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     <li><a href="#time-until-disconnect-notification">Time until Disconnect Notification</a>
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         <li><a href="#determining-a-devices-connection-status">Determining a Device&rsquo;s Connection Status</a></li>
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         <li><a href="#honos-http-protocol-adapter">Hono&rsquo;s HTTP protocol adapter</a></li>
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             <h1>

               Device Notifications
             </h1>


 <p><em>Business Applications</em> need to know when an attempt to send a command to device is feasible, e.g. because the device is then known to be connected to a protocol adapter. <em>Devices</em> and <em>Protocol Adapters</em> can indicate to <em>Business Applications</em> a device&rsquo;s intent to e.g. receive commands using specific <em>notifications</em>.</p>
 <h2 id="time-until-disconnect-notification">Time until Disconnect Notification</h2>
 <p><em>Devices</em> and <em>Protocol Adapters</em> can notify an application about the fact that a device is connected and ready to receive one or more commands by means of including a <em>time &lsquo;til disconnect</em> (<em>ttd</em>) property in telemetry or event messages.</p>
 <p>The <em>ttd</em> property value indicates the time that the device will stay connected to the protocol adapter.
 Using this value together with the <em>creation-time</em> of the message, an application can determine whether an attempt to send a command to the device has a reasonable chance of succeeding.
 The <em>ttd</em> property can be included in any regular telemetry or event message. However, if a device does not have any telemetry data or event to upload to the adapter, it can also use an <a href="/hono/docs/1.0/api/event/#empty-notification">empty notification</a> instead.</p>
 <p>Hono includes utility classes that application developers can use to register a callback to be notified when a device sends a <em>ttd</em> notification.
 See Hono&rsquo;s example module for details where such a notification callback is used.
 Please refer to the <a href="/hono/docs/1.0/api/telemetry/">Telemetry API</a> and the <a href="/hono/docs/1.0/api/event/">Event API</a> for further details.</p>
 <p>The following table defines the possible values of the <em>ttd</em> property and their semantics:</p>
 <table>
 <thead>
 <tr>
 <th style="text-align:left">TTD</th>
 <th style="text-align:left">Description</th>
 </tr>
 </thead>
 <tbody>
 <tr>
 <td style="text-align:left">&gt; 0</td>
 <td style="text-align:left">The value indicates the number of seconds that the device will stay connected. Devices using a stateless protocol like HTTP will be able to receive a single command only before disconnecting.</td>
 </tr>
 <tr>
 <td style="text-align:left">-1</td>
 <td style="text-align:left">The device is now connected (i.e. available to receive upstream messages) until further notice.</td>
 </tr>
 <tr>
 <td style="text-align:left">0</td>
 <td style="text-align:left">The device is now disconnected (i.e. not available anymore to receive upstream messages).</td>
 </tr>
 </tbody>
 </table>
 <h3 id="determining-a-devices-connection-status">Determining a Device&rsquo;s Connection Status</h3>
 <p>An application receiving a downstream message containing a <em>ttd</em> property can check if the device is currently connected
 (and thus ready to receive a command) by</p>
 <ul>
 <li>adding the <em>ttd</em> value to the <em>creation-time</em> to determine the <em>expiration</em> time, and then</li>
 <li>comparing the <em>current</em> time with the <em>expiration</em> time</li>
 </ul>
 <p>If the <em>current</em> time is <em>after</em> the <em>expiration</em> time, the device should be assumed to already have disconnected again.</p>
 <h3 id="source-of-the-ttd-value">Source of the <em>ttd</em> Value</h3>
 <p>While it seems to be natural that a device itself indicates when it is ready to receive a command, it may not always be
 possible or desirable to do so.
 A device could e.g. be not capable to specify the value for <em>ttd</em> in it&rsquo;s message, or all devices of a particular setup
 would always use the same value for <em>ttd</em>, so it would not make much sense to provide this value always again.
 Additionally different protocols may or may not let a sender set specific values for a message, so a device using a
 specific protocol may not be able to provide a value for the <em>ttd</em> property at all.
 For these reasons there are (resp. may be) additional ways of setting the value of <em>ttd</em>:</p>
 <ul>
 <li>Hono&rsquo;s Tenant and Device Registration APIs support the inclusion of default values for application-properties in the
 AMQP 1.0 message. By these means a device can be configured to always have a specific value for <em>ttd</em>.</li>
 <li>In a future extension there may be a configuration value per tenant and protocol adapter that sets the value of <em>ttd</em>
 if it was not provided by other means already (like provided to the protocol adapter or by setting a default value).</li>
 </ul>
 <h3 id="honos-http-protocol-adapter">Hono&rsquo;s HTTP protocol adapter</h3>
 <p>Hono&rsquo;s HTTP protocol adapter supports the setting of the <em>ttd</em> value in requests explicitly - please refer to the
 <a href="/hono/docs/1.0/user-guide/http-adapter/">HTTP Adapter</a> for details.
 Alternatively the default property values for devices from the Device Registry can be used (described above).</p>
 <h3 id="honos-mqtt-protocol-adapter">Hono&rsquo;s MQTT protocol adapter</h3>
 <p>The MQTT protocol adapter automatically sends a <em>Time until disconnect notification</em> with a <em>ttd</em> value of <code>-1</code>
 for a device that subscribes to the appropriate command topic (refer to the [MQTT Adapter user guide]
 (/hono/docs/1.0/user-guide/mqtt-adapter/) for details).</p>
 <p>When a device unsubscribes again, the adapter automatically sends a <em>Time until disconnect notification</em> with a <em>ttd</em> value of <code>0</code>.</p>
 <h3 id="examples">Examples</h3>
 <p>The following sequence diagram shows a <em>Time until disconnect notification</em> while sending a telemetry message downstream
 via the HTTP protocol adapter:</p>
 <figure>
     <img src="../device_commandReadinessImplicit.svg"/> <figcaption>
             <h4>Device command readiness with telemetry data</h4>
         </figcaption>
 </figure>

 <p>The following sequence diagram shows a <em>Time until disconnect notification</em> by sending an empty event message downstream
 via the HTTP protocol adapter:</p>
 <figure>
     <img src="../device_commandReadinessExplicit.svg"/> <figcaption>
             <h4>Device command readiness with explicit event</h4>
         </figcaption>
 </figure>

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