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| Jakarta |
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| Persistence |
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| Configuring Transports |
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| Detecting Dead Connections |
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| Detecting Slow Consumers |
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| Detecting Broker Issues (Critical Analysis) |
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| Resource Manager Configuration |
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| Flow Control |
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| Guarantees of sends and commits |
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| Message Redelivery and Undelivered Messages |
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| Message Expiry |
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| Large Messages |
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| Paging |
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| Scheduled Messages |
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| Last-Value Queues |
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| Non-Destructive Queues |
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| Ring Queues |
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| Retroactive Addresses |
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| Exclusive Queues |
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| Message Grouping |
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| Consumer Priority |
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| Extra Acknowledge Modes |
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| Management |
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| Management Console |
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| Metrics |
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| Security |
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| <a href="masking-passwords.html"> |
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| Masking Passwords |
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| </a> |
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| Broker Plugins |
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| </a> |
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| <a href="resource-limits.html"> |
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| Resource Limits |
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| </a> |
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| <a href="jms-bridge.html"> |
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| The JMS Bridge |
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| </a> |
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| <li class="chapter " data-level="1.55" data-path="client-reconnection.html"> |
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| <a href="client-reconnection.html"> |
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| Client Reconnection and Session Reattachment |
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| </a> |
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| <a href="diverts.html"> |
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| Diverting and Splitting Message Flows |
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| </a> |
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| <a href="core-bridges.html"> |
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| Core Bridges |
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| <a href="transformers.html"> |
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| Transformers |
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| </a> |
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| <li class="chapter " data-level="1.59" data-path="duplicate-detection.html"> |
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| <a href="duplicate-detection.html"> |
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| Duplicate Message Detection |
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| </a> |
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| <li class="chapter " data-level="1.60" data-path="clusters.html"> |
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| <a href="clusters.html"> |
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| Clusters |
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| </a> |
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| <li class="chapter " data-level="1.61" data-path="federation.html"> |
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| <a href="federation.html"> |
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| Federation |
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| <li class="chapter " data-level="1.61.1" data-path="federation-address.html"> |
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| <a href="federation-address.html"> |
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| Address Federation |
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| </a> |
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| <li class="chapter " data-level="1.61.2" data-path="federation-queue.html"> |
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| <a href="federation-queue.html"> |
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| Queue Federation |
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| </a> |
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| <li class="chapter " data-level="1.62" data-path="ha.html"> |
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| High Availability and Failover |
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| </a> |
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| Connection Routers |
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| </a> |
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| Graceful Server Shutdown |
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| </a> |
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| <li class="chapter " data-level="1.65" data-path="libaio.html"> |
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| <a href="libaio.html"> |
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| Libaio Native Libraries |
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| </a> |
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| <li class="chapter " data-level="1.66" data-path="thread-pooling.html"> |
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| <a href="thread-pooling.html"> |
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| Thread management |
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| </a> |
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| <a href="web-server.html"> |
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| Embedded Web Server |
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| </a> |
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| <a href="logging.html"> |
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| Logging |
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| </a> |
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| <a href="." >MQTT</a> |
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| <h1 id="mqtt">MQTT</h1> |
| <p>MQTT is a light weight, client to server, publish / subscribe messaging |
| protocol. MQTT has been specifically designed to reduce transport overhead |
| (and thus network traffic) and code footprint on client devices. For this |
| reason MQTT is ideally suited to constrained devices such as sensors and |
| actuators and is quickly becoming the defacto standard communication protocol |
| for IoT.</p> |
| <p>Apache ActiveMQ Artemis supports the following MQTT versions (with links to |
| their respective specifications):</p> |
| <ul> |
| <li><a href="https://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mqtt-v3r1.html" target="_blank">3.1</a></li> |
| <li><a href="https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html" target="_blank">3.1.1</a></li> |
| <li><a href="https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html" target="_blank">5.0</a></li> |
| </ul> |
| <p>By default there are <code>acceptor</code> elements configured to accept MQTT connections |
| on ports <code>61616</code> and <code>1883</code>.</p> |
| <p>See the general <a href="protocols-interoperability.html">Protocols and Interoperability</a> |
| chapter for details on configuring an <code>acceptor</code> for MQTT.</p> |
| <p>Refer to the MQTT examples for a look at some of this functionality in action.</p> |
| <h2 id="mqtt-quality-of-service">MQTT Quality of Service</h2> |
| <p>MQTT offers 3 quality of service levels.</p> |
| <p>Each message (or topic subscription) can define a quality of service that is |
| associated with it. The quality of service level defined on a topic is the |
| maximum level a client is willing to accept. The quality of service level on a |
| message is the desired quality of service level for this message. The broker |
| will attempt to deliver messages to subscribers at the highest quality of |
| service level based on what is defined on the message and topic subscription.</p> |
| <p>Each quality of service level offers a level of guarantee by which a message is |
| sent or received:</p> |
| <ul> |
| <li><p>QoS 0: <code>AT MOST ONCE</code></p> |
| <p>Guarantees that a particular message is only ever received by the subscriber |
| a maximum of one time. This does mean that the message may never arrive. The |
| sender and the receiver will attempt to deliver the message, but if something |
| fails and the message does not reach its destination (say due to a network |
| connection) the message may be lost. This QoS has the least network traffic |
| overhead and the least burden on the client and the broker and is often useful |
| for telemetry data where it doesn't matter if some of the data is lost.</p> |
| </li> |
| <li><p>QoS 1: <code>AT LEAST ONCE</code></p> |
| <p>Guarantees that a message will reach its intended recipient one or more |
| times. The sender will continue to send the message until it receives an |
| acknowledgment from the recipient, confirming it has received the message. The |
| result of this QoS is that the recipient may receive the message multiple |
| times, and also increases the network overhead than QoS 0, (due to acks). In |
| addition more burden is placed on the sender as it needs to store the message |
| and retry should it fail to receive an ack in a reasonable time.</p> |
| </li> |
| <li><p>QoS 2: <code>EXACTLY ONCE</code></p> |
| <p>The most costly of the QoS (in terms of network traffic and burden on sender |
| and receiver) this QoS will ensure that the message is received by a recipient |
| exactly one time. This ensures that the receiver never gets any duplicate |
| copies of the message and will eventually get it, but at the extra cost of |
| network overhead and complexity required on the sender and receiver.</p> |
| </li> |
| </ul> |
| <h2 id="mqtt-retain-messages">MQTT Retain Messages</h2> |
| <p>MQTT has an interesting feature in which messages can be "retained" for a |
| particular address. This means that once a retain message has been sent to an |
| address, any new subscribers to that address will receive the last sent retain |
| message before any others messages, this happens even if the retained message |
| was sent before a client has connected or subscribed. An example of where this |
| feature might be useful is in environments such as IoT where devices need to |
| quickly get the current state of a system when they are on boarded into a |
| system.</p> |
| <h2 id="will-messages">Will Messages</h2> |
| <p>A will message can be sent when a client initially connects to a broker. |
| Clients are able to set a "will message" as part of the connect packet. If the |
| client abnormally disconnects, say due to a device or network failure the |
| broker will proceed to publish the will message to the specified address (as |
| defined also in the connect packet). Other subscribers to the will topic will |
| receive the will message and can react accordingly. This feature can be useful |
| in an IoT style scenario to detect errors across a potentially large scale |
| deployment of devices.</p> |
| <h2 id="debug-logging">Debug Logging</h2> |
| <p>Detailed protocol logging (e.g. packets in/out) can be activated by turning |
| on <code>TRACE</code> logging for <code>org.apache.activemq.artemis.core.protocol.mqtt</code>. Follow |
| <a href="logging.html#activating-trace-for-a-specific-logger">these steps</a> to configure |
| logging appropriately.</p> |
| <p>The MQTT specification doesn't dictate the format of the payloads which clients |
| publish. As far as the broker is concerned a payload is just an array of |
| bytes. However, to facilitate logging the broker will encode the payloads as |
| UTF-8 strings and print them up to 256 characters. Payload logging is limited |
| to avoid filling the logs with potentially hundreds of megabytes of unhelpful |
| information.</p> |
| <h2 id="wildcard-subscriptions">Wildcard subscriptions</h2> |
| <p>MQTT addresses are hierarchical much like a file system, and they use a special |
| character (i.e. <code>/</code> by default) to separate hierarchical levels. Subscribers |
| are able to subscribe to specific topics or to whole branches of a hierarchy.</p> |
| <p>To subscribe to branches of an address hierarchy a subscriber can use wild |
| cards. There are 2 types of wildcards in MQTT:</p> |
| <ul> |
| <li><p><strong>Multi level</strong> (<code>#</code>)</p> |
| <p>Adding this wildcard to an address would match all branches of the address |
| hierarchy under a specified node. For example: <code>/uk/#</code> Would match |
| <code>/uk/cities</code>, <code>/uk/cities/newcastle</code> and also <code>/uk/rivers/tyne</code>. Subscribing to |
| an address <code>#</code> would result in subscribing to all topics in the broker. This |
| can be useful, but should be done so with care since it has significant |
| performance implications.</p> |
| </li> |
| <li><p><strong>Single level</strong> (<code>+</code>)</p> |
| <p>Matches a single level in the address hierarchy. For example <code>/uk/+/stores</code> |
| would match <code>/uk/newcastle/stores</code> but not <code>/uk/cities/newcastle/stores</code>.</p> |
| </li> |
| </ul> |
| <p>These MQTT-specific wildcards are automatically <em>translated</em> into the wildcard |
| syntax used by ActiveMQ Artemis. These wildcards are configurable. See the |
| <a href="wildcard-syntax.html#customizing-the-syntax">Wildcard Syntax</a> chapter for details about |
| how to configure custom wildcards. </p> |
| <h2 id="web-sockets">Web Sockets</h2> |
| <p>Apache ActiveMQ Artemis also supports MQTT over <a href="https://html.spec.whatwg.org/multipage/web-sockets.html" target="_blank">Web |
| Sockets</a>. Modern web |
| browsers which support Web Sockets can send and receive MQTT messages.</p> |
| <p>MQTT over Web Sockets is supported via a normal MQTT acceptor:</p> |
| <pre><code class="lang-xml"><span class="hljs-tag"><<span class="hljs-name">acceptor</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"mqtt-ws-acceptor"</span>></span>tcp://localhost:1883?protocols=MQTT<span class="hljs-tag"></<span class="hljs-name">acceptor</span>></span> |
| </code></pre> |
| <p>With this configuration, Apache ActiveMQ Artemis will accept MQTT connections |
| over Web Sockets on the port <code>1883</code>. Web browsers can then connect to |
| <code>ws://<server>:1883</code> using a Web Socket to send and receive MQTT messages.</p> |
| <h2 id="automatic-subscription-clean-up">Automatic Subscription Clean-up</h2> |
| <p>Sometimes MQTT clients don't clean up their subscriptions. In such situations |
| the <code>auto-delete-queues-delay</code> and <code>auto-delete-queues-message-count</code> |
| address-settings can be used to clean up the abandoned subscription queues. |
| However, the MQTT session meta-data is still present in memory and needs to be |
| cleaned up as well. The URL parameter <code>defaultMqttSessionExpiryInterval</code> can be |
| configured on the MQTT <code>acceptor</code> to deal with this situation.</p> |
| <p>MQTT 5 added a new <a href="https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901048" target="_blank">session expiry interval</a> |
| property with the same basic semantics. The broker will use the client's value |
| for this property if it is set. If it is not set then it will apply the |
| <code>defaultMqttSessionExpiryInterval</code>.</p> |
| <p>The default <code>defaultMqttSessionExpiryInterval</code> is <code>-1</code> which means no MQTT 3.x |
| session states will be expired and no MQTT 5 session states which do not pass |
| their own session expiry interval will be expired. Otherwise it represents the |
| number of <strong>seconds</strong> which must elapse after the client has disconnected |
| before the broker will remove the session state.</p> |
| <p>MQTT session state is scanned every 5,000 milliseconds by default. This can be |
| changed using the <code>mqtt-session-scan-interval</code> element set in the <code>core</code> section |
| of <code>broker.xml</code>.</p> |
| <h2 id="flow-control">Flow Control</h2> |
| <p>MQTT 5 introduced a simple form of <a href="https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Flow_Control" target="_blank">flow control</a>. |
| In short, a broker can tell a client how many QoS 1 & 2 messages it can receive |
| before being acknowledged and vice versa.</p> |
| <p>This is controlled on the broker by setting the <code>receiveMaximum</code> URL parameter on |
| the MQTT <code>acceptor</code> in <code>broker.xml</code>.</p> |
| <p>The default value is <code>65535</code> (the maximum value of the 2-byte integer used by |
| MQTT). </p> |
| <p>A value of <code>0</code> is prohibited by the MQTT 5 specification.</p> |
| <p>A value of <code>-1</code> will prevent the broker from informing the client of any receive |
| maximum which means flow-control will be disabled from clients to the broker. |
| This is effectively the same as setting the value to <code>65535</code>, but reduces the size |
| of the <code>CONNACK</code> packet by a few bytes.</p> |
| <h2 id="topic-alias-maximum">Topic Alias Maximum</h2> |
| <p>MQTT 5 introduced <a href="https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Topic_Alias" target="_blank">topic aliasing</a>. |
| This is an optimization for the size of <code>PUBLISH</code> control packets as a 2-byte |
| integer value can now be substituted for the <em>name</em> of the topic which can |
| potentially be quite long.</p> |
| <p>Both the client and the broker can inform each other about the <em>maximum</em> alias |
| value they support (i.e. how many different aliases they support). This is |
| controlled on the broker using the <code>topicAliasMaximum</code> URL parameter on the |
| <code>acceptor</code> used by the MQTT client.</p> |
| <p>The default value is <code>65535</code> (the maximum value of the 2-byte integer used by |
| MQTT). </p> |
| <p>A value of <code>0</code> will disable topic aliasing from clients to the broker.</p> |
| <p>A value of <code>-1</code> will prevent the broker from informing the client of any topic |
| alias maximum which means aliasing will be disabled from clients to the broker. |
| This is effectively the same as setting the value to <code>0</code>, but reduces the size |
| of the <code>CONNACK</code> packet by a few bytes.</p> |
| <h2 id="maximum-packet-size">Maximum Packet Size</h2> |
| <p>MQTT 5 introduced the <a href="https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901086" target="_blank">maximum packet size</a>. |
| This is the maximum packet size the server or client is willing to accept.</p> |
| <p>This is controlled on the broker by setting the <code>maximumPacketSize</code> URL parameter |
| on the MQTT <code>acceptor</code> in <code>broker.xml</code>.</p> |
| <p>The default value is <code>268435455</code> (i.e. 256MB - the maximum value of the variable |
| byte integer used by MQTT). </p> |
| <p>A value of <code>0</code> is prohibited by the MQTT 5 specification.</p> |
| <p>A value of <code>-1</code> will prevent the broker from informing the client of any maximum |
| packet size which means no limit will be enforced on the size of incoming packets. |
| This also reduces the size of the <code>CONNACK</code> packet by a few bytes.</p> |
| <h2 id="server-keep-alive">Server Keep Alive</h2> |
| <p>All MQTT versions support a connection keep alive value defined by the <em>client</em>. |
| MQTT 5 introduced a <a href="https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901094" target="_blank">server keep alive</a> |
| value so that a broker can define the value that the client should use. The |
| primary use of the server keep alive is for the server to inform the client that |
| it will disconnect the client for inactivity sooner than the keep alive specified |
| by the client.</p> |
| <p>This is controlled on the broker by setting the <code>serverKeepAlive</code> URL parameter |
| on the MQTT <code>acceptor</code> in <code>broker.xml</code>.</p> |
| <p>The default value is <code>60</code> and is measured in <strong>seconds</strong>.</p> |
| <p>A value of <code>0</code> completely disables keep alives no matter the client's keep alive |
| value. This is <strong>not recommended</strong> because disabling keep alives is generally |
| considered dangerous since it could lead to resource exhaustion.</p> |
| <p>A value of <code>-1</code> means the broker will <em>always</em> accept the client's keep alive |
| value (even if that value is <code>0</code>).</p> |
| <p>Any other value means the <code>serverKeepAlive</code> will be applied if it is <em>less than</em> |
| the client's keep alive value <strong>unless</strong> the client's keep alive value is <code>0</code> in |
| which case the <code>serverKeepAlive</code> is applied. This is because a value of <code>0</code> would |
| disable keep alives and disabling keep alives is generally considered dangerous |
| since it could lead to resource exhaustion.</p> |
| <h2 id="enhanced-authentication">Enhanced Authentication</h2> |
| <p>MQTT 5 introduced <a href="https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901256" target="_blank">enhanced authentication</a> |
| which extends the existing name & password authentication to include challenge / |
| response style authentication.</p> |
| <p>However, there are currently no challenge / response mechanisms implemented so if |
| a client passes the "Authentication Method" property in its <code>CONNECT</code> packet it will |
| receive a <code>CONNACK</code> with a reason code of <code>0x8C</code> (i.e. bad authentication method) |
| and the network connection will be closed.</p> |
| <h2 id="publish-authorization-failures">Publish Authorization Failures</h2> |
| <p>The MQTT 3.1.1 specification is ambiguous regarding the broker's behavior when |
| a <code>PUBLISH</code> packet fails due to a lack of authorization. In <a href="http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718042" target="_blank">section 3.3.5</a> |
| it says:</p> |
| <blockquote> |
| <p>If a Server implementation does not authorize a PUBLISH to be performed by a |
| Client; it has no way of informing that Client. It MUST either make a positive |
| acknowledgement, according to the normal QoS rules, or close the Network |
| Connection</p> |
| </blockquote> |
| <p>By default the broker will close the network connection. However if you'd rather |
| have the broker make a positive acknowledgement then set the URL parameter |
| <code>closeMqttConnectionOnPublishAuthorizationFailure</code> to <code>false</code> on the relevant |
| MQTT <code>acceptor</code> in <code>broker.xml</code>, e.g.:</p> |
| <pre><code class="lang-xml"><span class="hljs-tag"><<span class="hljs-name">acceptor</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"mqtt"</span>></span>tcp://0.0.0:1883?protocols=MQTT;closeMqttConnectionOnPublishAuthorizationFailure=false<span class="hljs-tag"></<span class="hljs-name">acceptor</span>></span> |
| </code></pre> |
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