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+      <li data-nav-id="/hono/docs/admin-guide/common-config/" title="Common Configuration" class="dd-item ">
+        <a href="/hono/docs/admin-guide/common-config/">
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+      <li data-nav-id="/hono/docs/admin-guide/device-registry-config/" title="Device Registry Configuration" class="dd-item ">
+        <a href="/hono/docs/admin-guide/device-registry-config/">
+        Device Registry Configuration
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+      <li data-nav-id="/hono/docs/admin-guide/amqp-network-config/" title="AMQP 1.0 Messaging Network Configuration" class="dd-item ">
+        <a href="/hono/docs/admin-guide/amqp-network-config/">
+        AMQP 1.0 Messaging Network Configuration
+        
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+        Monitoring &amp; Tracing
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+          &nbsp;<i class='fas fa-plug'></i>&nbsp;API
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+      <li data-nav-id="/hono/docs/api/telemetry-api/" title="Telemetry API Specification" class="dd-item ">
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+        Telemetry API
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+
+            
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+  
+    
+      <li data-nav-id="/hono/docs/api/event-api/" title="Event API Specification" class="dd-item ">
+        <a href="/hono/docs/api/event-api/">
+        Event API
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+     
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+
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+  
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+      <li data-nav-id="/hono/docs/api/command-and-control-api/" title="Command &amp; Control API Specification" class="dd-item ">
+        <a href="/hono/docs/api/command-and-control-api/">
+        Command &amp; Control API
+        
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+    </li>
+     
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+
+            
+          
+            
+            
+
+
+ 
+  
+    
+      <li data-nav-id="/hono/docs/api/tenant-api/" title="Tenant API Specification" class="dd-item ">
+        <a href="/hono/docs/api/tenant-api/">
+        Tenant API
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+     
+  
+ 
+
+            
+          
+            
+            
+
+
+ 
+  
+    
+      <li data-nav-id="/hono/docs/api/device-connection-api/" title="Device Connection API Specification" class="dd-item ">
+        <a href="/hono/docs/api/device-connection-api/">
+        Device Connection API
+        
+        </a>
+    </li>
+     
+  
+ 
+
+            
+          
+            
+            
+
+
+ 
+  
+    
+      <li data-nav-id="/hono/docs/api/device-registration-api/" title="Device Registration API Specification" class="dd-item ">
+        <a href="/hono/docs/api/device-registration-api/">
+        Device Registration API
+        
+        </a>
+    </li>
+     
+  
+ 
+
+            
+          
+            
+            
+
+
+ 
+  
+    
+      <li data-nav-id="/hono/docs/api/credentials-api/" title="Credentials API Specification" class="dd-item ">
+        <a href="/hono/docs/api/credentials-api/">
+        Credentials API
+        
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+    </li>
+     
+  
+ 
+
+            
+          
+            
+            
+
+
+ 
+  
+    
+      <li data-nav-id="/hono/docs/api/authentication-api/" title="Authentication API Specification" class="dd-item ">
+        <a href="/hono/docs/api/authentication-api/">
+        Authentication API
+        
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+    </li>
+     
+  
+ 
+
+            
+          
+            
+            
+
+
+ 
+  
+    
+      <li data-nav-id="/hono/docs/api/metrics/" title="Metrics" class="dd-item ">
+        <a href="/hono/docs/api/metrics/">
+        Metrics
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+     
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+ 
+
+            
+          
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+              
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+  
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+    <li data-nav-id="/hono/docs/deployment/" title="Deployment" class="dd-item 
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+      <li data-nav-id="/hono/docs/deployment/helm-based-deployment/" title="Helm based Deployment" class="dd-item ">
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+      <li data-nav-id="/hono/docs/deployment/openshift/" title="OpenShift / OKD" class="dd-item ">
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+        OpenShift / OKD
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+
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+          
+            
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+
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+ 
+  
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+      <li data-nav-id="/hono/docs/deployment/create-kubernetes-cluster/" title="Setting up a Kubernetes Cluster" class="dd-item ">
+        <a href="/hono/docs/deployment/create-kubernetes-cluster/">
+        Setting up a Kubernetes Cluster
+        
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+
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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&rsquo;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&rsquo;s HTTP protocol adapter</a></li>
+<li><a href="#hono-s-mqtt-protocol-adapter">Hono&rsquo;s MQTT protocol adapter</a></li>
+<li><a href="#examples">Examples</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>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 &lsquo;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&rsquo;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">&gt; 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&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.</p>
+
+<p>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.</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&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).
+<br /></li>
+</ul>
+
+<h3 id="hono-s-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/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&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 <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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