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     <li><a href="#southbound-operations">Southbound Operations</a>
       <ul>
         <li><a href="#produce-telemetry-data">Produce Telemetry Data</a></li>
       </ul>
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     <li><a href="#northbound-operations">Northbound Operations</a>
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         <li><a href="#consume-telemetry-data">Consume Telemetry Data</a></li>
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             <h1>

               Telemetry API for Kafka Specification
             </h1>


 <p>The <em>Telemetry</em> API is used by <em>Protocol Adapters</em> to send telemetry data downstream.
 <em>Business Applications</em> and other consumers use the API to receive data published by devices belonging to a particular tenant.</p>
 <p>The Telemetry API for Kafka is an alternative to the <a href="/hono/docs/1.7/api/telemetry/">Telemetry API for AMQP</a>.
 With this API, clients publish telemetry data to an Apache Kafka® cluster instead of an AMQP Messaging Network.</p>
 <p>See <a href="/hono/docs/1.7/api/kafka-api/">Kafka-based APIs</a> for fundamental information about Hono&rsquo;s Kafka-based APIs.
 The statements there apply to this specification.</p>
 <div class="alert alert-notice">
     <h4 class="alert-heading"><i class="fas fa-info-circle"></i> Tech preview</h4>
     <div>The support of Kafka as a messaging system is currently a preview and not yet ready for production. The APIs are subject to change without prior notice.</div>
 </div>
 <h2 id="southbound-operations">Southbound Operations</h2>
 <p>The following operation can be used by <em>Protocol Adapters</em> to send telemetry data received from devices to downstream consumers like <em>Business Applications</em>.</p>
 <h3 id="produce-telemetry-data">Produce Telemetry Data</h3>
 <p>The protocol adapter writes messages to the tenant-specific topic <code>hono.telemetry.${tenant_id}</code> where <code>${tenant_id}</code> is the ID of the tenant that the client wants to upload telemetry data for.</p>
 <p><strong>Preconditions</strong></p>
 <ol>
 <li>Either the topic <code>hono.telemetry.${tenant_id}</code> exists, or the broker is configured to automatically create topics on demand.</li>
 <li>The client is authorized to write to the topic.</li>
 <li>The device for which the adapter wants to send telemetry data has been registered (see <a href="/hono/docs/1.7/api/device-registration/">Device Registration API</a>).</li>
 </ol>
 <p><strong>Message Flow</strong></p>
 <p>It is up to the discretion of the protocol adapter whether it wants to use <em>AT LEAST ONCE</em> or <em>AT MOST ONCE</em> delivery semantics
 according to <a href="/hono/docs/1.7/api/kafka-api/#at-least-once-producers">AT LEAST ONCE Producers</a>
 respectively <a href="/hono/docs/1.7/api/kafka-api/#at-most-once-producers">AT MOST ONCE Producers</a>.</p>
 <p>The recommended Kafka producer properties for <em>AT LEAST ONCE</em> delivery are: <code>enable.idempotence=true</code>, <code>acks=all</code>, <code>max.in.flight.requests.per.connection=5</code>,
 leaving <em>retries</em> unset, and setting <em>delivery.timeout.ms</em> to a reasonable value (supported from Kafka version 1.0.0 on).</p>
 <p>The following sequence diagram illustrates the flow of messages involved in the <em>HTTP Adapter</em> producing a telemetry data message to the Kafka cluster implementing <em>AT MOST ONCE</em> delivery semantics.</p>
 <figure>
     <img src="produce_kafka_qos0.svg"/> <figcaption>
             <h4>Produce telemetry data flow (AT MOST ONCE)</h4>
         </figcaption>
 </figure>

 <ol>
 <li><em>Device</em> <code>4711</code> PUTs telemetry data to the <em>HTTP Adapter</em>
 <ol>
 <li><em>HTTP Adapter</em> produces telemetry data to the <em>Kafka Cluster</em>.</li>
 <li><em>HTTP Adapter</em> acknowledges the reception of the data to the <em>Device</em>.</li>
 </ol>
 </li>
 </ol>
 <p>The following sequence diagram illustrates the flow of messages involved in the <em>HTTP Adapter</em> producing a telemetry data message to the Kafka cluster implementing <em>AT LEAST ONCE</em> delivery semantics.</p>
 <figure>
     <img src="produce_kafka_qos1.svg"/> <figcaption>
             <h4>Produce telemetry data flow (AT LEAST ONCE)</h4>
         </figcaption>
 </figure>

 <ol>
 <li><em>Device</em> <code>4711</code> PUTs telemetry data to the <em>HTTP Adapter</em>, indicating <em>QoS Level</em> 1.
 <ol>
 <li><em>HTTP Adapter</em> produces telemetry data to the <em>Kafka Cluster</em>.</li>
 <li><em>Kafka Cluster</em> acknowledges reception of the message.</li>
 <li><em>HTTP Adapter</em> acknowledges the reception of the data to the <em>Device</em>.</li>
 </ol>
 </li>
 </ol>
 <p>When a Kafka producer raises an exception while sending a telemetry message to Kafka, the protocol adapter MUST NOT try to re-send such rejected messages but SHOULD indicate the failed transfer to the device if the transport protocol provides means to do so.</p>
 <p><strong>Use <em>AT LEAST ONCE</em> and <em>AT MOST ONCE</em> in a Protocol Adapter</strong></p>
 <p>If a protocol adapter should support both delivery semantics, a single producer MUST be used and it MUST be configured
 for <em>AT LEAST ONCE</em>. It may not wait for acknowledgements of <em>AT MOST ONCE</em> messages.</p>
 <p>A protocol adapter MUST NOT use two producers sending telemetry data for the same device.
 Otherwise, the messages from a device with different QoS levels could be out of order, as Kafka does not guarantee the
 message order between multiple producer instances.</p>
 <p><strong>Message Format</strong></p>
 <p>The key of the message MUST be the device ID.</p>
 <p>Metadata MUST be set as Kafka headers on a message.
 The following table provides an overview of the headers a client needs to set on a <em>Produce Telemetry Data</em> message.</p>
 <table>
 <thead>
 <tr>
 <th style="text-align:left">Name</th>
 <th style="text-align:center">Mandatory</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>content-type</em></td>
 <td style="text-align:center">yes</td>
 <td style="text-align:left"><em>string</em></td>
 <td style="text-align:left">A content type indicating the type and characteristics of the data contained in the message value as a valid MIME type, e.g. <code>text/plain; charset=&quot;utf-8&quot;</code> for a text message or <code>application/json</code> etc. The value may be set to <code>application/octet-stream</code> if the message message value is to be considered <em>opaque</em> binary data. See <a href="https://www.ietf.org/rfc/rfc2046.txt">RFC 2046</a> for details.</td>
 </tr>
 <tr>
 <td style="text-align:left"><em>content-encoding</em></td>
 <td style="text-align:center">no</td>
 <td style="text-align:left"><em>string</em></td>
 <td style="text-align:left">The content encoding as defined in section 3.5 of <a href="https://www.ietf.org/rfc/rfc2616.txt">RFC 2616</a>.</td>
 </tr>
 <tr>
 <td style="text-align:left"><em>creation-time</em></td>
 <td style="text-align:center">no</td>
 <td style="text-align:left"><em>long</em></td>
 <td style="text-align:left">The instant in time (milliseconds since the Unix epoch) when the message has been created. This header is mandatory if <em>ttd</em> or <em>ttl</em> is set, otherwise, it is optional.</td>
 </tr>
 <tr>
 <td style="text-align:left"><em>device_id</em></td>
 <td style="text-align:center">yes</td>
 <td style="text-align:left"><em>string</em></td>
 <td style="text-align:left">The identifier of the device that the data in the message value is originating from.</td>
 </tr>
 <tr>
 <td style="text-align:left"><em>qos</em></td>
 <td style="text-align:center">no</td>
 <td style="text-align:left"><em>int</em></td>
 <td style="text-align:left">The quality of service level of the message. Supported values are <code>0</code> for AT MOST ONCE and <code>1</code> for AT LEAST ONCE.</td>
 </tr>
 <tr>
 <td style="text-align:left"><em>ttd</em></td>
 <td style="text-align:center">no</td>
 <td style="text-align:left"><em>int</em></td>
 <td style="text-align:left">The <em>time &lsquo;til disconnect</em> indicates the number of seconds that the device will remain connected to the protocol adapter. The value of this header must be interpreted relative to the message&rsquo;s <em>creation-time</em>. A value of <code>-1</code> is used to indicate that the device will remain connected until further notice, i.e. until another message indicates a <em>ttd</em> value of <code>0</code>. In absence of this property, the connection status of the device is to be considered indeterminate. <em>Backend Applications</em> might use this information to determine a time window during which the device will be able to receive a command.</td>
 </tr>
 <tr>
 <td style="text-align:left"><em>ttl</em></td>
 <td style="text-align:center">no</td>
 <td style="text-align:left"><em>long</em></td>
 <td style="text-align:left">The <em>time-to-live</em> in milliseconds. A consumer of the message SHOULD discard the message if the sum of <em>creation-time</em> and <em>ttl</em> is greater than the current time (milliseconds since the Unix epoch).</td>
 </tr>
 </tbody>
 </table>
 <p>Protocol adapters MAY add additional headers to the Kafka record.</p>
 <p>The value of the message MUST consist of a byte array containing the telemetry data. The format and encoding of the data MUST be indicated by the <em>content-type</em> and (optional) <em>content-encoding</em> headers of the message.</p>
 <h2 id="northbound-operations">Northbound Operations</h2>
 <p>The following operation can be used by <em>Business Applications</em> to receive telemetry data from Kafka.</p>
 <h3 id="consume-telemetry-data">Consume Telemetry Data</h3>
 <p>Hono delivers messages containing telemetry data reported by a particular device in the same order that they have been received in (using the <a href="#produce-telemetry-data">Produce Telemetry Data</a> operation).
 <em>Business Applications</em> consume messages from the tenant-specific topic <code>hono.telemetry.${tenant_id}</code> where <code>${tenant_id}</code> represents the ID of the tenant the client wants to retrieve telemetry data for.</p>
 <p><strong>Preconditions</strong></p>
 <ol>
 <li>Either the topic <code>hono.telemetry.${tenant_id}</code> exists, or the broker is configured to automatically create topics on demand.</li>
 <li>The client is authorized to read from the topic.</li>
 <li>The client subscribes to the topic with a Kafka consumer.</li>
 </ol>
 <p>A consumer can provide <em>AT MOST ONCE</em> or <em>AT LEAST ONCE</em> delivery semantics depending on the offset commit strategy it implements.
 If a message has the <em>qos</em> Kafka header with the value <code>1</code>, the consumer is expected to provide proper
 <em>AT LEAST ONCE</em> semantics, and therefore MUST NOT commit the message <em>offset</em> before it has been fully processed.</p>
 <p><strong>Message Flow</strong></p>
 <p>The following sequence diagram illustrates the flow of messages involved in a <em>Business Application</em> consuming a telemetry data message from the Kafka cluster. The delivery mode used is <em>AT LEAST ONCE</em>.</p>
 <figure>
     <img src="consume_kafka.svg"/> <figcaption>
             <h4>Consume Telemetry Data</h4>
         </figcaption>
 </figure>

 <ol>
 <li><em>Business Application</em> polls telemetry messages from <em>Kafka Cluster</em>.
 <ol>
 <li><em>Kafka Cluster</em> returns a batch of messages.</li>
 </ol>
 </li>
 <li><em>Business Application</em> processes the messages.</li>
 <li><em>Business Application</em> commits the offset of the last processed telemetry message.
 <ol>
 <li><em>Kafka Cluster</em> acknowledges the commit.</li>
 </ol>
 </li>
 </ol>
 <p><strong>Message Format</strong></p>
 <p>The format of the messages containing the telemetry data is the same as for the <a href="#produce-telemetry-data">Produce Telemetry Data operation</a>.</p>


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