# @title Ruby AMQP gem: Working with exchanges

h1. Working with exchanges

h2. This Documentation Has Moved to rubyamqp.info

amqp gem documentation guides are now hosted on "rubyamqp.info":http://rubyamqp.info.


h2. About this guide

This guide covers the use of exchanges according to the AMQP v0.9.1 specification including message publishing,
common usage scenarios and how to accomplish typical operations using the Ruby amqp gem. This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by/3.0/">Creative Commons Attribution 3.0 Unported License</a> (including images & stylesheets). The source is available "on Github":https://github.com/ruby-amqp/amqp/tree/master/docs.


h2. Which versions of the amqp gem does this guide cover?

This guide covers v0.8.0 and later of the "Ruby amqp gem":http://github.com/ruby-amqp/amqp.


h2. Exchanges in AMQP v0.9.1 - overview

h3. What are AMQP exchanges?

An _exchange_ accepts messages from a producer application and routes them to message queues. They can be thought of
as the "mailboxes" of the AMQP world. Unlike some other messaging middleware products and protocols, in AMQP,
messages are *not* published directly to queues. Messages are published to exchanges that route them to queue(s)
using pre-arranged criteria called _bindings_.

There are multiple exchange types in the AMQP v0.9.1 specification, each with its own routing semantics. Custom
exchange types can be created to deal with sophisticated routing scenarios (e.g. routing based on geolocation data
or edge cases) or just for convenience.


h3. Concept of bindings

A _binding_ is an association between a queue and an exchange. A queue must be bound to at least one exchange in
order to receive messages from publishers. Learn more about bindings in the {file:docs/Bindings.textile Bindings guide}.


h3. Exchange attributes

Exchanges have several attributes associated with them:

 * Name
 * Type (direct, fanout, topic, headers or some custom type)
 * Durability
 * Whether the exchange is auto-deleted when no longer used
 * Other metadata (sometimes known as _X-arguments_)


h2. Exchange types

There are 4 built-in exchange types in AMQP v0.9.1:

 * Direct
 * Fanout
 * Topic
 * Headers

As stated previously, each exchange type has its own routing semantics and new exchange types can be added by
extending brokers with plugins. Custom exchange types begin with "x-", much like custom HTTP headers, e.g.
"x-recent-history exchange":https://github.com/videlalvaro/rabbitmq-recent-history-exchange or
"x-random exchange":https://github.com/jbrisbin/random-exchange.


h2. Message attributes

Before we start looking at various exchange types and their routing semantics, we need to introduce message
attributes. Every AMQP message has a number of *attributes*. Some attributes are important and used very often,
others are rarely used. AMQP message attributes are metadata and are similar in purpose to HTTP request and
response headers.

Every AMQP v0.9.1 message has an attribute called *routing key*. The routing key is an "address" that the exchange
may use to decide how to route the message . This is similar to, but more generic than, a URL in HTTP. Most exchange
types use the routing key to implement routing logic, but some ignore it and use other criteria (e.g. message
content).


h2. Fanout exchanges

h3. How fanout exchanges route messages

A fanout exchange routes messages to all of the queues that are bound to it and the routing key is ignored. If N
queues are bound to a fanout exchange, when a new message is published to that exchange a *copy of the message* is
delivered to all N queues. Fanout exchanges are ideal for the
"broadcast routing":http://en.wikipedia.org/wiki/Broadcasting_%28computing%29 of messages.

Graphically this can be represented as:

!https://github.com/ruby-amqp/amqp/raw/master/docs/diagrams/004_fanout_exchange.png!


h3. Declaring a fanout exchange

There are two ways to declare a fanout exchange:

 * By instantiating an {AMQP::Exchange} and specifying the type as ":fanout"
 * By using the {AMQP::Channel#fanout} method

Here are two examples to demonstrate:

<pre>
<code>
exchange = AMQP::Exchange.new(channel, :fanout, "nodes.metadata")
</code>
</pre>

<pre>
<code>
exchange = channel.fanout("nodes.metadata")
</code>
</pre>

Both methods asynchronously declare a queue. Because the declaration necessitates a network round-trip, publishing
operations on {AMQP::Exchange} instances are delayed until the broker reply (`exchange.declare-ok`) is received.

Also, both methods let you pass a block to run a piece of code when the broker responds with an `exchange.declare-ok`
(meaning that the exchange has been successfully declared).

<pre>
<code>
channel.fanout("nodes.metadata") do |exchange|
  # exchange is declared and ready to be used.
end
</code>
</pre>

h3. Fanout routing example

To demonstrate fanout routing behavior we can declare 10 server-named exclusive queues, bind them all to one
fanout exchange and then publish a message to the exchange:

<pre>
<code>
exchange = channel.topic("amqpgem.examples.routing.fanout_routing", :auto_delete => true)

10.times do
  q = channel.queue("", :exclusive => true, :auto_delete => true).bind(exchange)
  q.subscribe do |payload|
    puts "Queue #{q.name} received #{payload}"
  end
end
</code>
</pre>

<pre>
<code>
# Publish some test data after all queues are declared and bound
EventMachine.add_timer(1.2) { exchange.publish "Hello, fanout exchanges world!" }
</code>
</pre>

When run, this example produces the following output:

<pre>
Queue amq.gen-0p/BjxGNCue42RcJhpUrdg== received Hello, fanout exchanges world!
Queue amq.gen-3GXULvZuYh1KsOD83yvlNg== received Hello, fanout exchanges world!
Queue amq.gen-4EcyydTfoZzXjNSSLsh09Q== received Hello, fanout exchanges world!
Queue amq.gen-B1isyTpR5svB6ClQ2TQEBQ== received Hello, fanout exchanges world!
Queue amq.gen-FwLLioB7Mk4LGA4yJ1Mo7A== received Hello, fanout exchanges world!
Queue amq.gen-OtBQokiA/DmNkB5bPzaRig== received Hello, fanout exchanges world!
Queue amq.gen-RYHQUrj3yihb0DRF7KVpRg== received Hello, fanout exchanges world!
Queue amq.gen-SZJ40mGwbhdcbOGeHMhUkg== received Hello, fanout exchanges world!
Queue amq.gen-sDeVZg9Vx1knq+n9EMi8tA== received Hello, fanout exchanges world!
Queue amq.gen-uWOuVaosW4bWAHqKG6pZVw== received Hello, fanout exchanges world!
</pre>

Each of the queues bound to the exchange receives a *copy* of the message.


Full example:
<script src="https://gist.github.com/1020181.js"> </script>


h3. Fanout use cases

Because a fanout exchange delivers a copy of a message to every queue bound to it, its use cases are quite similar:

 * Massively multiplayer online (MMO) games can use it for leaderboard updates or other global events
 * Sport news sites can use fanout exchanges for distributing score updates to mobile clients in near real-time
 * Distributed systems can broadcast various state and configuration updates
 * Group chats can distribute messages between participants using a fanout exchange (although AMQP does not have a built-in concept of presence, so "XMPP":http://xmpp.org may be a better choice)


h3. Pre-declared fanout exchanges

AMQP v0.9.1 brokers must implement a fanout exchange type and pre-declare one instance with the name of "amq.fanout".
Applications can rely on that exchange always being available to them. Each vhost has a separate instance of that exchange, it is *not shared across vhosts* for obvious reasons.


h2. Direct exchanges

h3. How direct exchanges route messages

A direct exchange delivers messages to queues based on a *message routing key*, an attribute that every AMQP v0.9.1
message contains.

Here is how it works:

 * A queue binds to the exchange with a routing key K
 * When a new message with routing key R arrives at the direct exchange, the exchange routes it to the queue if K = R

A direct exchange is ideal for the "unicast routing":http://en.wikipedia.org/wiki/Unicast of messages (although they
can be used for "multicast routing":http://en.wikipedia.org/wiki/Multicast as well).

Here is a graphical representation:

!https://github.com/ruby-amqp/amqp/raw/master/docs/diagrams/005_direct_exchange.png!


h3. Declaring a direct exchange

There are two ways to declare a direct exchange:

 * By instantiating a {AMQP::Exchange} and specifying its type as ":direct"
 * By using the {AMQP::Channel#direct} method

Here are two examples to demonstrate:

<pre>
<code>
exchange = AMQP::Exchange.new(channel, :direct, "nodes.metadata")
</code>
</pre>

<pre>
<code>
exchange = channel.direct("nodes.metadata")
</code>
</pre>

Both methods asynchronously declare an exchange named "nodes.metadata". Because the declaration necessitates a network round trip, publishing
operations on {AMQP::Exchange} instances are delayed until a broker reply (`exchange.declare-ok`) is received.

Also, both methods let you pass a block to run a piece of code when the broker responds with `exchange.declare-ok`
(meaning that the exchange has been successfully declared).

<pre>
<code>
channel.direct("pages.content.extraction") do |exchange|
  # exchange is declared and ready to be used.
end
</code>
</pre>


h3. Direct routing example

Since direct exchanges use the *message routing key* for routing, message producers need to specify it:

<pre>
<code>
exchange.publish("Hello, direct exchanges world!", :routing_key => "amqpgem.examples.queues.shared")
</code>
</pre>

The routing key will then be compared for equality with routing keys on bindings, and consumers that subscribed with
the same routing key each get a copy of the message:

Full example:
<script src="https://gist.github.com/1041787.js"> </script>


h3. Direct exchanges and load balancing of messages

Direct exchanges are often used to distribute tasks between multiple workers (instances of the same application) in
a round robin manner. When doing so, it is important to understand that, in AMQP v0.9.1, *messages are load balanced
between consumers and not between queues*.

The Ruby amqp gem historically has a limitation that only one consumer (message handler) is allowed per {AMQP::Queue}
instance, however, this limitation will be addressed in the future. With the amqp gem v0.8.x, if you want to load
balance messages between multiple consumers in the same application/OS process, then you need to use a separate
channel for each of the consumers.

The {file:docs/Queues.textile Working With Queues} and {file:docs/PatternsAndUseCases.textile Patterns and Use Cases}
guides provide more information on this subject.


h3. Pre-declared direct exchanges

AMQP v0.9.1 brokers must implement a direct exchange type and pre-declare two instances:

 * *amq.direct*
 * *""* exchange known as *default exchange* (unnamed, referred to as an empty string by many clients including amqp Ruby gem)

Applications can rely on those exchanges always being available to them. Each vhost has separate instances of those
exchanges, they are *not shared across vhosts* for obvious reasons.


h3. Default exchange

The default exchange is a direct exchange with no name (the amqp gem refers to it using an empty string) pre-declared
by the broker. It has one special property that makes it very useful for simple applications, namely that *every
queue is automatically bound to it with a routing key which is the same as the queue name*.

For example, when you declare a queue with the name of "search.indexing.online", the AMQP broker will bind it to the
default exchange using "search.indexing.online" as the routing key. Therefore a message published to the default
exchange with routing key = "search.indexing.online" will be routed to the queue "search.indexing.online". In other
words, the default exchange makes it *seem like it is possible to deliver messages directly to queues*, even though
that is not technically what is happening.


The amqp gem offers two ways of obtaining a reference to the default exchange:

 * Using the {AMQP::Channel#default_exchange} method
 * Using the {AMQP::Channel#direct} method with an empty string as the exchange name

{AMQP::Exchange#initialize} can also be used, but requires more coding effort and it offers no benefits over instance
methods on {AMQP::Channel} in this particular case.

Some examples of usage:

<pre>
<code>
exchange = AMQP::Exchange.new(channel, :direct, "")
</code>
</pre>

<pre>
<code>
exchange = channel.default_exchange
</code>
</pre>

<pre>
<code>
exchange = channel.direct("")
</code>
</pre>


The default exchange is used by the "Hello, World" example:
<script src="https://gist.github.com/998691.js"> </script>

Additionally, the routing example above can be rewritten to use the default exchange:
<script src="https://gist.github.com/1041778.js"> </script>


h3. Direct exchange use cases

Direct exchanges can be used in a wide variety of cases:

 * Direct (near real-time) messages to individual players in an MMO game
 * Delivering notifications to specific geographic locations (for example, points of sale)
 * Distributing tasks between multiple instances of the same application all having the same function, for example, image processors
 * Passing data between workflow steps, each having an identifier (also consider using headers exchange)
 * Delivering notifications to individual software services in the network


h2. Topic exchanges

h3. How topic exchanges route messages

Topic exchanges route messages to one or many queues based on matching between a message routing key and the pattern
that was used to bind a queue to an exchange. The topic exchange type is often used to implement various
"publish/subscribe pattern":http://en.wikipedia.org/wiki/Publish/subscribe variations.

Topic exchanges are commonly used for the "multicast routing":http://en.wikipedia.org/wiki/Multicast of messages.

!http://upload.wikimedia.org/wikipedia/commons/thumb/3/30/Multicast.svg/500px-Multicast.svg.png!

Topic exchanges can be used for "broadcast routing":http://en.wikipedia.org/wiki/Broadcasting_%28computing%29, but
fanout exchanges are usually more efficient for this use case.


h3. Topic exchange routing example

Two classic examples of topic-based routing are stock price updates and location-specific data (for instance, weather
broadcasts). Consumers indicate which topics they are interested in (think of it like subscribing to a feed for an
individual tag of your favourite blog as opposed to the full feed). The routing is enabled by specifying a _routing
pattern_ to the {AMQP::Queue#bind} method, for example:

<pre>
<code>
channel.queue("americas.south").bind(exchange, :routing_key => "americas.south.#").subscribe do |headers, payload|
  puts "An update for South America: #{payload}, routing key is #{headers.routing_key}"
end
</code>
</pre>

In the example above we bind a queue with the name of "americas.south" to the topic exchange declared earlier using
the {AMQP::Queue#bind} method. This means that only messages with a routing key matching "americas.south.#" will be
routed to the "americas.south" queue.

A routing pattern consists of several words separated by dots, in a similar way to URI path segments being joined by
slash. A few of examples:

 * asia.southeast.thailand.bangkok
 * sports.basketball
 * usa.nasdaq.aapl
 * tasks.search.indexing.accounts

The following routing keys match the "americas.south.#" pattern:

 * americas.south
 * americas.south.*brazil*
 * americas.south.*brazil.saopaolo*
 * americas.south.*chile.santiago*

In other words, the "#" part of the pattern matches 0 or more words.

For the pattern "americas.south.*", some matching routing keys are:

 * americas.south.*brazil*
 * americas.south.*chile*
 * americas.south.*peru*

but not

 * americas.south
 * americas.south.chile.santiago

As you can see, the "*" part of the pattern matches 1 word only.


Full example:
<script src="https://gist.github.com/1020308.js"> </script>


h3. Topic exchange use cases

Topic exchanges have a very broad set of use cases. Whenever a problem involves multiple consumers/applications that
selectively choose which type of messages they want to receive, the use of topic exchanges should be considered.
To name a few examples:

 * Distributing data relevant to specific geographic location, for example, points of sale
 * Background task processing done by multiple workers, each capable of handling specific set of tasks
 * Stocks price updates (and updates on other kinds of financial data)
 * News updates that involve categorization or tagging (for example, only for a particular sport or team)
 * Orchestration of services of different kinds in the cloud
 * Distributed architecture/OS-specific software builds or packaging where each builder can handle only one architecture or OS


h2. Declaring/Instantiating exchanges

With the Ruby amqp gem, exchanges can be declared in two ways:

 * By using the {AMQP::Exchange#initialize} method that takes an optional callback
 * By using a number of convenience methods on {AMQP::Channel} instances:

  ** {AMQP::Channel#direct}
  ** {AMQP::Channel#default_exchange}
  ** {AMQP::Channel#topic}
  ** {AMQP::Channel#fanout}
  ** {AMQP::Channel#headers}

The previous sections on specific exchange types (direct, fanout, headers, etc.) provide plenty of examples of how
these methods can be used.


h2. Publishing messages

To publish a message to an AMQP exchange, use {AMQP::Exchange#publish}:

<pre>
<code>
exchange.publish("Some payload")
</code>
</pre>


{AMQP::Exchange#publish} can accept any object that responds to the `to_s` method, not just string instances:
<script src="https://gist.github.com/1020336.js"> </script>

The message payload is completely opaque to the library and is not modified in any way.


h3. Data serialization

You are encouraged to take care of data serialization before publishing (i.e. by using JSON, Thrift, Protocol Buffers
or some other serialization library). Note that because AMQP is a binary protocol, text formats like JSON largely
lose their advantage of being easy to inspect as data travels across the network, so consider using
"BSON":http://bsonspec.org instead.

A few popular options for data serialization are:

 * JSON: "json gem":https://rubygems.org/gems/json (part of standard Ruby library on Ruby 1.9) or "yajl-ruby":https://rubygems.org/gems/yajl-ruby (Ruby bindings to YAJL)
 * BSON: "bson gem":https://rubygems.org/gems/bson for JRuby (implemented as a Java extension) and "bson_ext":https://rubygems.org/bson_ext (C extension) for C-based Rubies
 * "Message Pack":http://msgpack.org has Ruby bindings but currently does not provide Java implementation for JRuby
 * XML: "Nokogiri":https://nokogiri.org is a swiss army knife for XML processing with Ruby, built on top of libxml2
 * Protocol Buffers: "beefcake":https://github.com/bmizerany/beefcake
 * Thrift: "thrift-client":https://github.com/fauna/thrift_client


h3. Message metadata

AMQP messages have various metadata attributes that can be set when a message is published. Some of the attributes are
well-known and mentioned in the AMQP v0.9.1 specification, others are specific to a particular application.
Well-known attributes are listed here as options that {AMQP::Exchange#publish} takes:

 * :routing_key
 * :persistent
 * :immediate
 * :mandatory
 * :content_type
 * :content_encoding
 * :priority
 * :message_id
 * :correlation_id
 * :reply_to
 * :type
 * :user_id
 * :app_id
 * :timestamp
 * :expiration

All other attributes can be added to a _headers table_ (in Ruby parlance, headers hash) that {AMQP::Exchange#publish}
accepts as the ":headers" argument.

An example to show how message metadata attributes are passed to {AMQP::Exchange#publish}:
<script src="https://gist.github.com/1020424.js"> </script>

<dl>
  <dt>:routing_key</dt>
  <dd>Used for routing messages depending on the exchange type and configuration.</dd>

  <dt>:persistent</dt>
  <dd>When set to true, AMQP broker will persist message to disk.</dd>

  <dt>:immediate</dt>
  <dd>
  This flag tells the server how to react if the message cannot be routed to a queue consumer immediately. If this flag is set to true, the server will return
  an undeliverable message to the producer with a basic.return AMQP method. If this flag is set to false, the server will queue the message, but with no guarantee
  that it will ever be consumed.
  </dd>

  <dt>:mandatory</dt>
  <dd>
  This flag tells the server how to react if the message cannot be routed to a queue. If this flag is set to true, the server will return an unroutable message
  to the producer with a basic.return AMQP method. If this flag is set to false, the server silently drops the message.
  </dd>

  <dt>:content_type</dt>
  <dd>MIME content type of message payload. Has the same purpose/semantics as HTTP Content-Type header.</dd>

  <dt>:content_encoding</dt>
  <dd>MIME content encoding of message payload. Has the same purpose/semantics as HTTP Content-Encoding header.</dd>

  <dt>:priority</dt>
  <dd>Message priority, from 0 to 9.</dd>

  <dt>:message_id</dt>
  <dd>
    Message identifier as a string. If applications need to identify messages, it is recommended that they use this attribute instead of putting it
    into the message payload.
  </dd>

  <dt>:reply_to</dt>
  <dd>
    Commonly used to name a reply queue (or any other identifier that helps a consumer application to direct its response).
    Applications are encouraged to use this attribute instead of putting this information into the message payload.
  </dd>

  <dt>:correlation_id</dt>
  <dd>
    ID of the message that this message is a reply to. Applications are encouraged to use this attribute instead of putting this information
    into the message payload.
  </dd>

  <dt>:type</dt>
  <dd>Message type as a string. Recommended to be used by applications instead of including this information into the message payload.</dd>

  <dt>:user_id</dt>
  <dd>
  Sender's identifier. Note that RabbitMQ will check that the "value of this attribute is the same as username AMQP connection was authenticated with":http://www.rabbitmq.com/extensions.html#validated-user-id, it SHOULD NOT be used to transfer, for example, other application user ids or be used as a basis for some kind of Single Sign-On solution.
  </dd>

  <dt>:app_id</dt>
  <dd>Application identifier string, for example, "eventoverse" or "webcrawler"</dd>

  <dt>:timestamp</dt>
  <dd>Timestamp of the moment when message was sent, in seconds since the Epoch</dd>

  <dt>:expiration</dt>
  <dd>Message expiration specification as a string</dd>

  <dt>:headers</dt>
  <dd>Ruby hash of any additional attributes that the application needs. Nested hashes are supported.</dd>
</dl>

It is recommended that application authors use well-known message attributes when applicable instead of relying on custom headers or placing
information into the message body. For example, if your application messages have priority, publishing timestamp, type and content type,
you should use the respective AMQP message attributes instead of reinventing the wheel.


h3. Validated user_id

In some scenarios it is useful for consumers to be able to know the identity of the user who published a message. RabbitMQ implements a feature known as
"validated User ID":http://www.rabbitmq.com/extensions.html#validated-user-id. If this property is set by a publisher, its value must be the same as the name of the user used to open the connection.
If the user-id property is not set, the publisher's identity is not validated and remains private.


h3. Publishing callback and reliable delivery in distributed environments

Sometimes it is convenient to execute an operation after publishing a message. For this, {AMQP::Exchange#publish} provides an optional
callback. It is important to clear up some expectations of when exactly it is run and how it is related to topics of message persistence,
delivery reliability and so on.

<pre>
<code>
exchange.publish(payload, :persistent => true, :type => "reports.done") do
  # ...
end
</code>
</pre>

A common expectation of the code above is that it is run after the message "has been sent", or even "has been delivered". Unfortunately, neither of
these expectations can be met by the Ruby amqp gem alone. Message publishing happens in several steps:

 * {AMQP::Exchange#publish} takes a message and various metadata attributes
 * {AMQP::Exchange#publish} internally calls #to_s on the message argument to get message payload
 * Resulting payload is staged for writing
 * On the next event loop tick, data is transferred to the OS kernel using one of the underlying system calls ("epoll":http://en.wikipedia.org/wiki/Epoll, "kqueue":http://en.wikipedia.org/wiki/Kqueue and so on)
   or NIO channels (in the case of JRuby)
 * OS kernel buffers data before sending it
 * Network driver may also employ buffering

<span class="note">
As you can see, "when data is sent" is a complicated issue and while methods to flush buffers certainly exist on various platforms, doing so in
a cross-platform way that *includes the JVM* (that EventMachine also runs on) is non-trivial. In addition, even flushing buffers does not guarantee that the
data was received by the broker because it might have crashed while data was travelling down the wire. The only way to reliably know whether data was
received by the broker or a peer application is to use message acknowledgements. This is how TCP works and this approach is proven to work at the
enormous scale of the modern Internet. AMQP (the protocol) fully embraces this fact and the amqp gem follows.
</span>

Given all of this, you may ask "when does the {AMQP::Exchange#publish} callback fire?" The answer is on the next event loop tick. By then the data is pushed down to
the OS kernel. As far as the Ruby library is concerned, it is reasonably safe behavior.

<span class="note">
The AMQP::Exchange#publish callback is fired on the next event loop tick. Data is staged for delivery immediately. Applications MUST NOT assume that
by the time the callback has fired, the data is guaranteed to leave the local machine networking stack, reach the AMQP broker or any peer applications that the
message needs to be routed to.
</span>

In cases when you cannot afford to lose a single message, AMQP v0.9.1 applications can use one (or a combination of) the following protocol features:

 * Publisher confirms (a RabbitMQ-specific extension to AMQP v0.9.1)
 * Publishing messages as immediate and/or mandatory
 * Transactions (these introduce noticeable overhead and have a relatively narrow set of use cases)

A more detailed overview of the pros and cons of each option can be found in a "blog post that introduces Publisher Confirms extension":http://bit.ly/rabbitmq-publisher-confirms by the RabbitMQ team.
The next sections of this guide will describe how the features above can be used with the Ruby amqp gem.


h3. Publishing messages as immediate

When publishing messages, it is possible to use the ":immediate" option to publish a message as "immediate". When an immediate message cannot be
delivered to any consumer (meaning that one or more queues to which the message was routed have no active consumers), then the message is returned to the
producer.

An example of {AMQP::Exchange#publish} being used to publish an immediate message:

<pre>
<code>
exchange.publish("Message ##{i}", :immediate => true)
</code>
</pre>

The following code example demonstrates a message that is published as immediate but cannot be immediately consumed (no consumers) and thus is returned
back to the producer:
<script src="https://gist.github.com/1025700.js"></script>


h3. Publishing messages as mandatory

When publishing messages, it is possible to use the ":mandatory" option to publish a message as "mandatory". When a mandatory message cannot be
*routed* to any queue (for example, there are no bindings or none of the bindings match), the message is returned to the producer.

The following code example demonstrates a message that is published as mandatory but cannot be routed (no bindings) and thus is returned
back to the producer:
<script src="https://gist.github.com/1041835.js"> </script>


h3. Returned messages

When a message is returned, the application that produced it can handle that message in different ways:

 * Store it for later redelivery in a persistent store
 * Publish it to a different destination
 * Log the event and discard the message

Returned messages contain information about the exchange they were published to. For convenience, the amqp gem associates returned message callbacks with {AMQP::Exchange}
instances. To handle returned messages, use {AMQP::Exchange#on_return}:

<pre>
<code>
exchange.on_return do |basic_return, metadata, payload|
  puts "#{payload} was returned! reply_code = #{basic_return.reply_code}, reply_text = #{basic_return.reply_text}"
end
</code>
</pre>

A returned message handler has access to AMQP method (basic.return) information, message metadata and payload. The metadata and message body are returned without modifications so that the application can store the message for later redelivery.


h3. Publishing persistent messages

Messages potentially spend some time in the queues to which they were routed before they are consumed. During this period of time, the broker may crash or
experience a restart. To survive it, messages must be persisted to disk. This has a negative effect on performance, especially with network attached
storage like NAS devices and Amazon EBS. AMQP v0.9.1 lets applications trade off performance for durability, or vice versa, on a message-by-message basis.

To publish a persistent message, use the ":persistent" option that {AMQP::Exchange#publish} accepts:

<pre>
<code>
exchange.publish(payload, :persistent => true)
</code>
</pre>

<span class="note">
Note that in order to survive a broker crash, both the message and the queue that it was routed to must be persistent/durable.
</span>

{file:docs/Durability.textile Durability and Message Persistence} provides more information on the subject.


h3. Publishing In Multi-threaded Environments

When using amqp gem in multi-threaded environments, the rule of thumb is: avoid sharing {AMQP::Channel} instances
across threads.

Starting with 0.8.0.RC14, {AMQP::Exchange#publish} synchronizes data delivery on the channel object associated
with exchange. This protects application developers from the most common problems related to publishing messages on a
shared channel from multiple threads, however, by no means protects from every possible concurrency hazard.

<span class="note">
When using amqp gem in multi-threaded environments, the rule of thumb is: avoid sharing {AMQP::Channel} instances
across threads.
</span>



h3. Sending one-off messages

The following example publishes a message and *safely* closes the AMQP connection afterwards by passing a block
to {AMQP::Exchange#publish}:

<script src="https://gist.github.com/1020425.js"> </script>


h2. Headers exchanges

Now that message attributes and publishing have been introduced, it is time to take a look at one more core exchange type in AMQP v0.9.1. It is called the
_headers_ exchange type and is quite powerful.

h3. How headers exchanges route messages

h4. An example problem definition

The best way to explain headers-based routing is with an example. Imagine a distributed "continuous integration":http://martinfowler.com/articles/continuousIntegration.html system
that distributes builds across multiple machines with different hardware architectures (x86, IA-64, AMD64, ARM family and so on) and operating systems.
It strives to provide a way for a community to contribute machines to run tests on and a nice build matrix like "the one WebKit uses":http://build.webkit.org/waterfall?category=core.
One key problem such systems face is build distribution. It would be nice if a messaging broker could figure out
which machine has which OS, architecture or combination of the two and route build request messages accordingly.

A headers exchange is designed to help in situations like this by routing on multiple attributes that are more easily expressed as
message metadata attributes (headers) rather than a routing key string.

h4. Routing on multiple message attributes

Headers exchanges route messages based on message header matching. Headers exchanges ignore the routing key attribute. Instead, the attributes used for routing
are taken from the "headers" attribute. When a queue is bound to a headers exchange, the ":arguments" attribute is used to define matching rules:

<pre>
<code>
# when binding to a headers exchange, :arguments parameter is used to specify matching rules
@channel.queue("", :auto_delete => true).bind(exchange, :arguments => { :os => 'linux' })
</code>
</pre>

When matching on one header, a message is considered matching if the value of the header equals the value specified upon binding. Using the example above,
some messages that match would be:

<pre>
<code>
exchange.publish "For linux/IA64",   :headers => { :arch => "IA64", :os => 'linux' }
exchange.publish "For linux/x86",    :headers => { :arch => "x86",  :os => 'linux' }
exchange.publish "For any linux",    :headers => { :os => 'linux' }
</code>
</pre>

The following example demonstrates matching on integer values:

<pre>
<code>
# consumer part
@channel.queue("", :auto_delete => true).bind(exchange, :arguments => { :cores => 8 })

# ...

# producer part
exchange.publish "For ocotocore", :headers => { :cores => 8 }
</code>
</pre>

Matching on hashes (in AMQP v0.9.1 parlance - _attribute tables_) is also supported:

<pre>
<code>
# consumer part
channel.queue("", :auto_delete => true).bind(exchange, :arguments => { :package => { :name => 'riak', :version => '0.14.2' } })

# ...

# producer part
exchange.publish "For nodes with Riak 0.14.2", :headers => { :package => { :name => 'riak', :version => '0.14.2' } }
</code>
</pre>

h4. Matching all vs matching one

It is possible to bind a queue to a headers exchange using more than one header for matching. In this case, the broker needs one more piece of
information from the application developer, namely, should it consider messages with any of the headers matching, or all of them? This is what the "x-match"
binding argument is for:

<pre>
<code>
channel.queue("", :auto_delete => true).bind(exchange, :arguments => { 'x-match' => 'all', :arch => "ia64", :os => 'linux' })
</code>
</pre>

In the example above, only messages that have an "arch" header value equal to "ia64" and an "os" header value equal to "linux" will
be considered matching.

<pre>
<code>
channel.queue("", :auto_delete => true).bind(exchange, :arguments => { 'x-match' => 'any', :os => 'macosx', :cores => 8 })
</code>
</pre>

When the "x-match" argument is set to "any", just one matching header value is sufficient. So in the example above, any message with a
"cores" header value equal to 8 will be considered matching.

h4. More examples

TBD


h3. Declaring a headers exchange

There are two ways to declare a headers exchange:

 * By instantiating {AMQP::Exchange} and specifying type as ":headers"
 * By using the {AMQP::Channel#headers} method

Here are two examples to demonstrate:

<pre>
<code>
exchange = AMQP::Exchange.new(channel, :headers, "builds")
</code>
</pre>

<pre>
<code>
exchange = channel.headers("builds")
</code>
</pre>

Both methods asynchronously declare a queue. Because declaration necessitates a network round trip, publishing operations on {AMQP::Exchange} instances are
delayed until the broker reply (`exchange.declare-ok`) is received.

Both methods let you pass a block to run a piece of code when the broker responds with `exchange.declare-ok` (meaning that the exchange has been successfully declared).

<pre>
<code>
channel.headers("builds") do |exchange|
  # exchange is declared and ready to be used.
end
</code>
</pre>


h3. Headers exchange routing example

When there is just one queue bound to a headers exchange, messages are routed to it if one or all headers of the message match those specified
upon binding. Whether it is "any header" or "all of them" depends on the "x-match" header value.
In the case of multiple queues, a headers exchange will deliver a copy of a message to each queue, just like direct exchanges do. Distribution rules between consumers
on a particular queue are the same as for a direct exchange.

Full example:
<script src="https://gist.github.com/1034875.js"> </script>


h3. Headers exchange use cases

Headers exchanges can be looked upon as "direct exchanges on steroids". Because they route based on header values, they can be used as
direct exchanges where the routing key does not have to be a string; it could be an integer or a hash (dictionary) for example.

Some specific use cases:

 * Transfer of work between stages in a multi-step workflow ("routing slip pattern":http://eaipatterns.com/RoutingTable.html)
 * Distributed build/continuous integration systems can distribute builds based on multiple parameters (OS, CPU architecture, availability
   of a particular package).


h3. Pre-declared headers exchanges

AMQP v0.9.1 brokers should (as defined by "IETF RFC 2119":http://www.ietf.org/rfc/rfc2119.txt) implement a headers exchange type
and pre-declare one instance with the name of "amq.match". RabbitMQ also pre-declares one instance with the name of "amq.headers".
Applications can rely on that exchange always being available to them. Each vhost has a separate instance of those exchanges and
they are *not shared across vhosts* for obvious reasons.


h2. Custom exchange types

h3. x-random

The "x-random AMQP exchange type":https://github.com/jbrisbin/random-exchange is a custom exchange type developed as a
RabbitMQ plugin by Jon Brisbin. To quote from the project README:

bq. It is basically a direct exchange, with the exception that, instead of each consumer bound to that exchange with the
    same routing key getting a copy of the message, the exchange type randomly selects a queue to route to.

This plugin is licensed under "Mozilla Public License 1.1":http://www.mozilla.org/MPL/MPL-1.1.html, same as RabbitMQ.

h3. x-recent-history

The "x-recent-history AMQP exchange type":https://github.com/videlalvaro/rabbitmq-recent-history-exchange is a customer exchange type
implemented as a RabbitMQ plugin by Alvaro Videla, one of the authors of "RabbitMQ in action":http://bit.ly/rabbitmq.

This plugin is licensed under the "MIT license":https://github.com/videlalvaro/rabbitmq-recent-history-exchange/blob/master/LICENSE.md.


h2. Using the Publisher Confirms extension to AMQP v0.9.1

Please refer to {file:docs/VendorSpecificExtensions.textile Vendor-specific extensions to AMQP 0.9.1 spec}


h3. Message acknowledgements and their relationship to transactions and publisher confirms

Consumer applications (applications that receive and process messages) may occasionally fail to process individual messages, or might just
crash. Additionally, network issues might be experienced. This raises a question - "when should the AMQP broker remove messages from queues?" This
topic is covered in depth in the {file:docs/Queues.textile Working With Queues} guide, including prefetching and examples.

In this guide, we will only mention how message acknowledgements are related to AMQP transactions and the Publisher Confirms extension.
Let us consider a publisher application (P) that communications with a consumer (C) using AMQP v0.9.1. Their communication can be graphically
represented like this:

<pre>
-----       -----       -----
|   |   S1  |   |   S2  |   |
| P | ====> | B | ====> | C |
|   |       |   |       |   |
-----       -----       -----
</pre>

We have two network segments, S1 and S2. Each of them may fail. P is concerned with making sure that messages cross S1,
while the broker (B) and C are concerned with ensuring that messages cross S2 and are only removed from the queue when
they are processed successfully.

Message acknowledgements cover reliable delivery over S2 as well as successful processing. For S1, P has to use transactions (a heavyweight solution) or the more
lightweight Publisher Confirms, a RabbitMQ-specific extension.


h2. Using AMQP transactions

TBD


h2. Binding queues to exchanges

Queues are bound to exchanges using the {AMQP::Queue#bind} method. This topic is described in detail in the {file:docs/Queues.textile Working with queues}
documentation guide.


h2. Unbinding queues from exchanges

Queues are unbound from exchanges using the {AMQP::Queue#unbind} method. This topic is described in detail in the {file:docs/Queues.textile Working with queues}
documentation guide.


h2. Deleting exchange

h3. Explicitly deleting an exchange

Exchanges are deleted using the {AMQP::Exchange#delete} method:

<pre>
<code>
exchange.delete
</code>
</pre>

{AMQP::Exchange#delete} takes an optional callback that is run when a `exchange.delete-ok` reply arrives from the broker.

<pre>
<code>
exchange.delete do |delete_ok|
  # by now exchange is guaranteed to be deleted
end
</code>
</pre>

h3. Auto-deleted exchanges

Exchanges can be *auto-deleted*. To declare an exchange as auto-deleted, use the ":auto_delete" option on declaration:

<pre>
<code>
exchange = AMQP::Exchange.new(channel, :direct, "nodes.metadata", :auto_delete => true)
</code>
</pre>

<pre>
<code>
exchange = channel.direct("nodes.metadata", :auto_delete => true)
</code>
</pre>


Full example:
<script src="https://gist.github.com/1020226.js"> </script>

TBD: explain when exchange is considered to be "no longer in use"


h2. Objects as message producers.

Since Ruby is a genuine object-oriented language, it is important to demonstrate how the Ruby amqp gem can be integrated
into rich object-oriented code. This part of the guide focuses on exchanges and the problems/solutions concerning producer applications
(applications that primarily generate and publish messages, as opposed to consumers that receive and process them).

Full example:
<script src="https://gist.github.com/1009425.js"> </script>

TBD


h2. Exchange durability vs Message durability

See {file:docs/Durability.textile Durability guide}


h2. Error handling and recovery

See {file:docs/ErrorHandling.textile Error handling and recovery guide}


h2. Vendor-specific extensions related to exchanges

See {file:docs/VendorSpecificExtensions.textile Vendor-specific Extensions guide}


h2.  What to read next

Documentation is organized as several {file:docs/DocumentationGuidesIndex.textile documentation guides} that cover all kinds of
topics. Guides related to this one are

 * {file:docs/Durability.textile Durability and message persistence}
 * {file:docs/Bindings.textile Bindings}
 * {file:docs/PatternsAndUseCases.textile Patterns and Use Cases}
 * {file:docs/Queues.textile Working With Queues}
 * {file:docs/ErrorHandling.textile Error handling and recovery}


h2. Authors

This guide was written by "Michael Klishin":http://twitter.com/michaelklishin and edited by "Chris Duncan":https://twitter.com/celldee.



h2. Tell us what you think!

Please take a moment to tell us what you think about this guide "on Twitter":http://twitter.com/rubyamqp or the "Ruby AMQP mailing list":http://groups.google.com/group/ruby-amqp.
 Let us know what was unclear or what has not been covered. Maybe you do not like the guide style or grammar or discover spelling mistakes. Reader feedback is
key to making the documentation better.

If, for some reason, you cannot use the communication channels mentioned above, you can "contact the author of the guides directly":mailto:michael@novemberain.com?subject=amqp%20gem%20documentation


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