message (for example, routing key). @yieldparam [String] payload Message body (content). On Ruby 1.9, you may want to check or enforce content encoding.

@return [Queue] Self @api public

@see file:docs/Queues.textile Documentation guide on queues @see unsubscribe @see AMQP::Consumer

    # File lib/amqp/queue.rb, line 720
    def subscribe(opts = {}, &block)
      raise RuntimeError.new("This queue already has default consumer. Please instantiate AMQP::Consumer directly to register additional consumers.") if @default_consumer

      opts[:nowait] = false if (@on_confirm_subscribe = opts[:confirm])

      @channel.once_open do
        self.once_name_is_available do
          # guards against a pathological case race condition when a channel
          # is opened and closed before delayed operations are completed.
          self.consume(!opts[:ack], opts[:exclusive], (opts[:nowait] || block.nil?), opts[:no_local], nil, &opts[:confirm])

          self.on_delivery(&block)
        end
      end

      self
    end

subscribed?() click to toggle source

Boolean check to see if the current queue has already subscribed to messages delivery (has default consumer).

Attempts to {Queue#subscribe} multiple times to the same exchange will raise an Exception. If you need more than one consumer per queue, use {AMQP::Consumer} instead.

@return [Boolean] true if there is a consumer tag associated with this Queue instance @api public @deprecated

    # File lib/amqp/queue.rb, line 844
    def subscribed?
      @default_consumer && @default_consumer.subscribed?
    end

unbind(exchange, opts = {}, &block) click to toggle source

Remove the binding between the queue and exchange. The queue will not receive any more messages until it is bound to another exchange.

Due to the asynchronous nature of the protocol, it is possible for "in flight" messages to be received after this call completes. Those messages will be serviced by the last block used in a {Queue#subscribe} or {Queue#pop} call.

@param [Exchange] Exchange to unbind from. @option opts [String] :routing_key Binding routing key @option opts [Hash] :arguments Binding arguments @option opts [Boolean] :nowait (true) If set, the server will not respond to the method. The client should

    not wait for a reply method.  If the server could not complete the
    method it will raise a channel or connection exception.

@yield [] Since queue.unbind-ok carries no attributes, no parameters are yielded to the block.

@api public @see Queue#bind

    # File lib/amqp/queue.rb, line 350
    def unbind(exchange, opts = {}, &block)
      @channel.once_open do
        self.once_name_is_available do
          super(exchange, (opts[:key] || opts[:routing_key] || AMQ::Protocol::EMPTY_STRING), opts[:arguments], &block)
        end
      end
    end

unsubscribe(opts = {}, &block) click to toggle source

Removes the subscription from the queue and cancels the consumer. Once consumer is cancelled, messages will no longer be delivered to it, however, due to the asynchronous nature of the protocol, it is possible for “in flight” messages to be received after this call completes. Those messages will be serviced by the last block used in a {Queue#subscribe} or {Queue#pop} call.

Fetching messages with {AMQP::Queue#pop} is still possible even after consumer is cancelled.

Additionally, if the queue was created with autodelete set to true, the server will delete the queue after its wait period has expired unless the queue is bound to an active exchange.

The method accepts a block which will be executed when the unsubscription request is acknowledged as complete by the server.

@option opts [Boolean] :nowait (true) If set, the server will not respond to the method. The client should

    not wait for a reply method, the callback (if passed) will be ignored. If the server could not complete the
    method it will raise a channel or connection exception.

@yield [cancel_ok] @yieldparam [AMQP::Protocol::Basic::CancelOk] cancel_ok AMQP method basic.cancel-ok. You can obtain consumer tag from it.

@api public

    # File lib/amqp/queue.rb, line 792
    def unsubscribe(opts = {}, &block)
      @channel.once_open do
        self.once_name_is_available do
          if @default_consumer
            @default_consumer.cancel(opts.fetch(:nowait, true), &block); @default_consumer = nil
          end
        end
      end
    end

### Protected Instance Methods

once_name_is_available(&block) click to toggle source

    # File lib/amqp/queue.rb, line 900
    def once_name_is_available(&block)
      if server_named?
        self.once_declared do
          block.call
        end
      else
        block.call
      end
    end

[Validate]

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