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   <ul>
     <li><a href="#southbound-operations">Southbound Operations</a>
       <ul>
         <li><a href="#produce-event">Produce Event</a></li>
       </ul>
     </li>
     <li><a href="#northbound-operations">Northbound Operations</a>
       <ul>
         <li><a href="#consume-events">Consume Events</a></li>
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     <li><a href="#well-known-event-message-types">Well-known Event Message Types</a>
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         <li><a href="#empty-notification">Empty Notification</a></li>
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             <h1>

               Event API for Kafka Specification
             </h1>


 <p>The <em>Event</em> API is used by <em>Protocol Adapters</em> to send event messages downstream.
 <em>Business Applications</em> and other consumers use the API to receive messages published by devices belonging to a particular tenant.</p>
 <p>The Event API for Kafka is an alternative to the <a href="/hono/docs/1.7/api/event/">Event API for AMQP</a>.
 With this API, clients publish event messages 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 event messages received from devices to downstream consumers like <em>Business Applications</em>.</p>
 <h3 id="produce-event">Produce Event</h3>
 <p>The protocol adapter writes messages to the tenant-specific topic <code>hono.event.${tenant_id}</code> where <code>${tenant_id}</code> is the ID of the tenant that the client wants to upload event messages for.</p>
 <p><strong>Preconditions</strong></p>
 <ol>
 <li>Either the topic <code>hono.event.${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 event messages 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>Hono supports <em>AT LEAST ONCE</em> delivery of <em>Event</em> messages only, as defined in <a href="/hono/docs/1.7/api/kafka-api/#at-least-once-producers">Telemetry API for Kafka</a>.</p>
 <p>The following sequence diagram illustrates the flow of messages involved in the <em>MQTT Adapter</em> producing an event to the Kafka cluster.</p>
 <figure>
     <img src="produce_kafka.svg"/> <figcaption>
             <h4>Produce event flow</h4>
         </figcaption>
 </figure>

 <ol>
 <li><em>Device</em> <code>4711</code> publishes an event using MQTT QoS 1.
 <ol>
 <li><em>MQTT Adapter</em> produces an event message to the <em>Kafka Cluster</em>.</li>
 <li><em>Kafka cluster</em> acknowledges reception of the message.</li>
 <li><em>MQTT Adapter</em> acknowledges the reception of the message to the <em>Device</em>.</li>
 </ol>
 </li>
 </ol>
 <p>When a Kafka producer raises an exception while sending an event message to Kafka, the protocol adapter MUST NOT try to re-send such rejected messages but MUST indicate the failed transfer to the device if the transport protocol provides means to do so.</p>
 <p><strong>Message Format</strong></p>
 <p>See <a href="/hono/docs/1.7/api/telemetry-kafka/#produce-telemetry-data">Telemetry API for Kafka</a> for the definition of the message format.</p>
 <h2 id="northbound-operations">Northbound Operations</h2>
 <p>The following operation can be used by <em>Business Applications</em> to receive event messages from Kafka.</p>
 <h3 id="consume-events">Consume Events</h3>
 <p>Hono delivers messages containing events reported by a particular device in the same order that they have been received in (using the <a href="#produce-event">Produce Event</a> operation).
 <em>Business Applications</em> consume messages from the tenant-specific topic <code>hono.event.${tenant_id}</code> where <code>${tenant_id}</code> represents the ID of the tenant the client wants to retrieve event messages for.</p>
 <p><strong>Preconditions</strong></p>
 <ol>
 <li>Either the topic <code>hono.event.${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>Hono supports <em>AT LEAST ONCE</em> delivery of <em>Event</em> messages only. A consumer is expected to provide proper <em>AT LEAST ONCE</em>
 semantics and therefore MUST NOT commit offsets of messages that have not yet 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 an event message from the Kafka cluster.</p>
 <figure>
     <img src="consume_kafka.svg"/> <figcaption>
             <h4>Consume event flow (success)</h4>
         </figcaption>
 </figure>

 <ol>
 <li><em>Business Application</em> polls event 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 event message.
 <ol>
 <li><em>Kafka Cluster</em> acknowledges the commit.</li>
 </ol>
 </li>
 </ol>
 <p><strong>Message Format</strong></p>
 <p>See <a href="/hono/docs/1.7/api/telemetry-kafka/#produce-telemetry-data">Telemetry API for Kafka</a> for the definition of the message format.</p>
 <h2 id="well-known-event-message-types">Well-known Event Message Types</h2>
 <p>Hono defines several <em>well-known</em> event types which have specific semantics. Events of these types are identified by means of the message&rsquo;s <em>content-type</em> header.</p>
 <h3 id="empty-notification">Empty Notification</h3>
 <p>A Kafka message containing this type of event does not have any payload so the value of the message MUST be empty.</p>
 <p>The headers an event sender needs to set for an <em>empty notification</em> event are defined in the <a href="/hono/docs/1.7/api/telemetry-kafka/#produce-telemetry-data">Telemetry API for Kafka</a>.</p>
 <p>The relevant headers are listed again in the following table:</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">MUST be set to <em>application/vnd.eclipse-hono-empty-notification</em></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> as described in the <a href="/hono/docs/1.7/api/telemetry-kafka/#produce-telemetry-data">Telemetry API for Kafka</a>.</td>
 </tr>
 </tbody>
 </table>
 <p><strong>NB</strong> An empty notification can be used to indicate to a <em>Business Application</em> that a device is currently ready to receive an upstream message by setting the <em>ttd</em> property.
 <em>Backend Applications</em> may use this information to determine the time window during which the device will be able to receive a command.</p>
 <h3 id="connection-event">Connection Event</h3>
 <p>Protocol Adapters may send this type of event to indicate that a connection with a device has
 been established or has ended. Note that such events can only be sent for authenticated devices,
 though, because an event is always scoped to a tenant and the tenant of a device is
 established as part of the authentication process.</p>
 <p>The Kafka message for a connection event MUST contain the following headers in addition to the standard event headers:</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">MUST be set to  <em>application/vnd.eclipse-hono-dc-notification+json</em>.</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 ID of the authenticated device.</td>
 </tr>
 </tbody>
 </table>
 <p>Each connection event&rsquo;s message value MUST contain a UTF-8 encoded string representation of a single JSON object with 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">Type</th>
 <th style="text-align:left">Description</th>
 </tr>
 </thead>
 <tbody>
 <tr>
 <td style="text-align:left"><em>cause</em></td>
 <td style="text-align:center">yes</td>
 <td style="text-align:left"><em>string</em></td>
 <td style="text-align:left">The cause of the connection event. MUST be set to either <code>connected</code> or <code>disconnected</code>.</td>
 </tr>
 <tr>
 <td style="text-align:left"><em>remote-id</em></td>
 <td style="text-align:center">yes</td>
 <td style="text-align:left"><em>string</em></td>
 <td style="text-align:left">An identifier of the device that is the subject of this event, e.g. an IP address:port, client id etc. The format and semantics of this identifier is specific to the protocol adapter and the transport protocol it supports.</td>
 </tr>
 <tr>
 <td style="text-align:left"><em>source</em></td>
 <td style="text-align:center">yes</td>
 <td style="text-align:left"><em>string</em></td>
 <td style="text-align:left">The type name of the protocol adapter reporting the event, e.g. <code>hono-mqtt</code>.</td>
 </tr>
 <tr>
 <td style="text-align:left"><em>data</em></td>
 <td style="text-align:center">no</td>
 <td style="text-align:left"><em>object</em></td>
 <td style="text-align:left">An arbitrary JSON object which may contain additional information about the occurrence of the event.</td>
 </tr>
 </tbody>
 </table>
 <p>The example below might be used by the MQTT adapter to indicate that a connection with a device using client identifier <code>mqtt-client-id-1</code> has been established:</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;cause&#34;</span>: <span style="color:#e6db74">&#34;connected&#34;</span>,
   <span style="color:#f92672">&#34;remote-id&#34;</span>: <span style="color:#e6db74">&#34;mqtt-client-id-1&#34;</span>,
   <span style="color:#f92672">&#34;source&#34;</span>: <span style="color:#e6db74">&#34;hono-mqtt&#34;</span>,
   <span style="color:#f92672">&#34;data&#34;</span>: {
     <span style="color:#f92672">&#34;foo&#34;</span>: <span style="color:#e6db74">&#34;bar&#34;</span>
   }
 }
 </code></pre></div>

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