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<title> Weak acknowledgments to decouple signal publishers and subscribers </title>
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<h1 class="post-title" itemprop="name headline">Weak acknowledgments to decouple signal publishers and subscribers</h1>
<p class="post-meta">Published by <img src="https://www.gravatar.com/avatar/d923944b6a8c76e411ff838fb19acfa7?s=135" alt="A photo of Yufei Cai" style="width:50px;border-radius:50%;display:inline-block;margin-right:5px;" /><span itemprop="author" itemscope itemtype="http://schema.org/Person"><span itemprop="name"><a href="https://github.com/yufei-cai" target="_blank">Yufei Cai</a> </span></span> on <time datetime="2020-11-16T00:00:00+00:00" itemprop="datePublished">Nov 16, 2020</time> - Tags:
<a href="tag_blog.html">blog</a>
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<h2 id="motivation">Motivation</h2>
<p><a href="2020-08-31-release-announcement-120.html">Ditto 1.2.0</a> introduced at-least-once delivery via
<a href="basic-acknowledgements.html">acknowledgement requests</a>.<br />
It increased coupling between the publisher and the subscriber of signals in that the subscriber is no longer at the
liberty to filter for signals it is interested in. Instead, the subscriber must consume all signals in order to
fulfill acknowledgement requests and prevent endless redelivery.</p>
<p>To combat the problem,
<a href="2020-10-28-release-announcement-140.html">Ditto 1.4.0</a> made acknowledgement labels unique and introduced the requirement
to manage <a href="basic-acknowledgements.html#issuing-acknowledgements"><em>declared acknowledgements</em></a>, identifying of each
subscriber.<br />
It is now possible for Ditto to issue
<a href="basic-acknowledgements.html#weak-acknowledgements-wacks"><em>weak acknowledgements</em></a> on behalf of the subscriber
whenever it decides to not consume a signal. That allows subscribers to configure RQL and namespace filters freely
without causing any futile redelivery.</p>
<div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note:</b> Weak acknowledgements are available since Ditto 1.5.0.</div>
<h2 id="what-it-is">What it is</h2>
<p>A <a href="basic-acknowledgements.html#weak-acknowledgements-wacks"><em>weak acknowledgement</em></a> is issued by Ditto for any
<a href="basic-acknowledgements.html#requesting-acks">acknowledgement request</a> that will not be fulfilled now or ever without
configuration change.<br />
A weak acknowledgement is identified by the header <code class="highlighter-rouge">ditto-weak-ack: true</code>.</p>
<p>The status code of weak acknowledgements is <code class="highlighter-rouge">200 OK</code>; it signifies that any redelivery is not to be made on their
account.</p>
<p>A weak acknowledgement may look like this in Ditto protocol:</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">"topic"</span><span class="p">:</span><span class="w"> </span><span class="s2">"com.acme/xdk_53/things/twin/acks/my-mqtt-connection:my-mqtt-topic"</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">"ditto-weak-ack"</span><span class="p">:</span><span class="w"> </span><span class="kc">true</span><span class="w">
</span><span class="p">},</span><span class="w">
</span><span class="s2">"path"</span><span class="p">:</span><span class="w"> </span><span class="s2">"/"</span><span class="p">,</span><span class="w">
</span><span class="s2">"value"</span><span class="p">:</span><span class="w"> </span><span class="s2">"Acknowledgement was issued automatically, because the subscriber is not authorized to receive the signal."</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="w">
</span><span class="p">}</span><span class="w">
</span></code></pre></div></div>
<h2 id="how-it-works">How it works</h2>
<p>Since Ditto 1.4.0, subscribers of <em>twin events</em> or <em>live signals</em> are required to declare unique acknowledgement labels
they are allowed to send. The labels of acknowledgement requests are then identifying the intended subscribers.<br />
If the intended subscriber exists but does not receive the signal for non-transient reasons, Ditto issues
a weak acknowledgement for that subscriber.<br />
Such reasons may be:</p>
<ul>
<li>The intended subscriber <strong>is not authorized</strong> to receive the signal by policy;</li>
<li>The intended subscriber did not subscribe for the signal type (<em>twin event, live command, live event or live message</em>);</li>
<li>The intended subscriber filtered the signal out by its <a href="basic-changenotifications.html#filtering">namespace or RQL filter</a>;</li>
<li>The intended subscriber dropped the signal because its <a href="connectivity-mapping.html">payload mapper</a> produced nothing.</li>
</ul>
<h2 id="limitation">Limitation</h2>
<p>The distributed nature of cluster pub/sub means that weak acknowledgements are not always issued correctly.<br />
They are only <em>eventually correct</em> in the sense that some time after a change to the publisher-subscriber pair,
the issued weak acknowledgements will reflect the change.<br />
Such changes include:</p>
<ul>
<li>Opening and closing of Websocket or other connections acting as the subscriber;</li>
<li>Subscribing and unsubscribing for different signal types via Websocket;</li>
<li>Modification of connections via the <a href="connectivity-manage-connections.html">connectivity API</a>;</li>
<li>Migration of a connection from one Ditto cluster member to another due to load balancing.</li>
</ul>
<h2 id="feedback">Feedback?</h2>
<p>Please <a href="feedback.html">get in touch</a> if you have feedback or questions towards this new concept of weak
acknowledgements.</p>
<p><br />
<br /></p>
<figure><img class="docimage" src="images/ditto.svg" alt="Ditto" style="max-width: 500px" /></figure>
<p><br />
The Eclipse Ditto team</p>
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