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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: |
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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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