311 lines
14 KiB
Markdown
311 lines
14 KiB
Markdown
# Messaging Concepts
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Apache ActiveMQ Artemis is an asynchronous messaging system, an example of
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[Message Oriented
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Middleware](https://en.wikipedia.org/wiki/Message-oriented_middleware) , we'll
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just call them messaging systems in the remainder of this book.
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We'll first present a brief overview of what kind of things messaging systems
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do, where they're useful and the kind of concepts you'll hear about in the
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messaging world.
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If you're already familiar with what a messaging system is and what it's
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capable of, then you can skip this chapter.
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## General Concepts
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Messaging systems allow you to loosely couple heterogeneous systems together,
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whilst typically providing reliability, transactions and many other features.
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Unlike systems based on a [Remote Procedure
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Call](https://en.wikipedia.org/wiki/Remote_procedure_call) (RPC) pattern,
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messaging systems primarily use an asynchronous message passing pattern with no
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tight relationship between requests and responses. Most messaging systems also
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support a request-response mode but this is not a primary feature of messaging
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systems.
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Designing systems to be asynchronous from end-to-end allows you to really take
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advantage of your hardware resources, minimizing the amount of threads blocking
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on IO operations, and to use your network bandwidth to its full capacity. With
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an RPC approach you have to wait for a response for each request you make so
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are limited by the network round trip time, or *latency* of your network. With
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an asynchronous system you can pipeline flows of messages in different
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directions, so are limited by the network *bandwidth* not the latency. This
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typically allows you to create much higher performance applications.
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Messaging systems decouple the senders of messages from the consumers of
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messages. The senders and consumers of messages are completely independent and
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know nothing of each other. This allows you to create flexible, loosely coupled
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systems.
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Often, large enterprises use a messaging system to implement a message bus
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which loosely couples heterogeneous systems together. Message buses often form
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the core of an [Enterprise Service
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Bus](https://en.wikipedia.org/wiki/Enterprise_service_bus). (ESB). Using a
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message bus to de-couple disparate systems can allow the system to grow and
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adapt more easily. It also allows more flexibility to add new systems or retire
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old ones since they don't have brittle dependencies on each other.
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## Messaging styles
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Messaging systems normally support two main styles of asynchronous messaging:
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[message queue](https://en.wikipedia.org/wiki/Message_queue) messaging (also
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known as *point-to-point messaging*) and [publish
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subscribe](https://en.wikipedia.org/wiki/Publish_subscribe) messaging. We'll
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summarise them briefly here:
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### Point-to-Point
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With this type of messaging you send a message to a queue. The message is then
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typically persisted to provide a guarantee of delivery, then some time later
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the messaging system delivers the message to a consumer. The consumer then
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processes the message and when it is done, it acknowledges the message. Once
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the message is acknowledged it disappears from the queue and is not available
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to be delivered again. If the system crashes before the messaging server
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receives an acknowledgement from the consumer, then on recovery, the message
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will be available to be delivered to a consumer again.
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With point-to-point messaging, there can be many consumers on the queue but a
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particular message will only ever be consumed by a maximum of one of them.
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Senders (also known as *producers*) to the queue are completely decoupled from
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receivers (also known as *consumers*) of the queue - they do not know of each
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other's existence.
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A classic example of point to point messaging would be an order queue in a
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company's book ordering system. Each order is represented as a message which is
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sent to the order queue. Let's imagine there are many front end ordering
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systems which send orders to the order queue. When a message arrives on the
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queue it is persisted - this ensures that if the server crashes the order is
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not lost. Let's also imagine there are many consumers on the order queue - each
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representing an instance of an order processing component - these can be on
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different physical machines but consuming from the same queue. The messaging
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system delivers each message to one and only one of the ordering processing
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components. Different messages can be processed by different order processors,
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but a single order is only processed by one order processor - this ensures
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orders aren't processed twice.
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As an order processor receives a message, it fulfills the order, sends order
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information to the warehouse system and then updates the order database with
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the order details. Once it's done that it acknowledges the message to tell the
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server that the order has been processed and can be forgotten about. Often the
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send to the warehouse system, update in database and acknowledgement will be
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completed in a single transaction to ensure
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[ACID](https://en.wikipedia.org/wiki/ACID) properties.
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### Publish-Subscribe
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With publish-subscribe messaging many senders can send messages to an entity on
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the server, often called a *topic* (e.g. in the JMS world).
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There can be many *subscriptions* on a topic, a subscription is just another
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word for a consumer of a topic. Each subscription receives a *copy* of *each*
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message sent to the topic. This differs from the message queue pattern where
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each message is only consumed by a single consumer.
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Subscriptions can optionally be *durable* which means they retain a copy of
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each message sent to the topic until the subscriber consumes them - even if the
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server crashes or is restarted in between. Non-durable subscriptions only last
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a maximum of the lifetime of the connection that created them.
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An example of publish-subscribe messaging would be a news feed. As news
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articles are created by different editors around the world they are sent to a
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news feed topic. There are many subscribers around the world who are interested
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in receiving news items - each one creates a subscription and the messaging
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system ensures that a copy of each news message is delivered to each
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subscription.
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## Delivery guarantees
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A key feature of most messaging systems is *reliable messaging*. With reliable
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messaging the server gives a guarantee that the message will be delivered once
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and only once to each consumer of a queue or each durable subscription of a
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topic, even in the event of system failure. This is crucial for many
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businesses; e.g. you don't want your orders fulfilled more than once or any of
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your orders to be lost.
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In other cases you may not care about a once and only once delivery guarantee
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and are happy to cope with duplicate deliveries or lost messages - an example
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of this might be transient stock price updates - which are quickly superseded
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by the next update on the same stock. The messaging system allows you to
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configure which delivery guarantees you require.
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## Transactions
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Messaging systems typically support the sending and acknowledgement of multiple
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messages in a single local transaction. Apache ActiveMQ Artemis also supports
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the sending and acknowledgement of message as part of a large global
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transaction - using the Java mapping of XA: JTA.
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## Durability
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Messages are either durable or non durable. Durable messages will be persisted
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in permanent storage and will survive server failure or restart. Non durable
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messages will not survive server failure or restart. Examples of durable
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messages might be orders or trades, where they cannot be lost. An example of a
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non durable message might be a stock price update which is transitory and
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doesn't need to survive a restart.
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## Messaging APIs and protocols
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How do client applications interact with messaging systems in order to send and
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consume messages?
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Several messaging systems provide their own proprietary APIs with which the
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client communicates with the messaging system.
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There are also some standard ways of operating with messaging systems and some
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emerging standards in this space.
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Let's take a brief look at these:
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### Java Message Service (JMS)
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[JMS](https://en.wikipedia.org/wiki/Java_Message_Service) is part of Oracle's
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Java EE specification. It's a Java API that encapsulates both message queue and
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publish-subscribe messaging patterns. JMS is a lowest common denominator
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specification - i.e. it was created to encapsulate common functionality of the
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already existing messaging systems that were available at the time of its
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creation.
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JMS is a very popular API and is implemented by most messaging systems. JMS is
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only available to clients running Java.
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JMS does not define a standard wire format - it only defines a programmatic API
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so JMS clients and servers from different vendors cannot directly interoperate
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since each will use the vendor's own internal wire protocol.
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Apache ActiveMQ Artemis provides a fully compliant [JMS 1.1 and JMS 2.0 client
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implementation](using-jms.md).
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### System specific APIs
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Many systems provide their own programmatic API for which to interact with the
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messaging system. The advantage of this it allows the full set of system
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functionality to be exposed to the client application. API's like JMS are not
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normally rich enough to expose all the extra features that most messaging
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systems provide.
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Apache ActiveMQ Artemis provides its own core client API for clients to use if
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they wish to have access to functionality over and above that accessible via
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the JMS API.
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Please see [Core](core.md) for using the Core API with Apache ActiveMQ Artemis.
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### RESTful API
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[REST](https://en.wikipedia.org/wiki/Representational_State_Transfer)
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approaches to messaging are showing a lot interest recently.
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It seems plausible that API standards for cloud computing may converge on a
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REST style set of interfaces and consequently a REST messaging approach is a
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very strong contender for becoming the de-facto method for messaging
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interoperability.
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With a REST approach messaging resources are manipulated as resources defined
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by a URI and typically using a simple set of operations on those resources,
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e.g. PUT, POST, GET etc. REST approaches to messaging often use HTTP as their
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underlying protocol.
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The advantage of a REST approach with HTTP is in its simplicity and the fact
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the internet is already tuned to deal with HTTP optimally.
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Please see [Rest Interface](rest.md) for using Apache ActiveMQ Artemis's
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RESTful interface.
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### AMQP
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[AMQP](https://en.wikipedia.org/wiki/AMQP) is a specification for interoperable
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messaging. It also defines a wire format, so any AMQP client can work with any
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messaging system that supports AMQP. AMQP clients are available in many
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different programming languages.
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Apache ActiveMQ Artemis implements the [AMQP
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1.0](https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=amqp)
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specification. Any client that supports the 1.0 specification will be able to
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interact with Apache ActiveMQ Artemis.
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Please see [AMQP](amqp.md) for using AMQP with Apache ActiveMQ Artemis.
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### MQTT
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[MQTT](https://mqtt.org/) is a lightweight connectivity protocol. It is
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designed to run in environments where device and networks are constrained. Out
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of the box Apache ActiveMQ Artemis supports version MQTT 3.1.1. Any client
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supporting this version of the protocol will work against Apache ActiveMQ
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Artemis.
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Please see [MQTT](mqtt.md) for using MQTT with Apache ActiveMQ Artemis.
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### STOMP
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[Stomp](https://stomp.github.io/) is a very simple text protocol for
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interoperating with messaging systems. It defines a wire format, so
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theoretically any Stomp client can work with any messaging system that
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supports Stomp. Stomp clients are available in many different
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programming languages.
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Please see [Stomp](stomp.md) for using STOMP with Apache ActiveMQ Artemis.
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### OpenWire
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ActiveMQ 5.x defines its own wire protocol: OpenWire. In order to support
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ActiveMQ 5.x clients, Apache ActiveMQ Artemis supports OpenWire. Any ActiveMQ
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5.12.x or higher can be used with Apache ActiveMQ Artemis.
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Please see [OpenWire](openwire.md) for using OpenWire with Apache ActiveMQ
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Artemis.
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## High Availability
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High Availability (HA) means that the system should remain operational after
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failure of one or more of the servers. The degree of support for HA varies
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between various messaging systems.
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Apache ActiveMQ Artemis provides automatic failover where your sessions are
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automatically reconnected to the backup server on event of live server failure.
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For more information on HA, please see [High Availability and Failover](ha.md).
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## Clusters
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Many messaging systems allow you to create groups of messaging servers called
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*clusters*. Clusters allow the load of sending and consuming messages to be
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spread over many servers. This allows your system to scale horizontally by
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adding new servers to the cluster.
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Degrees of support for clusters varies between messaging systems, with some
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systems having fairly basic clusters with the cluster members being hardly
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aware of each other.
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Apache ActiveMQ Artemis provides very configurable state-of-the-art clustering
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model where messages can be intelligently load balanced between the servers in
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the cluster, according to the number of consumers on each node, and whether
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they are ready for messages.
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Apache ActiveMQ Artemis also has the ability to automatically redistribute
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messages between nodes of a cluster to prevent starvation on any particular
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node.
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For full details on clustering, please see [Clusters](clusters.md).
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## Bridges and routing
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Some messaging systems allow isolated clusters or single nodes to be bridged
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together, typically over unreliable connections like a wide area network (WAN),
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or the internet.
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A bridge normally consumes from a queue on one server and forwards messages to
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another queue on a different server. Bridges cope with unreliable connections,
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automatically reconnecting when the connections becomes available again.
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Apache ActiveMQ Artemis bridges can be configured with filter expressions to
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only forward certain messages, and transformation can also be hooked in.
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Apache ActiveMQ Artemis also allows routing between queues to be configured in
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server side configuration. This allows complex routing networks to be set up
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forwarding or copying messages from one destination to another, forming a
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global network of interconnected brokers.
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For more information please see [Core Bridges](core-bridges.md) and [Diverting
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and Splitting Message Flows](diverts.md).
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