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+<ul>
+<li><a href="#time-until-disconnect-notification">Time until Disconnect Notification</a>
+<ul>
+<li><a href="#determining-a-device-s-connection-status">Determining a Device’s Connection Status</a></li>
+<li><a href="#source-of-the-ttd-value">Source of the <em>ttd</em> Value</a></li>
+<li><a href="#hono-s-http-protocol-adapter">Hono’s HTTP protocol adapter</a></li>
+<li><a href="#hono-s-mqtt-protocol-adapter">Hono’s MQTT 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’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>Devicess</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 ‘til disconnect</em> (<em>ttd</em>) property in downstream AMQP 1.0 messages containing data originating from a connected device.</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.</p>
+
+<p>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/api/event-api/#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’s example module for details where such a notification callback is used.</p>
+
+<p>Please refer to the <a href="/hono/docs/api/telemetry-api/">Telemetry API</a> and the <a href="/hono/docs/api/event-api/">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 align="left">TTD</th>
+<th align="left">Description</th>
+</tr>
+</thead>
+
+<tbody>
+<tr>
+<td align="left">> 0</td>
+<td 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 align="left">-1</td>
+<td align="left">The device is now connected (i.e. available to receive upstream messages) until further notice.</td>
+</tr>
+
+<tr>
+<td align="left">0</td>
+<td align="left">The device is now disconnected (i.e. not available anymore to receive upstream messages).</td>
+</tr>
+</tbody>
+</table>
+
+<h3 id="determining-a-device-s-connection-status">Determining a Device’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.</p>
+
+<p>A device could e.g. be not capable to specify the value for <em>ttd</em> in it’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.</p>
+
+<p>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.</p>
+
+<p>For these reasons there are (resp. may be) additional ways of setting the value of <em>ttd</em>:</p>
+
+<ul>
+<li>Hono’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).
+<br /></li>
+</ul>
+
+<h3 id="hono-s-http-protocol-adapter">Hono’s HTTP protocol adapter</h3>
+
+<p>Hono’s HTTP protocol adapter supports the setting of the <em>ttd</em> value in requests explicitly - please refer to the
+<a href="/hono/docs/user-guide/http-adapter/">HTTP Adapter</a> for details.</p>
+
+<p>Alternatively the default property values for devices from the Device Registry can be used (described above).</p>
+
+<h3 id="hono-s-mqtt-protocol-adapter">Hono’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 <a href="/hono/docs/user-guide/mqtt-adapter/">MQTT Adapter user guide</a> 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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