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<title> Acknowledgements • Eclipse Ditto • a digital twin framework</title>
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<a href="#"><span></span>→ Things examples</a>
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<li><a href="protocol-examples-creatething.html">Create a Thing</a></li>
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<li><a href="protocol-examples-modifything.html">Modify a Thing</a></li>
<li><a href="protocol-examples-retrievething.html">Retrieve a Thing</a></li>
<li><a href="protocol-examples-retrievethings.html">Retrieve multiple Things</a></li>
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<li><a href="protocol-examples-retrieveproperty.html">Retrieve a single Property</a></li>
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<a href="#"><span></span>→ Policies examples</a>
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<li><a href="protocol-examples-policies-createpolicy.html">Create a Policy</a></li>
<li><a href="protocol-examples-policies-deletepolicy.html">Delete a Policy</a></li>
<li><a href="protocol-examples-policies-modifypolicy.html">Modify a Policy</a></li>
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<li><a href="protocol-examples-policies-modifypolicyentry.html">Modify a single entry</a></li>
<li><a href="protocol-examples-policies-retrievepolicyentry.html">Retrieve a single entry</a></li>
<li><a href="protocol-examples-policies-modifysubjects.html">Modify subjects</a></li>
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<li><a href="protocol-examples-policies-createsubject.html">Create a single subject</a></li>
<li><a href="protocol-examples-policies-deletesubject.html">Delete a single subject</a></li>
<li><a href="protocol-examples-policies-modifysubject.html">Modify a single subject</a></li>
<li><a href="protocol-examples-policies-retrievesubject.html">Retrieve a single subject</a></li>
<li><a href="protocol-examples-policies-modifyresources.html">Modify resources</a></li>
<li><a href="protocol-examples-policies-retrieveresources.html">Retrieve resources</a></li>
<li><a href="protocol-examples-policies-createresource.html">Create a single resource</a></li>
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<li><a href="protocol-examples-policies-modifyresource.html">Modify a single resource</a></li>
<li><a href="protocol-examples-policies-retrieveresource.html">Retrieve a single resource</a></li>
<li><a href="protocol-examples-policies-errorresponses.html">Error responses</a></li>
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<li><a href="protocol-examples-search.html">→ Search examples</a></li>
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<h1 class="post-title-main">Acknowledgements</h1>
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<div class="bs-callout bs-callout-primary">Available since Ditto <strong>1.1.0</strong></div>
<p>Acknowledgements are a concept in Ditto used to signal that an <a href="basic-signals-event.html">event</a> was successfully
received/processed by either an internal Ditto functionality or an external subscriber of that event.</p>
<p>They can be seen as (potentially multiple) responses to a single event (e.g. after a Thing was modified).</p>
<p>Acknowledgements can be used in order to solve the following cases:</p>
<ul>
<li>postpone a response to an API request (e.g. block an HTTP request) until one or more specific actions were performed in
Ditto (e.g. a modification was successfully persisted).</li>
<li>postpone a response until an external subscriber connected to Ditto reports that he successfully processed an
<a href="basic-signals-event.html">event</a> which e.g. resulted by a persistence change of Ditto.</li>
<li><em>Outlook</em> (to come in Ditto 1.2.0): provide a QoS (quality of service) guarantee of “at least once” when processing
messages in an end-2-end manner by e.g. technically acknowledging/settling a processed message from a message broker
(e.g. <a href="connectivity-protocol-bindings-amqp10.html">AMQP 1.0</a> or <a href="connectivity-protocol-bindings-mqtt.html">MQTT</a>) only
after it was successfully applied to Ditto and potentially also to 3rd parties.</li>
</ul>
<h2 id="acknowledgement-requests">Acknowledgement requests</h2>
<p>For each Ditto API invocation <em>modifying the state of a twin</em> with a <a href="basic-signals-command.html">command</a> it is
possible to define that certain “acknowledgements” are <em>requested</em>.<br />
This means that the Ditto collects all of those requested acknowledgements until the message is handled as successfully
processed, applying a specified timeout interval.</p>
<p>Acknowledgement requests are expressed as protocol specific header fields of commands, more on that
<a href="#requesting-acknowledgements">later</a>.</p>
<p>The <a href="basic-signals-event.html">events</a> emitted by Ditto will include the custom acknowledgement requests in the
<code class="highlighter-rouge">"requested-acks"</code> header.</p>
<h2 id="acknowledgement-labels">Acknowledgement labels</h2>
<p>Acknowledgement labels identify the acknowledgement. Some labels are already engaged by Ditto for other purposes,
and may therefore not be used when sending back a custom acknowledgement.</p>
<h3 id="built-in-acknowledgement-labels">Built-in acknowledgement labels</h3>
<p>Ditto provides built-in acknowledgements that are automatically issued on certain actions in the Ditto cluster:</p>
<ul>
<li><strong>twin-persisted</strong>: An acknowledgement with this label is issued, when a modifying command has successfully
updated the digital twin in Ditto’s persistence.</li>
</ul>
<h3 id="custom-acknowledgement-labels">Custom acknowledgement labels</h3>
<p>In addition to the <a href="#built-in-acknowledgement-labels">built-in</a> acknowledgement requests,
a received <a href="basic-signals-event.html">event</a> can contain
custom acknowledgement labels.
The event subscriber can detect that an acknowledgement was requested
(via the <code class="highlighter-rouge">"requested-acks"</code> header), and - as far as it feels responsible for handling it - it<br />
<a href="#issuing-acknowledgements">issues an acknowledgement</a>.</p>
<h2 id="acknowledgements">Acknowledgements</h2>
<p>A single acknowledgement contains the following information:</p>
<ul>
<li>Acknowledgement label (one of the requested labels of the <a href="#acknowledgement-requests">ack requests</a> of e.g. an event)</li>
<li>Header fields
<ul>
<li>mandatory: <strong>correlation-id</strong> the same correlation-id as the one of the event which requested the acknowledgement</li>
<li>optional: additional header fields</li>
</ul>
</li>
<li>Status code (HTTP status code semantic) defining whether an acknowledgement was successful or not</li>
<li>Optional payload as JSON</li>
</ul>
<p>The <a href="protocol-specification-acks.html">Ditto Protocol specification</a> describes in detail what is contained.<br />
An example of how acknowledgements in Ditto Protocol look like can be found at the
<a href="protocol-examples.html#acknowledgements-acks">acknowledgements acks examples</a> section.</p>
<h2 id="requesting-acknowledgements">Requesting acknowledgements</h2>
<p>With every API call <em>modifying the state of a twin</em> there is the option to provide requested acknowledgements.</p>
<h3 id="via-http">Via HTTP</h3>
<p>Either specify the following HTTP header fields:</p>
<ul>
<li><strong>requested-acks</strong>: a comma separated list of <a href="#acknowledgement-labels">acknowledgement labels</a><br />
Example: <em>requested-acks</em>: twin-persisted,my-custom-ack</li>
<li>timeout: an optional time (in ms, s or m) of how long the HTTP request should wait for acknowledgements and block. Default: <code class="highlighter-rouge">60s</code><br />
Examples: <em>timeout</em>: <code class="highlighter-rouge">42s</code>, <em>timeout</em>: <code class="highlighter-rouge">250ms</code>, <em>timeout</em>: <code class="highlighter-rouge">1m</code></li>
</ul>
<p>Or specify the header fields as query parameters to the HTTP params, e.g.:</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>PUT /api/2/things/org.eclipse.ditto:thing-1?requested-acks=twin-persisted,my-custom-ack&amp;timeout=42s
</code></pre></div></div>
<p>The response of an HTTP request, which requested several acknowledgements, will differ from the response,
where an HTTP request was not requesting acknowledgements.</p>
<p>Example response when 2 acknowledgements were requested and were successful. The overall HTTP status code will be
<code class="highlighter-rouge">200</code> (OK) in this case:</p>
<div class="language-json highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">{</span><span class="w">
</span><span class="s2">"twin-persisted"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"status"</span><span class="p">:</span><span class="w"> </span><span class="mi">201</span><span class="p">,</span><span class="w">
</span><span class="s2">"payload"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"thingId"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto:thing-1"</span><span class="p">,</span><span class="w">
</span><span class="s2">"policyId"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto:thing-1"</span><span class="w">
</span><span class="p">},</span><span class="w">
</span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"db878735-4957-4fd9-92dc-6f09bb12a093"</span><span class="p">,</span><span class="w">
</span><span class="s2">"version"</span><span class="p">:</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w">
</span><span class="s2">"etag"</span><span class="p">:</span><span class="w"> </span><span class="s2">"</span><span class="se">\"</span><span class="s2">rev:1</span><span class="se">\"</span><span class="s2">"</span><span class="p">,</span><span class="w">
</span><span class="s2">"location"</span><span class="p">:</span><span class="w"> </span><span class="s2">"http://127.0.0.1:8080/api/2/things/org.eclipse.ditto:thing-1"</span><span class="w">
</span><span class="p">}</span><span class="w">
</span><span class="p">},</span><span class="w">
</span><span class="s2">"my-custom-ack"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"status"</span><span class="p">:</span><span class="w"> </span><span class="mi">200</span><span class="p">,</span><span class="w">
</span><span class="s2">"payload"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"outcome"</span><span class="p">:</span><span class="w"> </span><span class="s2">"green"</span><span class="w">
</span><span class="p">},</span><span class="w">
</span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"version"</span><span class="p">:</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w">
</span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"db878735-4957-4fd9-92dc-6f09bb12a093"</span><span class="w">
</span><span class="p">}</span><span class="w">
</span><span class="p">}</span><span class="w">
</span><span class="p">}</span><span class="w">
</span></code></pre></div></div>
<p>Example response when 2 acknowledgements were requested and one lead to a timeout. The overall HTTP status code will be
<code class="highlighter-rouge">424</code> (Dependency failed) in this case:</p>
<div class="language-json highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">{</span><span class="w">
</span><span class="s2">"twin-persisted"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"status"</span><span class="p">:</span><span class="w"> </span><span class="mi">201</span><span class="p">,</span><span class="w">
</span><span class="s2">"payload"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"thingId"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto:thing-1"</span><span class="p">,</span><span class="w">
</span><span class="s2">"policyId"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto:thing-1"</span><span class="w">
</span><span class="p">},</span><span class="w">
</span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"db878735-4957-4fd9-92dc-6f09bb12a093"</span><span class="p">,</span><span class="w">
</span><span class="s2">"version"</span><span class="p">:</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w">
</span><span class="s2">"etag"</span><span class="p">:</span><span class="w"> </span><span class="s2">"</span><span class="se">\"</span><span class="s2">rev:1</span><span class="se">\"</span><span class="s2">"</span><span class="p">,</span><span class="w">
</span><span class="s2">"location"</span><span class="p">:</span><span class="w"> </span><span class="s2">"http://127.0.0.1:8080/api/2/things/org.eclipse.ditto:thing-1"</span><span class="w">
</span><span class="p">}</span><span class="w">
</span><span class="p">},</span><span class="w">
</span><span class="s2">"my-custom-ack"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"status"</span><span class="p">:</span><span class="w"> </span><span class="mi">408</span><span class="p">,</span><span class="w">
</span><span class="s2">"payload"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"status"</span><span class="p">:</span><span class="w"> </span><span class="mi">408</span><span class="p">,</span><span class="w">
</span><span class="s2">"error"</span><span class="p">:</span><span class="w"> </span><span class="s2">"acknowledgement:request.timeout"</span><span class="p">,</span><span class="w">
</span><span class="s2">"message"</span><span class="p">:</span><span class="w"> </span><span class="s2">"The acknowledgement request reached the specified timeout of 42,000ms."</span><span class="p">,</span><span class="w">
</span><span class="s2">"description"</span><span class="p">:</span><span class="w"> </span><span class="s2">"Try increasing the timeout and make sure that the requested acknowledgement is sent back in time."</span><span class="w">
</span><span class="p">},</span><span class="w">
</span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="s2">"version"</span><span class="p">:</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w">
</span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"db878735-4957-4fd9-92dc-6f09bb12a093"</span><span class="w">
</span><span class="p">}</span><span class="w">
</span><span class="p">}</span><span class="w">
</span><span class="p">}</span><span class="w">
</span></code></pre></div></div>
<h3 id="via-websocket">Via WebSocket</h3>
<p>Put the <code class="highlighter-rouge">"requested-acks"</code> (as JsonArray of strings) and the <code class="highlighter-rouge">"timeout"</code> headers in the
<a href="protocol-specification.html#headers">Ditto Protocol headers</a> section of Ditto Protocol JSON.</p>
<p>The response will be an <a href="protocol-specification-acks.html">acknowledgements</a> message.</p>
<h3 id="via-connections">Via Connections</h3>
<p>For messages received by connections via AMQP, MQTT, etc. put the <code class="highlighter-rouge">"requested-acks"</code> (as JsonArray of strings) and the
<code class="highlighter-rouge">"timeout"</code> headers in the <a href="protocol-specification.html#headers">Ditto Protocol headers</a> section of Ditto Protocol JSON.</p>
<p>The response will be an (aggregating) <a href="protocol-specification-acks.html#acknowledgements-aggregating">acknowledgements</a>
message.</p>
<h2 id="issuing-acknowledgements">Issuing acknowledgements</h2>
<p>In order to issue a single acknowledgement when requested by a published event, an
<a href="protocol-specification-acks.html#acknowledgement">Ditto Protocol acknowledgement</a> message has to be built and sent
back, using the same <code class="highlighter-rouge">"correlation-id"</code> in the <a href="protocol-specification.html#headers">protocol headers</a> as the event
contained.</p>
<div class="tags">
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