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<p><a href="basic-signals.html">Signals</a> already described what an <a href="basic-signals-event.html">Event</a> in Ditto is.
Events are emitted after an entity (either a <strong>digital twin</strong> or an actual device) was changed.</p>
<p>At the Ditto API there are different ways for getting notified of such events:</p>
<ul>
<li>Via the <a href="httpapi-protocol-bindings-websocket.html">WebSocket API</a> a WebSocket client gets all Events the authenticated subject
(e.g. a user) is <a href="basic-auth.html">authorized</a> to receive as <a href="protocol-overview.html">Ditto Protocol</a> messages.</li>
<li>Via <a href="httpapi-sse.html">HTTP SSEs</a> a consumer of the SSE <code class="highlighter-rouge">EventSource</code> gets all Events the authenticated subject
(e.g. a user) is <a href="basic-auth.html">authorized</a> to receive directly in the format of the changed entity
(e.g. as <a href="basic-thing.html#model-specification">Thing JSON</a> format).</li>
<li>Via an established <a href="basic-connections.html">connection</a> in the <a href="connectivity-overview.html">connectivity</a> service</li>
</ul>
<h2 id="filtering">Filtering</h2>
<p>In order to not get all of the events an authenticated <a href="basic-auth.html">subject</a> (e.g. a user added in nginx) is allowed
to see, but to filter for specific criteria, events may be filtered on the Ditto backend side.</p>
<p>The above mentioned different APIs provide their own mechanisms on how to define such filters, but they all share the
common functionality of based on which information events may be filtered.</p>
<div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note:</b> All filters are specified in an URL query format, therefore their values should be URL
encoded before sending them to the backend. The equal (=) and the ampersand (&amp;) character must be encoded in any RQL
filter!</div>
<h3 id="by-namespaces">By namespaces</h3>
<p>Filtering may be done based on a namespace name. Each Ditto <a href="basic-thing.html">Thing</a> has an ID containing a namespace
(see also the conventions for a <a href="basic-thing.html#thing-id">Thing ID</a>).</p>
<p>By providing the <code class="highlighter-rouge">namespaces</code> filter, a comma separated list of which namespaces to include in the result, only Things
in namespaces of interest are considered and thus only events of these Things are emitted at the API.</p>
<p>For example, one would only subscribe for events occurring in 2 specific namespaces by defining:</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>namespaces=org.eclipse.ditto.one,org.eclipse.ditto.two
</code></pre></div></div>
<h3 id="by-rql-expression">By RQL expression</h3>
<p>If filtering by namespaces it not sufficient, Ditto also allows to provide an <a href="basic-rql.html">RQL expression</a> specifying a
Thing payload based condition determining which events should be emitted and which don’t.</p>
<div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note:</b> This filter is applied on the modified data of a Thing, data which was not changed will
not be considered when applying the filter.</div>
<p>This provides the opportunity to formulate filters like the following:</p>
<h4 id="examples">Examples</h4>
<p>Only emit events when attribute “count” was changed to a value greater than 42:</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>filter=gt(attributes/count,42)
</code></pre></div></div>
<p>Only emit events for Things starting with myThing when a feature “lamp” was modified:</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>filter=and(like(thingId,"org.eclipse.ditto:myThing*"),exists(features/lamp))
</code></pre></div></div>
<p>Only emit events when the attribute “manufacturer” was changed to starting with “ACME &amp; Sons”.
The <code class="highlighter-rouge">&amp;</code> must be escaped in that case:</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>filter=like(attributes/manufacturer,"ACME %26 Sons*")
</code></pre></div></div>
<p>You get the idea of how mighty this becomes by utilizing Ditto’s <a href="basic-rql.html">RQL expressions</a>.</p>
<div class="tags">
<b>Tags: </b>
<a href="tag_model.html" class="btn btn-default navbar-btn cursorNorm" role="button">model</a>
<a href="tag_rql.html" class="btn btn-default navbar-btn cursorNorm" role="button">rql</a>
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