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| <a href="." >Messaging Concepts</a> |
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| <h1 id="messaging-concepts">Messaging Concepts</h1> |
| <p>Apache ActiveMQ Artemis is an asynchronous messaging system, an example of <a href="http://en.wikipedia.org/wiki/Message_oriented_middleware" target="_blank">Message |
| Oriented |
| Middleware</a> , |
| we'll just call them messaging systems in the remainder of this book.</p> |
| <p>We'll first present a brief overview of what kind of things messaging |
| systems do, where they're useful and the kind of concepts you'll hear |
| about in the messaging world.</p> |
| <p>If you're already familiar with what a messaging system is and what it's |
| capable of, then you can skip this chapter.</p> |
| <h2 id="messaging-concepts">Messaging Concepts</h2> |
| <p>Messaging systems allow you to loosely couple heterogeneous systems |
| together, whilst typically providing reliability, transactions and many |
| other features.</p> |
| <p>Unlike systems based on a <a href="http://en.wikipedia.org/wiki/Remote_procedure_call" target="_blank">Remote Procedure |
| Call</a> (RPC) pattern, |
| messaging systems primarily use an asynchronous message passing pattern |
| with no tight relationship between requests and responses. Most |
| messaging systems also support a request-response mode but this is not a |
| primary feature of messaging systems.</p> |
| <p>Designing systems to be asynchronous from end-to-end allows you to |
| really take advantage of your hardware resources, minimizing the amount |
| of threads blocking on IO operations, and to use your network bandwidth |
| to its full capacity. With an RPC approach you have to wait for a |
| response for each request you make so are limited by the network round |
| trip time, or <em>latency</em> of your network. With an asynchronous system you |
| can pipeline flows of messages in different directions, so are limited |
| by the network <em>bandwidth</em> not the latency. This typically allows you to |
| create much higher performance applications.</p> |
| <p>Messaging systems decouple the senders of messages from the consumers of |
| messages. The senders and consumers of messages are completely |
| independent and know nothing of each other. This allows you to create |
| flexible, loosely coupled systems.</p> |
| <p>Often, large enterprises use a messaging system to implement a message |
| bus which loosely couples heterogeneous systems together. Message buses |
| often form the core of an <a href="http://en.wikipedia.org/wiki/Enterprise_service_bus" target="_blank">Enterprise Service |
| Bus</a>. (ESB). Using |
| a message bus to de-couple disparate systems can allow the system to |
| grow and adapt more easily. It also allows more flexibility to add new |
| systems or retire old ones since they don't have brittle dependencies on |
| each other.</p> |
| <h2 id="messaging-styles">Messaging styles</h2> |
| <p>Messaging systems normally support two main styles of asynchronous |
| messaging: <a href="http://en.wikipedia.org/wiki/Message_queue" target="_blank">message queue</a> |
| messaging (also known as <em>point-to-point messaging</em>) and <a href="http://en.wikipedia.org/wiki/Publish_subscribe" target="_blank">publish |
| subscribe</a> messaging. |
| We'll summarise them briefly here:</p> |
| <h3 id="the-message-queue-pattern">The Message Queue Pattern</h3> |
| <p>With this type of messaging you send a message to a queue. The message |
| is then typically persisted to provide a guarantee of delivery, then |
| some time later the messaging system delivers the message to a consumer. |
| The consumer then processes the message and when it is done, it |
| acknowledges the message. Once the message is acknowledged it disappears |
| from the queue and is not available to be delivered again. If the system |
| crashes before the messaging server receives an acknowledgement from the |
| consumer, then on recovery, the message will be available to be |
| delivered to a consumer again.</p> |
| <p>With point-to-point messaging, there can be many consumers on the queue |
| but a particular message will only ever be consumed by a maximum of one |
| of them. Senders (also known as <em>producers</em>) to the queue are completely |
| decoupled from receivers (also known as <em>consumers</em>) of the queue - they |
| do not know of each other's existence.</p> |
| <p>A classic example of point to point messaging would be an order queue in |
| a company's book ordering system. Each order is represented as a message |
| which is sent to the order queue. Let's imagine there are many front end |
| ordering systems which send orders to the order queue. When a message |
| arrives on the queue it is persisted - this ensures that if the server |
| crashes the order is not lost. Let's also imagine there are many |
| consumers on the order queue - each representing an instance of an order |
| processing component - these can be on different physical machines but |
| consuming from the same queue. The messaging system delivers each |
| message to one and only one of the ordering processing components. |
| Different messages can be processed by different order processors, but a |
| single order is only processed by one order processor - this ensures |
| orders aren't processed twice.</p> |
| <p>As an order processor receives a message, it fulfills the order, sends |
| order information to the warehouse system and then updates the order |
| database with the order details. Once it's done that it acknowledges the |
| message to tell the server that the order has been processed and can be |
| forgotten about. Often the send to the warehouse system, update in |
| database and acknowledgement will be completed in a single transaction |
| to ensure <a href="http://en.wikipedia.org/wiki/ACID" target="_blank">ACID</a> properties.</p> |
| <h3 id="the-publish-subscribe-pattern">The Publish-Subscribe Pattern</h3> |
| <p>With publish-subscribe messaging many senders can send messages to an |
| entity on the server, often called a <em>topic</em> (e.g. in the JMS world).</p> |
| <p>There can be many <em>subscriptions</em> on a topic, a subscription is just |
| another word for a consumer of a topic. Each subscription receives a |
| <em>copy</em> of <em>each</em> message sent to the topic. This differs from the |
| message queue pattern where each message is only consumed by a single |
| consumer.</p> |
| <p>Subscriptions can optionally be <em>durable</em> which means they retain a copy |
| of each message sent to the topic until the subscriber consumes them - |
| even if the server crashes or is restarted in between. Non-durable |
| subscriptions only last a maximum of the lifetime of the connection that |
| created them.</p> |
| <p>An example of publish-subscribe messaging would be a news feed. As news |
| articles are created by different editors around the world they are sent |
| to a news feed topic. There are many subscribers around the world who |
| are interested in receiving news items - each one creates a subscription |
| and the messaging system ensures that a copy of each news message is |
| delivered to each subscription.</p> |
| <h2 id="delivery-guarantees">Delivery guarantees</h2> |
| <p>A key feature of most messaging systems is <em>reliable messaging</em>. With |
| reliable messaging the server gives a guarantee that the message will be |
| delivered once and only once to each consumer of a queue or each durable |
| subscription of a topic, even in the event of system failure. This is |
| crucial for many businesses; e.g. you don't want your orders fulfilled |
| more than once or any of your orders to be lost.</p> |
| <p>In other cases you may not care about a once and only once delivery |
| guarantee and are happy to cope with duplicate deliveries or lost |
| messages - an example of this might be transient stock price updates - |
| which are quickly superseded by the next update on the same stock. The |
| messaging system allows you to configure which delivery guarantees you |
| require.</p> |
| <h2 id="transactions">Transactions</h2> |
| <p>Messaging systems typically support the sending and acknowledgement of |
| multiple messages in a single local transaction. Apache ActiveMQ Artemis also supports |
| the sending and acknowledgement of message as part of a large global |
| transaction - using the Java mapping of XA: JTA.</p> |
| <h2 id="durability">Durability</h2> |
| <p>Messages are either durable or non durable. Durable messages will be |
| persisted in permanent storage and will survive server failure or |
| restart. Non durable messages will not survive server failure or |
| restart. Examples of durable messages might be orders or trades, where |
| they cannot be lost. An example of a non durable message might be a |
| stock price update which is transitory and doesn't need to survive a |
| restart.</p> |
| <h2 id="messaging-apis-and-protocols">Messaging APIs and protocols</h2> |
| <p>How do client applications interact with messaging systems in order to |
| send and consume messages?</p> |
| <p>Several messaging systems provide their own proprietary APIs with which |
| the client communicates with the messaging system.</p> |
| <p>There are also some standard ways of operating with messaging systems |
| and some emerging standards in this space.</p> |
| <p>Let's take a brief look at these:</p> |
| <h3 id="java-message-service-jms">Java Message Service (JMS)</h3> |
| <p><a href="http://en.wikipedia.org/wiki/Java_Message_Service" target="_blank">JMS</a> is part of |
| Oracle's JEE specification. It's a Java API that encapsulates both message |
| queue and publish-subscribe messaging patterns. JMS is a lowest common |
| denominator specification - i.e. it was created to encapsulate common |
| functionality of the already existing messaging systems that were |
| available at the time of its creation.</p> |
| <p>JMS is a very popular API and is implemented by most messaging systems. |
| JMS is only available to clients running Java.</p> |
| <p>JMS does not define a standard wire format - it only defines a |
| programmatic API so JMS clients and servers from different vendors |
| cannot directly interoperate since each will use the vendor's own |
| internal wire protocol.</p> |
| <p>Apache ActiveMQ Artemis provides a fully compliant JMS 1.1 and JMS 2.0 API.</p> |
| <h3 id="system-specific-apis">System specific APIs</h3> |
| <p>Many systems provide their own programmatic API for which to interact |
| with the messaging system. The advantage of this it allows the full set |
| of system functionality to be exposed to the client application. API's |
| like JMS are not normally rich enough to expose all the extra features |
| that most messaging systems provide.</p> |
| <p>Apache ActiveMQ Artemis provides its own core client API for clients to use if they |
| wish to have access to functionality over and above that accessible via |
| the JMS API.</p> |
| <h3 id="restful-api">RESTful API</h3> |
| <p><a href="http://en.wikipedia.org/wiki/Representational_State_Transfer" target="_blank">REST</a> |
| approaches to messaging are showing a lot interest recently.</p> |
| <p>It seems plausible that API standards for cloud computing may converge |
| on a REST style set of interfaces and consequently a REST messaging |
| approach is a very strong contender for becoming the de-facto method for |
| messaging interoperability.</p> |
| <p>With a REST approach messaging resources are manipulated as resources |
| defined by a URI and typically using a simple set of operations on those |
| resources, e.g. PUT, POST, GET etc. REST approaches to messaging often |
| use HTTP as their underlying protocol.</p> |
| <p>The advantage of a REST approach with HTTP is in its simplicity and the |
| fact the internet is already tuned to deal with HTTP optimally.</p> |
| <p>Please see <a href="rest.html">Rest Interface</a> for using Apache ActiveMQ Artemis's RESTful interface.</p> |
| <h3 id="amqp">AMQP</h3> |
| <p><a href="http://en.wikipedia.org/wiki/AMQP" target="_blank">AMQP</a> is a specification for |
| interoperable messaging. It also defines a wire format, so any AMQP |
| client can work with any messaging system that supports AMQP. AMQP |
| clients are available in many different programming languages.</p> |
| <p>Apache ActiveMQ Artemis implements the <a href="https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=amqp" target="_blank">AMQP |
| 1.0</a> |
| specification. Any client that supports the 1.0 specification will be |
| able to interact with Apache ActiveMQ Artemis.</p> |
| <h3 id="mqtt">MQTT</h3> |
| <p><a href="http://mqtt.org/" target="_blank">MQTT</a> is a lightweight connectivity protocol. It is designed |
| to run in environments where device and networks are constrained. Out of the box |
| Apache ActiveMQ Artemis supports version MQTT 3.1.1. Any client supporting this |
| version of the protocol will work against Apache ActiveMQ Artemis.</p> |
| <h3 id="stomp">STOMP</h3> |
| <p><a href="http://stomp.github.io/" target="_blank">Stomp</a> is a very simple text protocol for |
| interoperating with messaging systems. It defines a wire format, so |
| theoretically any Stomp client can work with any messaging system that |
| supports Stomp. Stomp clients are available in many different |
| programming languages.</p> |
| <p>Please see <a href="interoperability.md">Stomp</a> for using STOMP with Apache ActiveMQ Artemis.</p> |
| <h3 id="openwire">OPENWIRE</h3> |
| <p>ActiveMQ 5.x defines it's own wire Protocol "OPENWIRE". In order to support |
| ActiveMQ 5.x clients, Apache ActiveMQ Artemis supports OPENWIRE. Any ActiveMQ 5.12.x |
| or higher can be used with Apache ActiveMQ Artemis.</p> |
| <h2 id="high-availability">High Availability</h2> |
| <p>High Availability (HA) means that the system should remain operational |
| after failure of one or more of the servers. The degree of support for |
| HA varies between various messaging systems.</p> |
| <p>Apache ActiveMQ Artemis provides automatic failover where your sessions are |
| automatically reconnected to the backup server on event of live server |
| failure.</p> |
| <p>For more information on HA, please see <a href="ha.html">High Availability and Failover</a>.</p> |
| <h2 id="clusters">Clusters</h2> |
| <p>Many messaging systems allow you to create groups of messaging servers |
| called <em>clusters</em>. Clusters allow the load of sending and consuming |
| messages to be spread over many servers. This allows your system to |
| scale horizontally by adding new servers to the cluster.</p> |
| <p>Degrees of support for clusters varies between messaging systems, with |
| some systems having fairly basic clusters with the cluster members being |
| hardly aware of each other.</p> |
| <p>Apache ActiveMQ Artemis provides very configurable state-of-the-art clustering model |
| where messages can be intelligently load balanced between the servers in |
| the cluster, according to the number of consumers on each node, and |
| whether they are ready for messages.</p> |
| <p>Apache ActiveMQ Artemis also has the ability to automatically redistribute messages |
| between nodes of a cluster to prevent starvation on any particular node.</p> |
| <p>For full details on clustering, please see <a href="clusters.html">Clusters</a>.</p> |
| <h2 id="bridges-and-routing">Bridges and routing</h2> |
| <p>Some messaging systems allow isolated clusters or single nodes to be |
| bridged together, typically over unreliable connections like a wide area |
| network (WAN), or the internet.</p> |
| <p>A bridge normally consumes from a queue on one server and forwards |
| messages to another queue on a different server. Bridges cope with |
| unreliable connections, automatically reconnecting when the connections |
| becomes available again.</p> |
| <p>Apache ActiveMQ Artemis bridges can be configured with filter expressions to only |
| forward certain messages, and transformation can also be hooked in.</p> |
| <p>Apache ActiveMQ Artemis also allows routing between queues to be configured in server |
| side configuration. This allows complex routing networks to be set up |
| forwarding or copying messages from one destination to another, forming |
| a global network of interconnected brokers.</p> |
| <p>For more information please see <a href="core-bridges.html">Core Bridges</a> and <a href="diverts.html">Diverting and Splitting Message Flows</a>.</p> |
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