AMQP 0.9.1 Model Explained

About this guide

This guide explains the AMQP 0.9.1 Model used by RabbitMQ. Understanding the AMQP Model will make a lot of other documentation, both for the Ruby amqp gem and RabbitMQ itself, easier to follow. This work is licensed under a Creative Commons Attribution 3.0 Unported License (including images & stylesheets). The source is available on Github.

What this guide covers

High-level overview of the AMQP 0.9.1 Model
Key differences between the AMQP model and some other messaging models
What exchanges are
What queues are
What bindings are
How AMQP protocol is structured and what AMQP methods are
AMQP 0.9.1 message attributes
What message acknowledgements are
What negative message acknowledgements are
and a lot of other things

High-level overview of AMQP 0.9.1 and the AMQP Model

What is AMQP

AMQP (Advanced Message Queuing Protocol) is a networking protocol that enables conforming client applications to communicate with conforming messaging middleware brokers.

Why AMQP was created

Messaging solutions have been around since the 1970s with a view to solving the problem of integrating incompatible products from diverse vendors. Without the use of messaging middleware, the integration of heterogenous systems has proved to be very expensive and complex. However, messaging solutions, such as IBM Websphere MQ and Tibco Enterprise Message Service, are also very costly and tend to be exclusively employed by large companies (who can afford them), especially those in the financial services industry.

There is also a problem with interoperability between messaging solutions. Vendors have created their own proprietary messaging protocols which do not interoperate with others, therefore resulting in ‘vendor lock-in’.

AMQP has multiple design goals but two of the most important are:

To produce an open standard for messaging middleware
To enable interoperability between various technologies and platforms

There is a lot of software running on many operating systems built with multiple programming languages running on various hardware architectures and virtual machines. AMQP not only makes it possible for these disparate systems to communicate with one another, but also enables different products that implement AMQP to exchange information.

Brokers and their role

Messaging brokers receive messages from producers (applications that publish them) and route them to consumers (applications that process them).

If you imagine the human body, then brokers would be equivalent to centers of the nervous system and applications would be more like limbs.

AMQP 0.9.1 Model in brief

The AMQP 0.9.1 Model has the following view of the world: messages are published by producers to exchanges, often compared to post offices or mailboxes. Exchanges then distribute message copies to queues using rules called bindings. Then AMQP brokers either push messages to consumers subscribed to queues, or consumers fetch/pull messages from queues on demand.

When publishing a message, producers may specify various message attributes (message metadata). Some of this metadata may be used by the broker, however, the rest of it is completely opaque to the broker and is only used by applications that receive the message.

Networks are unreliable and applications may fail to process messages, therefore the AMQP Model has a notion of message acknowledgements: when a message is pushed down to a consumer, the consumer notifies the broker, either automatically, or as soon as the application developer chooses to do so. When message acknowledgements are in use, a broker will only completely remove a message from a queue when it receives a notification for that message (or group of messages).

In certain situations, for example, when a message cannot be routed, messages may be returned to producers, dropped, or, if the broker implements an extension, placed into a so-called “dead letter queue”. Producers choose how to handle situations like this by publishing messages using certain parameters.

Queues, exchanges and bindings are commonly referred to as AMQP entities.

AMQP is a Programmable Protocol

AMQP 0.9.1 is a programmable protocol in the sense that AMQP entities and routing schemes are defined by applications themselves, not a broker administrator. Accordingly, provision is made for protocol operations that declare queues and exchanges, define bindings between them, subscribe to queues and so on.

This gives application developers a lot of freedom but also requires them to be aware of potential definition conflicts. In practice, definition conflicts are rare and often indicate misconfigurations. This can be very useful as it is a good thing if misconfigurations are caught early.

Applications declare the AMQP entities that they need, define necessary routing schemes and may choose to delete AMQP entities when they are no longer used.

AMQP Exchanges and Exchange Types

Exchanges are AMQP entities where messages are sent. Exchanges then take a message and route it into one or more (or no) queues. The routing algorithm used depends on exchange type and rules called bindings. AMQP 0.9.1 brokers typically provide 4 exchange types out of the box:

Direct exchange (typically used for for 1-to-1 communication or unicasting)
Fanout exchange (1-to-n communication or broadcasting)
Topic exchange (1-to-n or n-to-m communication, multicasting)
Headers exchange (message metadata-based routing)

but it is possible to extend AMQP 0.9.1 brokers with custom exchange types, for example:

x-random exchange (randomly chooses a queue to route incoming messages to)
x-recent-history (a fanout exchange that also keeps N recent messages in memory)
regular expressions based variations of headers exchange

and so on.

Besides the type, exchanges have a number of attributes, most important of which are:

Name
Can be durable (information about them is persisted to disk and thus survives broker restarts) or non-durable (information is only kept in RAM)
Can have metadata associated with them on declaration

AMQP Queues

Queues in the AMQP Model are very similar to queues in other message and “task queueing” systems: they store messages that are consumed by applications. Like AMQP exchanges, an AMQP queue has a name and a durability property but also

Can be exclusive (used by only one connection)
Can be automatically deleted when last consumer unsubscribes
Can have metadata associated with them on declaration (some brokers use this to implement features like message TTL)

AMQP Bindings

Bindings are rules that exchanges use (among other things) to route messages to queues. To instruct an exchange E to route messages to a queue Q, Q has to be bound to E. Bindings may have an optional routing key attribute used by some exchange types. The purpose of the routing key is to selectively match only specific (matching) messages published to an exchange to the bound queue. In other words, the routing key acts like a filter.

To draw an analogy:

Queue is like your destination in New York city
Exchange is like JFK airport
Bindings are routes from JFK to your destination. There may be no way, or more than one way, to reach it

Having this layer of indirection enables routing scenarios that are impossible or very hard to implement using publishing directly to queues and also eliminates a certain amount of duplicated work that application developers have to do.

If an AMQP message cannot be routed to any queue (for example, because there are no bindings for the exchange it was published to), it is either dropped or returned to the publisher, depending on the message attributes that the publisher has set.

AMQP Message Consumers

Storing messages in queues is useless unless applications can consume them. In the AMQP 0.9.1 Model, there are two ways for applications to do this:

Have messages pushed to them (“push API”)
Fetch messages as needed (“pull API”)

With the “push API”, applications have to indicate interest in consuming messages from a particular queue. When they do so, we say that they register a consumer or, simply put, subscribe to a queue. It is possible to have more than one consumer per queue or to register an exclusive consumer (excludes all other consumers from the queue while it is consuming).

Each consumer (subscription) has an identifier called a consumer tag. This can be used to unsubscribe from messages. Consumer tags are just strings.

AMQP Message Attributes and Payload

Messages in the AMQP Model have attributes. Some attributes are so common that the AMQP 0.9.1 specification defines them and application developers do not have to think about the exact attribute name. Some examples are

Content type
Content encoding
Routing key
Delivery mode (persistent or not)
Message priority
Message publishing timestamp
Expiration period
Producer application id

Some attributes are used by AMQP brokers, but most are open to interpretation by applications that receive them. Some attributes are optional and known as headers. They are similar to X-Headers in HTTP. Message attributes are set when a message is published.

AMQP messages also have a payload (the data that they carry). Brokers treat this data as opaque (it is neither modified nor used by them). It is possible for messages to contain only attributes and no payload. It is common to use serialization formats like JSON, Thrift, Protocol Buffers and MessagePack to serialize structured data in order to publish it as an AMQP message payload.

AMQP Message Acknowledgements

Since networks are unreliable and applications fail, it is often necessary to have some kind of “processing acknowledgement”. Sometimes it is only necessary to acknowledge the fact that a message has been received. Sometimes acknowledgements mean that a message was validated and processed by a consumer, for example, verified as having mandatory data and persisted to a data store or indexed.

This situation is very common, so AMQP 0.9.1 has a built-in feature called message acknowledgements (sometimes referred to as acks) that consumers use to confirm message delivery and/or processing. If an application crashes (the AMQP broker notices this when the connection is closed), and if an acknowledgement for a message was expected but not received by the AMQP broker, the message is re-queued (and possibly immediately delivered to another consumer, if any exists).

Having acknowledgements built into the protocol helps developers to build more robust software.

AMQP 0.9.1 Methods

AMQP 0.9.1 is structured as a number of methods. Methods are operations (like HTTP methods) and have nothing in common with methods in object-oriented programming languages. AMQP methods are grouped into classes. Classes are just logical groupings of AMQP methods. The AMQP 0.9.1 reference can be found on the RabbitMQ website.

Let us take a look at the exchange.* class, a group of methods related to operations on exchanges. It includes the following operations:

exchange.declare
exchange.declare-ok
exchange.delete
exchange.delete-ok

The operations above form logical pairs: exchange.declare and exchange.declare-ok, exchange.delete and exchange.delete-ok. These operations are “requests” and “responses” .

As an example, the client asks the broker to declare a new exchange using the exchange.declare method:

As shown on the diagram above, exchange.declare carries several*parameters*. They enable the client to specify exchange name, type, durability flag and so on.

If the operation succeeds, the broker responds with the exchange.declare-ok method:

exchange.declare-ok does not carry any parameters except for the channel number.

The sequence of events is very similar for another method pair, queue.declare and queue.declare-ok:

Not all AMQP methods have counterparts. Some do not have corresponding “response” methods and some others have more than one possible “response”.

AMQP Connections

AMQP connections are typically long-lived. AMQP is an application level protocol that uses TCP for reliable delivery. AMQP connections use authentication and can be protected using TLS . When an application no longer needs to be connected to an AMQP broker, it should gracefully close the AMQP connection instead of abruptly closing the underlying TCP connection.

AMQP Channels

Some applications need multiple connections to an AMQP broker. However, it is undesirable to keep many TCP connections open at the same time because doing so consumes system resources and makes it more difficult to configure firewalls. AMQP 0.9.1 connections are multiplexed with*channels_ that can be thought of as “lightweight connections that share a single TCP connection”.

For applications that use multiple threads/processes/etc. for processing, it is very common to open a new channel per thread (process, etc.) and not share channels between them.

Communication on a particular channel is completely separate from communication on another channel, therefore every AMQP method also carries a channel number that clients use to figure out which channel the method is for (and thus, which event handler needs to be invoked).

AMQP Virtual Hosts (vhosts)

To make it possible for a single broker to host multiple isolated “environments” (groups of users, exchanges, queues and so on), AMQP includes the concept of virtual hosts (vhosts). They are similar to virtual hosts used by many popular Web servers and provide completely isolated environments in which AMQP entities live. AMQP clients specify the vhosts that they want to use during AMQP connection negotiation.

An AMQP 0.9.1 vhost name can be any non-blank string. Some of the most common use cases for vhosts are

To separate AMQP entities used by different groups of applications
To separate multiple installations/environments (e.g. production, staging) of one or more applications
To implement a multi-tenant environment

AMQP is Extensible

AMQP 0.9.1 has several extension points:

Custom exchange types let developers implement routing schemes that exchange types provided out-of-the-box do not cover well, for example, geodata-based routing.
Declaration of exchanges and queues can include additional attributes that the broker can use. For example, per-queue message TTL in RabbitMQ is implemented this way.
Broker-specific extensions to the protocol. See, for example, extensions RabbitMQ implements.
New AMQP 0.9.1 method classes can be introduced.
Brokers can be extended with additional plugins, for example, RabbitMQ management frontend and HTTP API are implemented as a plugin.

These features make the AMQP 0.9.1 Model even more flexible and applicable to a very broad range of problems.

Key differences from some other messaging models

One key difference to understand about the AMQP 0.9.1 model is that messages are not sent to queues. They are sent to exchanges that route them to queues according to rules called “bindings”. This means that routing is primarily handled by AMQP brokers and not applications themselves.

TBD

AMQP 0.9.1 clients ecosystem

Overview

There are many AMQP 0.9.1 clients for popular programming languages and platforms. Some of them follow AMQP terminology closely and only provide implementations of AMQP methods. Some others have additional features, convenience methods and abstractions. Some of the clients are asynchronous (non-blocking), some are synchronous (blocking), some support both models. Some clients support vendor-specific extensions (for example, RabbitMQ-specific extensions).

Because one of the main AMQP goals is interoperability, it is a good idea for developers to understand protocol operations and not limit themselves to the terminology of a particular client library. This way communicating with developers using different libraries will be significantly easier.

Wrapping up

This is the end of the AMQP 0.9.1 Model tutorial. Congratulations! Armed with this knowledge, you will find it easier to follow the rest of the amqp gem documentation as well as the rabbitmq.com documentation and the RabbitMQ mailing list.

To stay up to date with amqp gem development, follow @rubyamqp on Twitter and join our mailing list.

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