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| Management Console |
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| Metrics |
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| Security |
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| Masking Passwords |
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| Broker Plugins |
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| Resource Limits |
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| The JMS Bridge |
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| Client Reconnection and Session Reattachment |
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| Diverting and Splitting Message Flows |
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| Core Bridges |
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| Transformers |
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| Duplicate Message Detection |
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| Clusters |
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| Federation |
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| Address Federation |
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| Queue Federation |
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| High Availability and Failover |
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| Graceful Server Shutdown |
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| Libaio Native Libraries |
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| Thread management |
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| Embedded Web Server |
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| Logging |
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| REST Interface |
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| Embedding the Broker |
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| Apache Karaf |
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| Apache Tomcat |
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| Spring Integration |
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| CDI Integration |
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| Intercepting Operations |
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| Data Tools |
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| <h1> |
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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="https://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="general-concepts">General 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="https://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="https://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="https://en.wikipedia.org/wiki/Message_queue" target="_blank">message queue</a> messaging (also |
| known as <em>point-to-point messaging</em>) and <a href="https://en.wikipedia.org/wiki/Publish_subscribe" target="_blank">publish |
| subscribe</a> messaging. We'll |
| summarise them briefly here:</p> |
| <h3 id="point-to-point">Point-to-Point</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="https://en.wikipedia.org/wiki/ACID" target="_blank">ACID</a> properties.</p> |
| <h3 id="publish-subscribe">Publish-Subscribe</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="https://en.wikipedia.org/wiki/Java_Message_Service" target="_blank">JMS</a> is part of Oracle's |
| Java EE 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 <a href="using-jms.html">JMS 1.1 and JMS 2.0 client |
| implementation</a>.</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> |
| <p>Please see <a href="core.html">Core</a> for using the Core API with Apache ActiveMQ Artemis.</p> |
| <h3 id="restful-api">RESTful API</h3> |
| <p><a href="https://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="https://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> |
| <p>Please see <a href="amqp.html">AMQP</a> for using AMQP with Apache ActiveMQ Artemis.</p> |
| <h3 id="mqtt">MQTT</h3> |
| <p><a href="https://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> |
| <p>Please see <a href="mqtt.html">MQTT</a> for using MQTT with Apache ActiveMQ Artemis.</p> |
| <h3 id="stomp">STOMP</h3> |
| <p><a href="https://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="stomp.html">Stomp</a> for using STOMP with Apache ActiveMQ Artemis.</p> |
| <h3 id="openwire">OpenWire</h3> |
| <p>ActiveMQ 5.x defines its 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> |
| <p>Please see <a href="openwire.html">OpenWire</a> for using OpenWire 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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