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<a href='/hono/docs/1.7/'>Documentation</a> > <a href='/hono/docs/1.7/user-guide/'>User Guide</a> > MQTT Adapter
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<ul>
<li><a href="#authentication">Authentication</a>
<ul>
<li><a href="#client-certificate">Client Certificate</a></li>
<li><a href="#usernamepassword">Username/Password</a></li>
</ul>
</li>
<li><a href="#resource-limit-checks">Resource Limit Checks</a>
<ul>
<li><a href="#connection-limits">Connection Limits</a></li>
<li><a href="#connection-duration-limits">Connection Duration Limits</a></li>
<li><a href="#message-limits">Message Limits</a></li>
</ul>
</li>
<li><a href="#connection-events">Connection Events</a></li>
<li><a href="#publishing-telemetry-data">Publishing Telemetry Data</a></li>
<li><a href="#publish-telemetry-data-authenticated-device">Publish Telemetry Data (authenticated Device)</a></li>
<li><a href="#publish-telemetry-data-unauthenticated-device">Publish Telemetry Data (unauthenticated Device)</a></li>
<li><a href="#publish-telemetry-data-authenticated-gateway">Publish Telemetry Data (authenticated Gateway)</a></li>
<li><a href="#publishing-events">Publishing Events</a></li>
<li><a href="#publish-an-event-authenticated-device">Publish an Event (authenticated Device)</a></li>
<li><a href="#publish-an-event-unauthenticated-device">Publish an Event (unauthenticated Device)</a></li>
<li><a href="#publish-an-event-authenticated-gateway">Publish an Event (authenticated Gateway)</a></li>
<li><a href="#command--control">Command &amp; Control</a>
<ul>
<li><a href="#receiving-commands-authenticated-device">Receiving Commands (authenticated Device)</a></li>
<li><a href="#receiving-commands-unauthenticated-device">Receiving Commands (unauthenticated Device)</a></li>
<li><a href="#receiving-commands-authenticated-gateway">Receiving Commands (authenticated Gateway)</a></li>
<li><a href="#sending-a-response-to-a-command-authenticated-device">Sending a Response to a Command (authenticated Device)</a></li>
<li><a href="#sending-a-response-to-a-command-unauthenticated-device">Sending a Response to a Command (unauthenticated Device)</a></li>
<li><a href="#sending-a-response-to-a-command-authenticated-gateway">Sending a Response to a Command (authenticated Gateway)</a></li>
</ul>
</li>
<li><a href="#error-reporting-via-error-topic">Error Reporting via Error Topic</a>
<ul>
<li><a href="#receiving-error-messages-authenticated-device">Receiving Error Messages (authenticated Device)</a></li>
<li><a href="#receiving-error-messages-unauthenticated-device">Receiving Error Messages (unauthenticated Device)</a></li>
<li><a href="#receiving-error-messages-authenticated-gateway">Receiving Error Messages (authenticated Gateway)</a></li>
<li><a href="#error-message-payload">Error Message Payload</a></li>
</ul>
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<li><a href="#error-handling">Error Handling</a></li>
<li><a href="#custom-message-mapping">Custom Message Mapping</a></li>
<li><a href="#downstream-meta-data">Downstream Meta Data</a>
<ul>
<li><a href="#event-message-time-to-live">Event Message Time-to-live</a></li>
</ul>
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<li><a href="#tenant-specific-configuration">Tenant specific Configuration</a></li>
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<h1>
MQTT Adapter
</h1>
<p>The MQTT protocol adapter exposes an MQTT topic hierarchy for publishing telemetry data and events to downstream consumers and for receiving commands from applications and sending back responses.</p>
<p>The MQTT adapter is <strong>not</strong> a general purpose MQTT broker. In particular the adapter</p>
<ul>
<li>supports MQTT 3.1.1 only.</li>
<li>does not maintain session state for clients and thus always sets the <em>session present</em> flag in its CONNACK packet to <code>0</code>, regardless of the value of the <em>clean session</em> flag provided in a client&rsquo;s CONNECT packet.</li>
<li>ignores any <em>Will</em> included in a client&rsquo;s CONNECT packet.</li>
<li>only supports topic names/filters for devices to publish and subscribe to that are specific to Hono&rsquo;s functionality as described in the following sections.</li>
<li>does not support <em>retaining</em> messages. However, if an event or telemetry message&rsquo;s <em>retain</em> flag is set to <code>1</code> then the corresponding AMQP 1.0 message being sent downstream by the adapter will contain an <em>x-opt-retain</em> message annotation containing the boolean value <code>true</code>. A downstream consumer may then react according to the presence of this annotation.</li>
</ul>
<h2 id="authentication">Authentication</h2>
<p>The MQTT adapter by default requires clients (devices or gateway components) to authenticate during connection establishment.
The adapter supports both the authentication based on the username/password provided in an MQTT CONNECT packet as well as client
certificate based authentication as part of a TLS handshake for that purpose.</p>
<p>The adapter tries to authenticate the device using these mechanisms in the following order</p>
<h3 id="client-certificate">Client Certificate</h3>
<p>When a device uses a client certificate for authentication during the TLS handshake, the adapter tries to determine the tenant
that the device belongs to based on the <em>issuer DN</em> contained in the certificate.
In order for the lookup to succeed, the tenant&rsquo;s trust anchor needs to be configured by means of registering the
<a href="/hono/docs/1.7/api/tenant/#tenant-information-format">trusted certificate authority</a>. The device&rsquo;s client certificate will then be
validated using the registered trust anchor, thus implicitly establishing the tenant that the device belongs to.
In a second step, the adapter uses the Credentials API&rsquo;s <em>get</em> operation to retrieve the credentials on record, including the client
certificate&rsquo;s <em>subject DN</em> as the <em>auth-id</em>, <code>x509-cert</code> as the <em>type</em> of secret and the MQTT client identifier as <em>client-id</em> in the
request payload.</p>
<p><strong>NB</strong> The adapter needs to be <a href="/hono/docs/1.7/admin-guide/secure_communication/#mqtt-adapter">configured for TLS</a> in order to support this mechanism.</p>
<h3 id="usernamepassword">Username/Password</h3>
<p>When a device wants to authenticate using this mechanism, it needs to provide a <em>username</em> and a <em>password</em> in the MQTT <em>CONNECT</em> packet
it sends in order to initiate the connection. The <em>username</em> must have the form <em>auth-id@tenant</em>, e.g. <code>sensor1@DEFAULT_TENANT</code>.
The adapter verifies the credentials provided by the client against the credentials that the
<a href="/hono/docs/1.7/admin-guide/common-config/#credentials-service-connection-configuration">configured Credentials service</a> has on record for the client.
The adapter uses the Credentials API&rsquo;s <em>get</em> operation to retrieve the credentials on record, including the <em>tenant</em> and <em>auth-id</em> provided
by the client in the <em>username</em>, <code>hashed-password</code> as the <em>type</em> of secret and the MQTT client identifier as <em>client-id</em> in the request payload.</p>
<p>The examples below refer to devices <code>4711</code> and <code>gw-1</code> of tenant <code>DEFAULT_TENANT</code> using <em>auth-ids</em> <code>sensor1</code> and <code>gw1</code> and corresponding passwords.
The example deployment as described in the <a href="/hono/docs/1.7/deployment/">Deployment Guides</a> comes pre-configured with the corresponding entities in its device registry component.</p>
<p><strong>NB</strong> There is a subtle difference between the <em>device identifier</em> (<em>device-id</em>) and the <em>auth-id</em> a device uses for authentication.
See <a href="/hono/docs/1.7/concepts/device-identity/">Device Identity</a> for a discussion of the concepts.</p>
<h2 id="resource-limit-checks">Resource Limit Checks</h2>
<p>The adapter performs additional checks regarding <a href="/hono/docs/1.7/concepts/resource-limits/">resource limits</a> when a client tries to connect and/or
send a message to the adapter.</p>
<h3 id="connection-limits">Connection Limits</h3>
<p>The adapter rejects a client’s connection attempt with return code</p>
<ul>
<li><code>0x03</code> (<em>Connection Refused: server unavailable</em>), if the maximum number of connections per protocol adapter instance is reached</li>
<li><code>0x05</code> (<em>Connection Refused: not authorized</em>), if the maximum number of simultaneously connected devices for the tenant is reached.</li>
</ul>
<h3 id="connection-duration-limits">Connection Duration Limits</h3>
<p>The adapter rejects a client’s connection attempt with return code <code>0x05</code> (<em>Connection Refused: not authorized</em>), if the
<a href="/hono/docs/1.7/concepts/resource-limits/#connection-duration-limit">connection duration limit</a> that has been configured for the client’s tenant is exceeded.</p>
<h3 id="message-limits">Message Limits</h3>
<p>The adapter</p>
<ul>
<li>rejects a client&rsquo;s connection attempt with return code <code>0x05</code> (<em>Connection Refused: not authorized</em>),</li>
<li>discards any MQTT PUBLISH packet containing telemetry data or an event that is sent by a client and</li>
<li>rejects any AMQP 1.0 message containing a command sent by a north bound application</li>
</ul>
<p>if the <a href="/hono/docs/1.7/concepts/resource-limits/">message limit</a> that has been configured for the device’s tenant is exceeded.</p>
<h2 id="connection-events">Connection Events</h2>
<p>The adapter can emit <a href="/hono/docs/1.7/api/event/#connection-event">Connection Events</a> for client connections being established and/or terminated.
Please refer to the <a href="/hono/docs/1.7/admin-guide/common-config/#connection-event-producer-configuration">common configuration options</a>
for details regarding how to enable this behavior.</p>
<p>The adapter includes the <em>client identifier</em> from the client&rsquo;s MQTT CONNECT packet as the Connection Event&rsquo;s <em>remote-id</em>.</p>
<h2 id="publishing-telemetry-data">Publishing Telemetry Data</h2>
<p>The MQTT adapter supports the publishing of telemetry data by means of MQTT <em>PUBLISH</em> packets using either QoS 0 or QoS 1.
Using QoS 1 will result in the adapter sending an MQTT <em>PUBACK</em> packet to the client once the message has been settled with the <em>accepted</em> outcome by the AMQP 1.0 Messaging Network.</p>
<p>This requires that</p>
<ul>
<li>the AMQP 1.0 Messaging Network has capacity to process telemetry messages for the client&rsquo;s tenant and</li>
<li>the messages published by the client comply with the format defined by the Telemetry API.</li>
</ul>
<p>The protocol adapter checks the configured <a href="/hono/docs/1.7/concepts/resource-limits/">message limit</a> before accepting any telemetry messages. An exceeded message limit will cause an error.</p>
<p>Any kind of error when processing an incoming telemetry message will be reported back to the client if the client has subscribed on a dedicated error topic. See <a href="#error-reporting-via-error-topic">Error Reporting via Error Topic</a> for details.</p>
<p>If such an error subscription by the client exists, the error will by default be ignored after it got published on the error topic, otherwise the connection to the client will be closed. The handling of errors can further be controlled by means of an <em>on-error</em> property bag parameter set on the telemetry message topic. Refer to <a href="#error-handling">Error Handling</a> for details.</p>
<p>The devices can optionally indicate the content type of the payload by setting the <em>content-type</em> property explicitly in the <code>property-bag</code>. The <code>property-bag</code> is an optional collection of properties intended for the receiver of the message. A property bag is only allowed at the very end of a topic. It always starts with a <code>/?</code> character, followed by pairs of URL encoded property names and values that are separated by <code>&amp;</code>. For example, a property bag containing two properties <em>seqNo</em> and <em>importance</em> looks like this: <code>/topic/name/?seqNo=10034&amp;importance=high</code>.</p>
<h2 id="publish-telemetry-data-authenticated-device">Publish Telemetry Data (authenticated Device)</h2>
<ul>
<li>Topic: <code>telemetry</code> or <code>t</code></li>
<li>Authentication: required</li>
<li>Payload:
<ul>
<li>(required) Arbitrary payload</li>
</ul>
</li>
</ul>
<p>This is the preferred way for devices to publish telemetry data. It is available only if the protocol adapter is configured to require devices to authenticate (which is the default). When using this topic, the MQTT adapter determines the device&rsquo;s tenant and device identity as part of the authentication process.</p>
<p><strong>Example</strong></p>
<p>Publish some JSON data for device <code>4711</code>:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -u <span style="color:#e6db74">&#39;sensor1@DEFAULT_TENANT&#39;</span> -P hono-secret -t telemetry -m <span style="color:#e6db74">&#39;{&#34;temp&#34;: 5}&#39;</span>
</code></pre></div><p>Publish some JSON data for device <code>4711</code> using a client certificate for authentication:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh"><span style="color:#75715e"># in base directory of Hono repository:</span>
mosquitto_pub -p <span style="color:#ae81ff">8883</span> -t telemetry -m <span style="color:#e6db74">&#39;{&#34;temp&#34;: 5}&#39;</span> --cert demo-certs/certs/device-4711-cert.pem --key demo-certs/certs/device-4711-key.pem --cafile demo-certs/certs/trusted-certs.pem
</code></pre></div><p><strong>NB</strong> The example above assumes that the MQTT adapter is <a href="/hono/docs/1.7/admin-guide/secure_communication/#mqtt-adapter">configured for TLS</a> and the secure port is used.</p>
<h2 id="publish-telemetry-data-unauthenticated-device">Publish Telemetry Data (unauthenticated Device)</h2>
<ul>
<li>Topic: <code>telemetry/${tenant-id}/${device-id}</code> or <code>t/${tenant-id}/${device-id}</code></li>
<li>Authentication: none</li>
<li>Payload:
<ul>
<li>(required) Arbitrary payload</li>
</ul>
</li>
</ul>
<p>This topic can be used by devices that have not authenticated to the protocol adapter. Note that this requires the
<code>HONO_MQTT_AUTHENTICATION_REQUIRED</code> configuration property to be explicitly set to <code>false</code>.</p>
<p><strong>Examples</strong></p>
<p>Publish some JSON data for device <code>4711</code>:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -t telemetry/DEFAULT_TENANT/4711 -m <span style="color:#e6db74">&#39;{&#34;temp&#34;: 5}&#39;</span>
</code></pre></div><h2 id="publish-telemetry-data-authenticated-gateway">Publish Telemetry Data (authenticated Gateway)</h2>
<ul>
<li>Topic: <code>telemetry/${tenant-id}/${device-id}</code> or <code>t/${tenant-id}/${device-id}</code></li>
<li>Authentication: required</li>
<li>Payload:
<ul>
<li>(required) Arbitrary payload</li>
</ul>
</li>
</ul>
<p>This topic can be used by <em>gateway</em> components to publish data <em>on behalf of</em> other devices which do not connect to a protocol adapter directly but instead are connected to the gateway, e.g. using some low-bandwidth radio based technology like <a href="https://www.sigfox.com">SigFox</a> or <a href="https://lora-alliance.org/">LoRa</a>. In this case the credentials provided by the gateway during connection establishment with the protocol adapter are used to authenticate the gateway whereas the parameters from the topic name are used to identify the device that the gateway publishes data for.</p>
<p>The protocol adapter checks the gateway&rsquo;s authority to publish data on behalf of the device implicitly by means of retrieving a <em>registration assertion</em> for the device from the <a href="/hono/docs/1.7/admin-guide/common-config/#device-registration-service-connection-configuration">configured Device Registration service</a>.</p>
<p><strong>Examples</strong></p>
<p>Publish some JSON data for device <code>4712</code> via gateway <code>gw-1</code>:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -u <span style="color:#e6db74">&#39;gw@DEFAULT_TENANT&#39;</span> -P gw-secret -t telemetry/DEFAULT_TENANT/4712 -m <span style="color:#e6db74">&#39;{&#34;temp&#34;: 5}&#39;</span>
</code></pre></div><p><strong>NB</strong> The example above assumes that a gateway device with ID <code>gw-1</code> has been registered with <code>hashed-password</code> credentials with <em>auth-id</em> <code>gw</code> and password <code>gw-secret</code>.</p>
<h2 id="publishing-events">Publishing Events</h2>
<p>The MQTT adapter supports the publishing of events by means of MQTT <em>PUBLISH</em> packets using QoS 1 only.
The adapter will send an MQTT <em>PUBACK</em> packet to the client once the event has been settled with the <em>accepted</em> outcome by the AMQP 1.0 Messaging Network.</p>
<p>This requires that</p>
<ul>
<li>the AMQP 1.0 Messaging Network has capacity to process events for the client&rsquo;s tenant and</li>
<li>the events published by the client comply with the format defined by the Event API.</li>
</ul>
<p>The protocol adapter checks the configured <a href="/hono/docs/1.7/concepts/resource-limits/">message limit</a> before accepting any event messages. An exceeded message limit will cause an error.</p>
<p>Any kind of error when processing an incoming event message will be reported back to the client if the client has subscribed on a dedicated error topic. See <a href="#error-reporting-via-error-topic">Error Reporting via Error Topic</a> for details.</p>
<p>If such an error subscription by the client exists, the error will by default be ignored after it got published on the error topic, otherwise the connection to the client will be closed. The handling of errors can further be controlled by means of an <em>on-error</em> property bag parameter set on the event message topic. Refer to <a href="#error-handling">Error Handling</a> for details.</p>
<p>The devices can optionally indicate a <em>time-to-live</em> duration for event messages and the content type of the payload by setting the <em>hono-ttl</em> and <em>content-type</em> properties explicitly in the <code>property-bag</code>. The <code>property-bag</code> is an optional collection of properties intended for the receiver of the message. A property bag is only allowed at the very end of a topic. It always starts with a <code>/?</code> character, followed by pairs of URL encoded property names and values that are separated by <code>&amp;</code>. For example, a property bag containing two properties <em>seqNo</em> and <em>importance</em> looks like this: <code>/topic/name/?seqNo=10034&amp;importance=high</code>.</p>
<p>The MQTT adapter currently does not use any properties except <em>hono-ttl</em>.</p>
<h2 id="publish-an-event-authenticated-device">Publish an Event (authenticated Device)</h2>
<ul>
<li>Topic: <code>event</code> or <code>e</code></li>
<li>Authentication: required</li>
<li>Payload:
<ul>
<li>(required) Arbitrary payload</li>
</ul>
</li>
</ul>
<p>This is the preferred way for devices to publish events. It is available only if the protocol adapter has been configured to require devices to authenticate (which is the default).</p>
<p><strong>Example</strong></p>
<p>Upload a JSON string for device <code>4711</code>:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -u <span style="color:#e6db74">&#39;sensor1@DEFAULT_TENANT&#39;</span> -P hono-secret -t event -q <span style="color:#ae81ff">1</span> -m <span style="color:#e6db74">&#39;{&#34;alarm&#34;: 1}&#39;</span>
</code></pre></div><p>Upload a JSON string for device <code>4711</code> with <code>time-to-live</code> as 10 seconds:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -u <span style="color:#e6db74">&#39;sensor1@DEFAULT_TENANT&#39;</span> -P hono-secret -t event/?hono-ttl<span style="color:#f92672">=</span><span style="color:#ae81ff">10</span> -q <span style="color:#ae81ff">1</span> -m <span style="color:#e6db74">&#39;{&#34;alarm&#34;: 1}&#39;</span>
</code></pre></div><h2 id="publish-an-event-unauthenticated-device">Publish an Event (unauthenticated Device)</h2>
<ul>
<li>Topic: <code>event/${tenant-id}/${device-id}</code> or <code>e/${tenant-id}/${device-id}</code></li>
<li>Authentication: none</li>
<li>Payload:
<ul>
<li>(required) Arbitrary payload</li>
</ul>
</li>
</ul>
<p>This topic can be used by devices that have not authenticated to the protocol adapter. Note that this requires the
<code>HONO_MQTT_AUTHENTICATION_REQUIRED</code> configuration property to be explicitly set to <code>false</code>.</p>
<p><strong>Examples</strong></p>
<p>Publish some JSON data for device <code>4711</code>:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -t event/DEFAULT_TENANT/4711 -q <span style="color:#ae81ff">1</span> -m <span style="color:#e6db74">&#39;{&#34;alarm&#34;: 1}&#39;</span>
</code></pre></div><p>Publish some JSON data for device <code>4711</code> with <code>time-to-live</code> as 15 seconds:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -t event/DEFAULT_TENANT/4711/?hono-ttl<span style="color:#f92672">=</span><span style="color:#ae81ff">15</span> -q <span style="color:#ae81ff">1</span> -m <span style="color:#e6db74">&#39;{&#34;alarm&#34;: 1}&#39;</span>
</code></pre></div><h2 id="publish-an-event-authenticated-gateway">Publish an Event (authenticated Gateway)</h2>
<ul>
<li>Topic: <code>event/${tenant-id}/${device-id}</code> or <code>e/${tenant-id}/${device-id}</code></li>
<li>Authentication: required</li>
<li>Payload:
<ul>
<li>(required) Arbitrary payload</li>
</ul>
</li>
</ul>
<p>This topic can be used by <em>gateway</em> components to publish data <em>on behalf of</em> other devices which do not connect to a protocol adapter directly but instead are connected to the gateway, e.g. using some low-bandwidth radio based technology like <a href="https://www.sigfox.com">SigFox</a> or <a href="https://lora-alliance.org/">LoRa</a>. In this case the credentials provided by the gateway during connection establishment with the protocol adapter are used to authenticate the gateway whereas the parameters from the topic name are used to identify the device that the gateway publishes data for.</p>
<p>The protocol adapter checks the gateway&rsquo;s authority to publish data on behalf of the device implicitly by means of retrieving a <em>registration assertion</em> for the device from the <a href="/hono/docs/1.7/admin-guide/common-config/#device-registration-service-connection-configuration">configured Device Registration service</a>.</p>
<p><strong>Examples</strong></p>
<p>Publish some JSON data for device <code>4712</code> via gateway <code>gw-1</code>:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -u <span style="color:#e6db74">&#39;gw@DEFAULT_TENANT&#39;</span> -P gw-secret -t event/DEFAULT_TENANT/4712 -q <span style="color:#ae81ff">1</span> -m <span style="color:#e6db74">&#39;{&#34;temp&#34;: 5}&#39;</span>
</code></pre></div><p><strong>NB</strong> The example above assumes that a gateway device with ID <code>gw-1</code> has been registered with <code>hashed-password</code> credentials with <em>auth-id</em> <code>gw</code> and password <code>gw-secret</code>.</p>
<h2 id="command--control">Command &amp; Control</h2>
<p>The MQTT adapter enables devices to receive commands that have been sent by business applications by means of sending an MQTT <em>SUBSCRIBE</em> packet containing a device specific <em>topic filter</em> as described below. Devices can subscribe with QoS 1 or QoS 0. The adapter indicates the outcome of the subscription request by sending back a corresponding <em>SUBACK</em> packet. The SUBACK packet will contain <em>Success - QoS 0</em> (<code>0x00</code>) or <em>Success - QoS 1</em> (<code>0x01</code>) for a valid command topic filter indicating QoS 0 or 1 and will contain the <em>Failure</em> (<code>0x80</code>) value for an invalid or unsupported filter. When a device no longer wants to receive commands anymore, it can send an MQTT <em>UNSUBSCRIBE</em> packet to the adapter, including the same topic filter that has been used to subscribe.</p>
<p>When a device has successfully subscribed, the adapter sends an <a href="/hono/docs/1.7/api/event/#empty-notification">empty notification</a> on behalf of the device to the downstream AMQP 1.0 Messaging Network with the <em>ttd</em> header set to <code>-1</code>, indicating that the device will be ready to receive commands until further notice. Analogously, the adapter sends an empty notification with the <em>ttd</em> header set to <code>0</code> when a device unsubscribes from commands.</p>
<p>Commands can be sent following a <em>request/response</em> pattern or being <em>one-way</em>.</p>
<p>For <em>Request/Response</em> commands, devices send their responses to commands by means of sending an MQTT <em>PUBLISH</em> message to a topic that is specific to the command that has been executed. The MQTT adapter accepts responses being published using either QoS 0 or QoS 1.</p>
<p>The MQTT adapter checks the configured <a href="/hono/docs/1.7/concepts/resource-limits/">message limit</a> before accepting any command requests and responses. In case of incoming command requests from business applications, if the message limit is exceeded, the Adapter rejects the message with the reason <code>amqp:resource-limit-exceeded</code>. And for the incoming command responses from devices, the Adapter rejects the message and closes the connection to the client.</p>
<p>The following sections define the topic filters/names to use for subscribing to and responding to commands. The following <em>shorthand</em> versions of topic path segments are supported:</p>
<ul>
<li><code>c</code> instead of <code>command</code></li>
<li><code>q</code> instead of <code>req</code></li>
<li><code>s</code> instead of <code>res</code></li>
</ul>
<p>The following variables are used:</p>
<ul>
<li><code>${command}</code> : An arbitrary string that indicates the command to execute, e.g. <code>setBrightness</code>. The command is provided by the application that sends the command.</li>
<li><code>${req-id}</code> (only for <em>Request/Response</em> commands) : The unique identifier of the command execution request. The identifier is passed to the device as part of the name of the topic that the command is published to. The device needs to publish its response to the command to a topic which includes this identifier, thus allowing the adapter to correlate the response with the request.</li>
<li><code>${status}</code> : The HTTP status code indicating the outcome of executing the command. This status code is passed on to the application in the AMQP message&rsquo;s <em>status</em> application property.</li>
</ul>
<h3 id="receiving-commands-authenticated-device">Receiving Commands (authenticated Device)</h3>
<p>An authenticated device MUST use the following topic filter for subscribing to commands:</p>
<p><code>command/[${tenant-id}]/[${device-id}]/req/#</code></p>
<p>Both the tenant and the device ID are optional. If specified, they MUST match the authenticated device&rsquo;s tenant and/or device ID.
Note that the <em>authentication identifier</em> used in the device&rsquo;s credentials is <em>not</em> necessarily the same as the device ID.</p>
<p>The protocol adapter will publish commands for the device to the following topic names</p>
<ul>
<li><strong>one-way</strong> <code>command/${tenant-id}/${device-id}/req//${command}</code></li>
<li><strong>request-response</strong> <code>command/${tenant-id}/${device-id}/req/${req-id}/${command}</code></li>
</ul>
<p>The <em>tenant-id</em> and/or <em>device-id</em> will be included in the topic name if the tenant and/or device ID had been included
in the topic filter used for subscribing to commands.</p>
<div class="alert alert-notice">
<h4 class="alert-heading"><i class="fas fa-info-circle"></i> Deprecation</h4>
<div>Previous versions of Hono required authenticated devices to use <code>command/+/+/req/#</code> for subscribing to commands.
This old topic filter is deprecated. Devices MAY still use it until support for it will be removed in a future Hono version.</div>
</div>
<p><strong>Examples</strong></p>
<p>The following command can be used to subscribe to commands resulting in command messages being published to a
topic that does not include tenant nor device ID:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_sub -v -u <span style="color:#e6db74">&#39;sensor1@DEFAULT_TENANT&#39;</span> -P hono-secret -t command///req/#
</code></pre></div><p>A request/response command with name <code>setBrightness</code> from an application might look like this:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-plaintext" data-lang="plaintext">command///req/1010f8ab0b53-bd96-4d99-9d9c-56b868474a6a/setBrightness
{
&#34;brightness&#34;: 79
}
</code></pre></div><p>A corresponding <em>one-way</em> command might look like this:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-plaintext" data-lang="plaintext">command///req//setBrightness
{
&#34;brightness&#34;: 79
}
</code></pre></div><p>Note that the topic in the latter case doesn&rsquo;t contain a request identifier.</p>
<p>The following command can be used to subscribe to commands resulting in command messages being published to a
topic that includes the tenant ID:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_sub -v -u <span style="color:#e6db74">&#39;sensor1@DEFAULT_TENANT&#39;</span> -P hono-secret -t c/DEFAULT_TENANT//q/#
</code></pre></div><p>Note the usage of the abbreviated names (<code>c</code> and <code>q</code> instead of <code>command</code> and <code>req</code>) and the inclusion of the tenant ID
in the topic filter.</p>
<p>A corresponding request/response command with name <code>setBrightness</code> from an application might look like this:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-plaintext" data-lang="plaintext">c/DEFAULT_TENANT//q/1010f8ab0b53-bd96-4d99-9d9c-56b868474a6a/setBrightness
{
&#34;brightness&#34;: 79
}
</code></pre></div><p>A corresponding <em>one-way</em> command might look like this:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-plaintext" data-lang="plaintext">c/DEFAULT_TENANT//q//setBrightness
{
&#34;brightness&#34;: 79
}
</code></pre></div><p>Note that the topic also includes the abbreviated names and the tenant identifier because the
topic filter used for subscribing did contain the tenant ID as well.</p>
<h3 id="receiving-commands-unauthenticated-device">Receiving Commands (unauthenticated Device)</h3>
<p>An unauthenticated device MUST use the topic filter <code>command/${tenant-id}/${device-id}/req/#</code> to subscribe to commands.</p>
<p><strong>Example</strong></p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_sub -v -t command/DEFAULT_TENANT/4711/req/#
</code></pre></div><p>The adapter will then publish <em>Request/Response</em> commands for the device to topic <code>command/${tenant-id}/${device-id}/req/${req-id}/${command}</code>
and <em>one-way</em> commands to topic <code>command/${tenant-id}/${device-id}/req//${command}</code>.</p>
<p>For example, a request/response command with name <code>setBrightness</code> from an application might look like this:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-plaintext" data-lang="plaintext">command/DEFAULT_TENANT/4711/req/1010f8ab0b53-bd96-4d99-9d9c-56b868474a6a/setBrightness
{
&#34;brightness&#34;: 79
}
</code></pre></div><p>A corresponding <em>one-way</em> command might look like this:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-plaintext" data-lang="plaintext">command/DEFAULT_TENANT/4711/req//setBrightness
{
&#34;brightness&#34;: 79
}
</code></pre></div><p>Note that the topic in the latter case doesn&rsquo;t contain a request identifier.</p>
<h3 id="receiving-commands-authenticated-gateway">Receiving Commands (authenticated Gateway)</h3>
<p><em>Gateway</em> components can receive commands for devices which do not connect to a protocol adapter directly but instead are connected to the gateway, e.g. using some low-bandwidth radio based technology like <a href="https://www.sigfox.com">SigFox</a> or <a href="https://lora-alliance.org/">LoRa</a>. Corresponding devices have to be configured so that they can be used with a gateway. See <a href="/hono/docs/1.7/admin-guide/file-based-device-registry-config/#configuring-gateway-devices">Configuring Gateway Devices</a> for details.</p>
<p>An authenticated gateway MUST use one of the following topic filters for subscribing to commands:</p>
<table>
<thead>
<tr>
<th style="text-align:left">Topic Filter</th>
<th style="text-align:left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left"><code>command//+/req/#</code></td>
<td style="text-align:left">Subscribe to commands for all devices that the gateway is authorized to act on behalf of.</td>
</tr>
<tr>
<td style="text-align:left"><code>command/${tenant-id}/+/req/#</code></td>
<td style="text-align:left">Subscribe to commands for all devices that the gateway is authorized to act on behalf of.</td>
</tr>
<tr>
<td style="text-align:left"><code>command//${device-id}/req/#</code></td>
<td style="text-align:left">Subscribe to commands for a specific device that the gateway is authorized to act on behalf of.</td>
</tr>
<tr>
<td style="text-align:left"><code>command/${tenant-id}/${device-id}/req/#</code></td>
<td style="text-align:left">Subscribe to commands for a specific device that the gateway is authorized to act on behalf of.</td>
</tr>
</tbody>
</table>
<p>The protocol adapter will publish commands for devices to the following topic names</p>
<ul>
<li><strong>one-way</strong> <code>command//${device-id}/req//${command}</code> or <code>command/${tenant-id}/${device-id}/req//${command}</code></li>
<li><strong>request-response</strong> <code>command//${device-id}/req/${req-id}/${command}</code> or <code>command/${tenant-id}/${device-id}/req/${req-id}/${command}</code></li>
</ul>
<p>The <code>${tenant-id}</code> will be included in the topic name if the tenant ID had been included in the topic filter used for subscribing to commands.</p>
<div class="alert alert-notice">
<h4 class="alert-heading"><i class="fas fa-info-circle"></i> Deprecation</h4>
<div>Previous versions of Hono required authenticated gateways to use <code>command/+/+/req/#</code> for subscribing to commands.
This old topic filter is deprecated. Gateways MAY still use it until support for it will be removed in a future Hono version.</div>
</div>
<p>When processing an incoming command message, the protocol adapter will give precedence to a device-specific command subscription matching the command target device, whether the subscription comes from a gateway or the device itself. If there are multiple such subscriptions from multiple gateways and/or from the device itself, the subscription initiated last will get the command messages.</p>
<p>If no device-specific command subscription exists for a command target device, but <em>one</em> gateway, that may act on behalf of the device, has subscribed to commands for all its devices, then the command message is sent to that gateway.</p>
<p>If <em>multiple</em> gateways have initiated such generic subscriptions, the protocol adapter may have to decide to which gateway a particular command message will be sent to.
In case the command target device has already sent a telemetry, event or command response message via a gateway and if that gateway has created such a command subscription, that gateway will be chosen. Otherwise one gateway that may act on behalf of the command target device and that has an open subscription will be chosen randomly to receive the command message.</p>
<p><strong>Subscribe to all Devices</strong></p>
<p>A subscription to commands for all devices that a gateway acts on behalf of looks like this:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_sub -v -u <span style="color:#e6db74">&#39;gw@DEFAULT_TENANT&#39;</span> -P gw-secret -t command/DEFAULT_TENANT/+/req/#
</code></pre></div><p>A request/response command for device <code>4711</code> with name <code>setBrightness</code> from an application might then look like this:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-plaintext" data-lang="plaintext">command/DEFAULT_TENANT/4711/req/1010f8ab0b53-bd96-4d99-9d9c-56b868474a6a/setBrightness
{
&#34;brightness&#34;: 79
}
</code></pre></div><p>Note that the tenant identifier is included in the topic name that the command has been published to because it had been
included in the topic filter used for subscribing to the commands.</p>
<p><strong>Subscribe to a specific Device</strong></p>
<p>A subscription to commands for a specific device can be done like this:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_sub -v -u <span style="color:#e6db74">&#39;gw@DEFAULT_TENANT&#39;</span> -P gw-secret -t c//4711/q/#
</code></pre></div><p>Note the usage of the abbreviated names (<code>c</code> and <code>q</code> instead of <code>command</code> and <code>req</code>) in the topic filter.</p>
<p>A corresponding <em>one-way</em> command might look like this:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-plaintext" data-lang="plaintext">c//4711/q//setBrightness
{
&#34;brightness&#34;: 79
}
</code></pre></div><p>Note that the topic also includes the abbreviated names and does not include the tenant identifier because the
topic filter used for subscribing did not contain the tenant ID either.</p>
<h3 id="sending-a-response-to-a-command-authenticated-device">Sending a Response to a Command (authenticated Device)</h3>
<p>An authenticated device MUST send the response to a previously received command to the topic <code>command///res/${req-id}/${status}</code>.</p>
<p><strong>Example</strong></p>
<p>After a command has arrived as in the above example, you send a response using the arrived <code>${req-id}</code>:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -u <span style="color:#e6db74">&#39;sensor1@DEFAULT_TENANT&#39;</span> -P hono-secret -t command///res/1010f8ab0b53-bd96-4d99-9d9c-56b868474a6a/200 -m <span style="color:#e6db74">&#39;{&#34;lumen&#34;: 200}&#39;</span>
</code></pre></div><h3 id="sending-a-response-to-a-command-unauthenticated-device">Sending a Response to a Command (unauthenticated Device)</h3>
<p>An unauthenticated device MUST send the response to a previously received command to the topic <code>command/${tenant-id}/${device-id}/res/${req-id}/${status}</code>.</p>
<p><strong>Example</strong></p>
<p>After a command has arrived as in the above example, you send a response using the arrived <code>${req-id}</code>:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -t command/DEFAULT_TENANT/4711/res/1010f8ab0b53-bd96-4d99-9d9c-56b868474a6a/200 -m <span style="color:#e6db74">&#39;{&#34;lumen&#34;: 200}&#39;</span>
</code></pre></div><h3 id="sending-a-response-to-a-command-authenticated-gateway">Sending a Response to a Command (authenticated Gateway)</h3>
<p>An authenticated gateway MUST send a device&rsquo;s response to a command it has received on behalf of the device to the topic <code>command//${device-id}/res/${req-id}/${status}</code>.</p>
<p><strong>Example</strong></p>
<p>After a command has arrived as in the above example, the response is sent using the <code>${req-id}</code> from the topic that the command had been published to:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mosquitto_pub -u <span style="color:#e6db74">&#39;gw@DEFAULT_TENANT&#39;</span> -P gw-secret -t command//4711/res/1010f8ab0b53-bd96-4d99-9d9c-56b868474a6a/200 -m <span style="color:#e6db74">&#39;{&#34;lumen&#34;: 200}&#39;</span>
</code></pre></div><h2 id="error-reporting-via-error-topic">Error Reporting via Error Topic</h2>
<p>The default behaviour when an error occurs while publishing telemetry, event or command response messages is for the MQTT adapter to close the network connection to the device, as mandated by the MQTT 3.1.1 spec.</p>
<p>An alternative way of dealing with errors involves keeping the connection intact and letting the MQTT adapter publish a corresponding error message on a specific error topic to the device. To enable that behaviour, the device sends an MQTT <em>SUBSCRIBE</em> packet with a topic filter as described below on <em>the same MQTT connection</em> that is also used for publishing the telemetry, event or command response messages. Devices can subscribe with QoS 0 only. The adapter indicates the outcome of the subscription request by sending back a corresponding <em>SUBACK</em> packet. The SUBACK packet will contain <em>Success - QoS 0</em> (<code>0x00</code>) for a valid error topic filter and will contain the <em>Failure</em> (<code>0x80</code>) value for an invalid or unsupported filter. In order to again activate the default error handling behaviour, the device can send an MQTT <em>UNSUBSCRIBE</em> packet to the adapter, including the same topic filter that has been used to subscribe.</p>
<p>The following sections define the topic filters to use for subscribing to error messages and the resulting error message topic. Instead of the <code>error</code> topic path segment, the shorthand version <code>e</code> is also supported.</p>
<p>The following variables are used:</p>
<ul>
<li><code>${endpoint-type}</code>: The endpoint type of the device message that caused the error. Its value is either <code>telemetry</code>, <code>event</code> or the respective shorthand version. In case of a command response device message <code>command-response</code> or <code>c-s</code> is used.</li>
<li><code>${correlation-id}</code>: The identifier that may be used to correlate the error message with the device message that caused the error. The identifier is either the value of a <em>correlation-id</em> property bag value contained in the device message topic, or the identifier is the <em>packet-id</em> of the device message if it was sent with QoS 1. Otherwise, a value of <code>-1</code> is used.</li>
<li><code>${error-status}</code>: The HTTP status code of the error that was caused by the device message.</li>
</ul>
<div class="alert alert-notice">
<h4 class="alert-heading"><i class="fas fa-info-circle"></i> Examples</h4>
<div>Since the subscription on the error topic needs to be done on the same MQTT connection that is also used for publishing the telemetry, event or command response messages, the Mosquitto MQTT Command Line Client cannot be used. The <a href="https://hivemq.github.io/mqtt-cli/">MQTT CLI</a> tool with its <a href="https://hivemq.github.io/mqtt-cli/docs/shell.html">shell mode</a> is an alternative that supports using one MQTT connection for both subscribing and publishing.</div>
</div>
<h3 id="receiving-error-messages-authenticated-device">Receiving Error Messages (authenticated Device)</h3>
<p>An authenticated device MUST use the following topic filter for subscribing to error messages:</p>
<p><code>error/[${tenant-id}]/[${device-id}]/#</code></p>
<p>Both the tenant and the device ID are optional. If specified, they MUST match the authenticated device&rsquo;s tenant and/or device ID.
Note that the <em>authentication identifier</em> used in the device&rsquo;s credentials is <em>not</em> necessarily the same as the device ID.</p>
<p>The protocol adapter will publish error messages for the device to the following topic name</p>
<p><code>error/[${tenant-id}]/[${device-id}]/${endpoint-type}/${correlation-id}/${error-status}</code></p>
<p>The <em>tenant-id</em> and/or <em>device-id</em> will be included in the topic name if the tenant and/or device ID had been included
in the topic filter used for subscribing to error messages.</p>
<p><strong>Example</strong></p>
<p>An example using the <a href="https://hivemq.github.io/mqtt-cli/">MQTT CLI</a> that will produce an error output provided there is no downstream consumer for the device messages.</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">mqtt shell
con -V <span style="color:#ae81ff">3</span> -h <span style="color:#f92672">[</span>MQTT_ADAPTER_IP<span style="color:#f92672">]</span> -u <span style="color:#f92672">[</span>DEVICE<span style="color:#f92672">]</span>@<span style="color:#f92672">[</span>TENANT<span style="color:#f92672">]</span> -pw <span style="color:#f92672">[</span>PWD<span style="color:#f92672">]</span>
sub -t error///# --qos <span style="color:#ae81ff">0</span> --outputToConsole
pub -t telemetry -m <span style="color:#e6db74">&#39;{&#34;temp&#34;: 5}&#39;</span> --qos <span style="color:#ae81ff">1</span>
</code></pre></div><p>Using an explicit correlation id:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-sh" data-lang="sh">pub -t telemetry/?correlation-id<span style="color:#f92672">=</span><span style="color:#ae81ff">123</span> -m <span style="color:#e6db74">&#39;{&#34;temp&#34;: 5}&#39;</span> --qos <span style="color:#ae81ff">1</span>
</code></pre></div><h3 id="receiving-error-messages-unauthenticated-device">Receiving Error Messages (unauthenticated Device)</h3>
<p>An unauthenticated device MUST use the following topic filter for subscribing to error messages:</p>
<p><code>error/${tenant-id}/${device-id}/#</code></p>
<p>The protocol adapter will publish error messages for the device to the following topic name</p>
<p><code>error/${tenant-id}/${device-id}/${endpoint-type}/${correlation-id}/${error-status}</code></p>
<h3 id="receiving-error-messages-authenticated-gateway">Receiving Error Messages (authenticated Gateway)</h3>
<p>An authenticated gateway MUST use one of the following topic filters for subscribing to error messages:</p>
<table>
<thead>
<tr>
<th style="text-align:left">Topic Filter</th>
<th style="text-align:left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left"><code>error//+/#</code><br/><code>error/${tenant-id}/+/#</code></td>
<td style="text-align:left">Subscribe to error messages for all devices that the gateway is authorized to act on behalf of.</td>
</tr>
<tr>
<td style="text-align:left"><code>error//${device-id}/#</code><br/><code>error/${tenant-id}/${device-id}/#</code></td>
<td style="text-align:left">Subscribe to error messages for a specific device that the gateway is authorized to act on behalf of.</td>
</tr>
</tbody>
</table>
<p>The protocol adapter will publish error messages for the device to the following topic name</p>
<p><code>error/[${tenant-id}]/[${device-id}]/${endpoint-type}/${correlation-id}/${error-status}</code></p>
<p>The <em>tenant-id</em> and/or <em>device-id</em> will be included in the topic name if the tenant and/or device ID had been included
in the topic filter used for subscribing to error messages.</p>
<h3 id="error-message-payload">Error Message Payload</h3>
<p>The MQTT adapter publishes error messages with a UTF-8 encoded JSON payload containing the following fields:</p>
<table>
<thead>
<tr>
<th style="text-align:left">Name</th>
<th style="text-align:center">Mandatory</th>
<th style="text-align:left">JSON Type</th>
<th style="text-align:left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left"><em>code</em></td>
<td style="text-align:center"><em>yes</em></td>
<td style="text-align:left"><em>number</em></td>
<td style="text-align:left">The HTTP error status code. See the table below for possible values.</td>
</tr>
<tr>
<td style="text-align:left"><em>message</em></td>
<td style="text-align:center"><em>yes</em></td>
<td style="text-align:left"><em>string</em></td>
<td style="text-align:left">The error detail message.</td>
</tr>
<tr>
<td style="text-align:left"><em>timestamp</em></td>
<td style="text-align:center"><em>yes</em></td>
<td style="text-align:left"><em>string</em></td>
<td style="text-align:left">The date and time the error message was published by the MQTT adapter. The value is an <a href="https://en.wikipedia.org/wiki/ISO_8601#Combined_date_and_time_representations">ISO 8601 compliant <em>combined date and time representation in extended format</em></a>.</td>
</tr>
<tr>
<td style="text-align:left"><em>correlation-id</em></td>
<td style="text-align:center"><em>yes</em></td>
<td style="text-align:left"><em>string</em></td>
<td style="text-align:left">The identifier that may be used to correlate the error message with the device message that caused the error. The identifier is either the value of a <em>correlation-id</em> property bag value contained in the device message topic, or the identifier is the <em>packet-id</em> of the device message if it was sent with QoS 1. Otherwise a value of <code>-1</code> is used.</td>
</tr>
</tbody>
</table>
<p>The error message&rsquo;s <em>code</em> field may contain the following HTTP status codes:</p>
<table>
<thead>
<tr>
<th style="text-align:left">Code</th>
<th style="text-align:left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left"><em>400</em></td>
<td style="text-align:left">Bad Request, the request cannot be processed. A possible reason for this is an invalid <em>PUBLISH</em> topic.</td>
</tr>
<tr>
<td style="text-align:left"><em>403</em></td>
<td style="text-align:left">Forbidden, the device&rsquo;s registration status cannot be asserted.</td>
</tr>
<tr>
<td style="text-align:left"><em>404</em></td>
<td style="text-align:left">Not Found, the device is disabled or does not exist.</td>
</tr>
<tr>
<td style="text-align:left"><em>413</em></td>
<td style="text-align:left">Request Entity Too Large, the request body exceeds the maximum supported size.</td>
</tr>
<tr>
<td style="text-align:left"><em>429</em></td>
<td style="text-align:left">Too Many Requests, the tenant&rsquo;s message limit for the current period is exceeded.</td>
</tr>
<tr>
<td style="text-align:left"><em>503</em></td>
<td style="text-align:left">Service Unavailable, the request cannot be processed. Possible reasons for this include:<ul><li>There is no consumer of telemetry data for the given tenant connected to Hono, or the consumer has not indicated that it may receive further messages (not giving credits). </li><li>If the QoS level header is set to <code>1</code> (<em>at least once</em> semantics), the reason may be: <ul><li>The consumer has indicated that it didn&rsquo;t process the telemetry data.</li> <li>The consumer failed to indicate in time whether it has processed the telemetry data.</li></ul></li></ul></td>
</tr>
</tbody>
</table>
<p>Example payload:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-json" data-lang="json">{
<span style="color:#f92672">&#34;code&#34;</span>: <span style="color:#ae81ff">400</span>,
<span style="color:#f92672">&#34;message&#34;</span>: <span style="color:#e6db74">&#34;malformed topic name&#34;</span>,
<span style="color:#f92672">&#34;timestamp&#34;</span>: <span style="color:#e6db74">&#34;2020-12-24T19:00:00+0100&#34;</span>,
<span style="color:#f92672">&#34;correlation-id&#34;</span>: <span style="color:#e6db74">&#34;5&#34;</span>
}
</code></pre></div><h2 id="error-handling">Error Handling</h2>
<p>When a device publishes a telemetry, event or command response message and there is an error processing the message, the handling of the error depends on whether there is a <a href="#error-reporting-via-error-topic">error topic subscription</a> for the device and whether a <em>on-error</em> property bag parameter was set on the topic used for sending the message.</p>
<p>If no error subscription is in place and no <em>on-error</em> parameter was set, the default error handling behaviour is to close the MQTT connection to the device.
If the device has a subscription on the error topic (on the same MQTT connection the device uses for sending messages), the default behaviour is to keep the
MQTT connection open unless a terminal error happens. The errors that are classified as terminal are listed below.</p>
<ul>
<li>The adapter is disabled for the tenant that the client belongs to.</li>
<li>The authenticated device or gateway is disabled or not registered.</li>
<li>The tenant is disabled or does not exist.</li>
</ul>
<div class="alert alert-notice">
<h4 class="alert-heading"><i class="fas fa-info-circle"></i> Note</h4>
<div>When a terminal error occurs, the connection will always be closed irrespective of any <em>on-error</em> parameter or error subscription.</div>
</div>
<p>The following table lists the different behaviours based on the value of the <em>on-error</em> property bag parameter and the existence of an error subscription:</p>
<table>
<thead>
<tr>
<th style="text-align:left"><em>on-error</em> topic parameter</th>
<th style="text-align:left">Error subscription exists</th>
<th style="text-align:left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left"><em>default</em> or value not set</td>
<td style="text-align:left">no</td>
<td style="text-align:left">The connection to the device will get closed (like with the <em>disconnect</em> option).</td>
</tr>
<tr>
<td style="text-align:left"><em>disconnect</em></td>
<td style="text-align:left">no</td>
<td style="text-align:left">The connection to the device will get closed.</td>
</tr>
<tr>
<td style="text-align:left"><em>ignore</em></td>
<td style="text-align:left">no</td>
<td style="text-align:left">The error will be ignored and a <em>PUBACK</em> for the message that caused the error will get sent.</td>
</tr>
<tr>
<td style="text-align:left"><em>skip-ack</em></td>
<td style="text-align:left">no</td>
<td style="text-align:left">The error will be ignored and no <em>PUBACK</em> for the message that caused the error will get sent.</td>
</tr>
<tr>
<td style="text-align:left"><em>default</em> or value not set</td>
<td style="text-align:left">yes</td>
<td style="text-align:left">After having sent an error message on the error topic, the error will be ignored and a <em>PUBACK</em> for the message that caused the error will get sent (like with the <em>ignore</em> option).</td>
</tr>
<tr>
<td style="text-align:left"><em>disconnect</em></td>
<td style="text-align:left">yes</td>
<td style="text-align:left">After having sent an error message on the error topic, the connection to the device will get closed.</td>
</tr>
<tr>
<td style="text-align:left"><em>ignore</em></td>
<td style="text-align:left">yes</td>
<td style="text-align:left">After having sent an error message on the error topic, the error will be ignored and a <em>PUBACK</em> for the message that caused the error will get sent.</td>
</tr>
<tr>
<td style="text-align:left"><em>skip-ack</em></td>
<td style="text-align:left">yes</td>
<td style="text-align:left">After having sent an error message on the error topic, the error will be ignored and no <em>PUBACK</em> for the message that caused the error will get sent.</td>
</tr>
</tbody>
</table>
<p><strong>Example</strong></p>
<p>An authenticated device wanting to have errors always be ignored can for example publish telemetry messages on this topic:</p>
<p><code>telemetry/?on-error=ignore</code></p>
<h2 id="custom-message-mapping">Custom Message Mapping</h2>
<p>This protocol adapter supports transformation of messages that have been uploaded by devices before they get forwarded to downstream consumers.</p>
<div class="alert alert-notice">
<h4 class="alert-heading"><i class="fas fa-info-circle"></i> Experimental</h4>
<div>This is an experimental feature. The names of the configuration properties, potential values and the overall functionality are therefore
subject to change without prior notice.</div>
</div>
<p>This feature is useful in scenarios where devices are connected to the adapter via a gateway but the gateway is not able to include
the device ID in the topic that the gateway publishes data to. The gateway will use the plain <code>telemetry</code> or <code>event</code> topics in this case.
The message payload will usually contain the identifier of the device that the data originates from.</p>
<p>The same functionality can also be used to transform the payload of messages uploaded by a device. This can be used for example to
transform binary encoded data into a JSON document which can be consumed more easily by downstream consumers.</p>
<p>The mechanism works as follows:</p>
<ol>
<li>
<p>A client uploads a message to the MQTT adapter.</p>
</li>
<li>
<p>The adapter invokes the Device Registration service&rsquo;s <em>assert Registration</em> operation using either
the authenticated device&rsquo;s identifier, if the topic does not contain a device ID, or the device ID from the
topic.</p>
</li>
<li>
<p>If the assertion succeeds, the adapter creates the downstream message using the original message&rsquo;s payload and
the asserted device ID as the origin device.</p>
</li>
<li>
<p>If the <em>assert Registration</em> response payload contains a value for the <em>mapper</em> property, the adapter tries to
find a <em>mapper endpoint</em> configuration for the given value. If a mapper endpoint with a matching name has been
configured for the adapter,</p>
<ol>
<li>the adapter sends an HTTP request to the endpoint which contains the original message&rsquo;s payload in the request body.</li>
<li>If the response body is not empty, it is used as the downstream message&rsquo;s payload, replacing the original payload.</li>
<li>If the response contains a <em>device_id</em> header and its value is different from the original device ID, then the adapter
invokes the <em>assert Registration</em> operation again, this time using the mapped device ID instead of the original device ID.
If the assertion succeeds, the adapter uses the asserted (mapped) device ID for the downstream message.</li>
</ol>
</li>
<li>
<p>The adapter forwards the downstream message.</p>
</li>
</ol>
<p>Please refer to the <a href="/hono/docs/1.7/api/management/#/devices/createDeviceRegistration">Device Registry Management API</a>
for how to register a <em>mapper</em> for a device.
Please refer to the <a href="/hono/docs/1.7/admin-guide/mqtt-adapter-config/#custom-message-mapping">MQTT Adapter Admin Guide</a>
for how to configure custom mapper endpoints.</p>
<h2 id="downstream-meta-data">Downstream Meta Data</h2>
<p>The adapter includes the following meta data in messages being sent downstream:</p>
<table>
<thead>
<tr>
<th style="text-align:left">Name</th>
<th style="text-align:left">Location</th>
<th style="text-align:left">Type</th>
<th style="text-align:left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left"><em>device_id</em></td>
<td style="text-align:left"><em>application</em></td>
<td style="text-align:left"><em>string</em></td>
<td style="text-align:left">The identifier of the device that the message originates from.</td>
</tr>
<tr>
<td style="text-align:left"><em>orig_adapter</em></td>
<td style="text-align:left"><em>application</em></td>
<td style="text-align:left"><em>string</em></td>
<td style="text-align:left">Contains the adapter&rsquo;s <em>type name</em> which can be used by downstream consumers to determine the protocol adapter that the message has been received over. The MQTT adapter&rsquo;s type name is <code>hono-mqtt</code>.</td>
</tr>
<tr>
<td style="text-align:left"><em>orig_address</em></td>
<td style="text-align:left"><em>application</em></td>
<td style="text-align:left"><em>string</em></td>
<td style="text-align:left">Contains the name of the MQTT topic that the device has originally published the data to.</td>
</tr>
<tr>
<td style="text-align:left"><em>x-opt-retain</em></td>
<td style="text-align:left"><em>message-annotations</em></td>
<td style="text-align:left"><em>boolean</em></td>
<td style="text-align:left">Contains <code>true</code> if the device has published an event or telemetry message with its <em>retain</em> flag set to <code>1</code></td>
</tr>
</tbody>
</table>
<p>The adapter also considers <em>defaults</em> registered for the device at either the <a href="/hono/docs/1.7/api/tenant/#tenant-information-format">tenant</a>
or the <a href="/hono/docs/1.7/api/device-registration/#assert-device-registration">device level</a>.
The values of the default properties are determined as follows:</p>
<ol>
<li>If the message already contains a non-empty property of the same name, the value if unchanged.</li>
<li>Otherwise, if a default property of the same name is defined in the device&rsquo;s registration information, that value is used.</li>
<li>Otherwise, if a default property of the same name is defined for the tenant that the device belongs to, that value is used.</li>
</ol>
<p>Note that of the standard AMQP 1.0 message properties only the <em>content-type</em> and <em>ttl</em> can be set this way to a default value.</p>
<h3 id="event-message-time-to-live">Event Message Time-to-live</h3>
<p>Events published by devices will usually be persisted by the AMQP Messaging Network in order to support deferred delivery to downstream consumers.
In most cases the AMQP Messaging Network can be configured with a maximum <em>time-to-live</em> to apply to the events so that the events will be removed
from the persistent store if no consumer has attached to receive the event before the message expires.</p>
<p>In order to support environments where the AMQP Messaging Network cannot be configured accordingly, the MQTT protocol adapter supports setting a
downstream event message&rsquo;s <em>ttl</em> property based on the <em>hono-ttl</em> property set as <em>property-bag</em> at the end of the event topic.
Also the default <em>ttl</em> and <em>max-ttl</em> values can be configured for a tenant/device as described in the <a href="/hono/docs/1.7/api/tenant/#resource-limits-configuration-format">Tenant API</a>.</p>
<h2 id="tenant-specific-configuration">Tenant specific Configuration</h2>
<p>The adapter uses the <a href="/hono/docs/1.7/api/tenant/#get-tenant-information">Tenant API</a> to retrieve <em>tenant specific configuration</em> for adapter type <code>hono-mqtt</code>.
The following properties are (currently) supported in the <em>Adapter</em> object:</p>
<table>
<thead>
<tr>
<th style="text-align:left">Name</th>
<th style="text-align:left">Type</th>
<th style="text-align:left">Default Value</th>
<th style="text-align:left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left"><em>enabled</em></td>
<td style="text-align:left"><em>boolean</em></td>
<td style="text-align:left"><code>true</code></td>
<td style="text-align:left">If set to <code>false</code> the adapter will reject all data from devices belonging to the tenant.</td>
</tr>
</tbody>
</table>
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