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| <h1 class="post-title-main">HTTP API server sent events (SSE)</h1> |
| </div> |
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| <p>HTML5 <a href="#" data-toggle="tooltip" data-original-title="Server Sent Events - one way messaging/push from a HTTP server to a consuming client (e.g. a browser).">SSEs</a> may be used in order |
| to get notified when the state of <strong>digital twins</strong> change.</p> |
| |
| <p>This is the second mechanism of Ditto in order to get <a href="basic-changenotifications.html">change notifications</a>. |
| The benefit of this mechanism in contrary to the <a href="httpapi-protocol-bindings-websocket.html">WebSocket</a> channel is that it is |
| even easier to open a SSE connection from the client than a WebSocket and that in Ditto’s interpretation of SSEs the |
| events sent back from the backend have the same JSON structure as the HTTP API on which they are invoked.</p> |
| |
| <h2 id="server-sent-events">Server-Sent Events</h2> |
| |
| <p>Different as WebSockets, Server-Sent Events are unidirectional originating from the back-end towards the client. Via SSEs |
| the client can only be notified, it cannot send data back (it can use plain HTTP therefore).</p> |
| |
| <p>For a detailed introduction into SSEs, please visit |
| the <a href="https://html.spec.whatwg.org/multipage/server-sent-events.html">HTML5 specification</a>.</p> |
| |
| <div class="alert alert-danger" role="alert"><i class="fa fa-exclamation-circle"></i> <b>Warning:</b> Although SSEs are a HTML5 standard, they still cannot be used in every browser: |
| see <a href="https://caniuse.com/#search=Server-sent%20events">caniuse.com</a> - Microsoft IE11 and Edge browser still doesn’t support them.</div> |
| |
| <h3 id="sses-in-javascript">SSEs in JavaScript</h3> |
| |
| <p>Using the <code class="highlighter-rouge">EventSource</code> object in JavaScript is also well covered in the <a href="https://html.spec.whatwg.org/multipage/server-sent-events.html#server-sent-events-intro">HTML5 specification</a>.</p> |
| |
| <h2 id="sse-api-things">SSE API <code class="highlighter-rouge">/things</code></h2> |
| |
| <p>Currently the only supported HTTP API for receiving <a href="#" data-toggle="tooltip" data-original-title="Server Sent Events - one way messaging/push from a HTTP server to a consuming client (e.g. a browser).">SSEs</a> |
| is the <code class="highlighter-rouge">/things</code> endpoint:</p> |
| <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>http://localhost:8080/api/<1|2>/things |
| </code></pre></div></div> |
| |
| <p>When the endpoint is invoked with an HTTP header <code class="highlighter-rouge">Accept</code> with value <code class="highlighter-rouge">text/event-stream</code>, an Server-Sent Event stream of |
| <a href="basic-changenotifications.html">change notifications</a> is created by Ditto and for each notification for which the caller |
| has READ permissions (see <a href="basic-auth.html#authorization">authorization</a>), a message is sent to the client.</p> |
| |
| <p>The format of the message at the <code class="highlighter-rouge">/things</code> endpoint is always in the form of a <a href="basic-thing.html#model-specification">Thing JSON</a> |
| (in API 1 format or API 2 format depending on which edpoint the SSE was created).</p> |
| |
| <p>For partial updates to a <code class="highlighter-rouge">Thing</code> however, only the changed part is sent back via the SSE, not the complete <code class="highlighter-rouge">Thing</code>.</p> |
| |
| <h3 id="only-get-notified-about-certain-changes">Only get notified about certain changes</h3> |
| |
| <p>In order to apply a server side filtering of which Server-Sent Events should be emitted to a consumer, Ditto provides |
| several possibilities listed in the sections below.</p> |
| |
| <p>All of the query parameters below can be combined, so that you can for example formulate that only events from a certain |
| namespace with a specific RQL expression should be emitted which could look like:</p> |
| <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>http://localhost:8080/api/<1|2>/things?namespaces=org.eclipse.ditto.one,org.eclipse.test&filter=gt |
| (attributes/counter,42) |
| </code></pre></div></div> |
| |
| <h4 id="specify-the-thing-ids">Specify the Thing IDs</h4> |
| |
| <p>When the <code class="highlighter-rouge">/things</code> endpoint is used for connecting to the SSE stream, all for the authenticated user visible <code class="highlighter-rouge">Things</code> are |
| included in the stream. If only specific <code class="highlighter-rouge">Things</code> should be watched, the query parameter <code class="highlighter-rouge">ids</code> can be added:</p> |
| <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>http://localhost:8080/api/<1|2>/things?ids=<thingId1>,<thingId2> |
| </code></pre></div></div> |
| |
| <h4 id="fields-projection">Fields projection</h4> |
| |
| <p>Additionally using the <code class="highlighter-rouge">fields</code> parameter of the <a href="httpapi-concepts.html#partial-requests">partial request</a> feature, only |
| specific parts can be watched for changes, e.g.:</p> |
| <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>http://localhost:8080/api/<1|2>/things?fields=thingId,attributes |
| </code></pre></div></div> |
| |
| <div class="alert alert-success" role="alert"><i class="fa fa-check-square-o"></i> <b>Tip:</b> The <code class="highlighter-rouge">thingId</code> should always be included in the <code class="highlighter-rouge">fields</code> query, otherwise it is no longer |
| reproducible, for which Thing the change was made.</div> |
| |
| <h4 id="filtering-by-namespaces">Filtering by namespaces</h4> |
| |
| <p>As described in <a href="basic-changenotifications.html#by-namespaces">change notifications</a>, it is possible to only subscribe |
| for changes done in certain namespaces. At the SSE API, simply specify the <code class="highlighter-rouge">namespaces</code> parameter and provide a comma |
| separated list of which namespaces to select, e.g.:</p> |
| <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>http://localhost:8080/api/<1|2>/things?namespaces=org.eclipse.ditto.one,org.eclipse.test |
| </code></pre></div></div> |
| |
| <h4 id="filtering-by-rql-expression">Filtering by RQL expression</h4> |
| |
| <p>As also described in <a href="basic-changenotifications.html#by-rql-expression">change notifications</a>, it is additionally possible |
| to specify an RQL expression expressing on which occasions to emit an event via the SSE API. Simply specify the <code class="highlighter-rouge">filter</code> |
| parameter with an <a href="basic-rql.html">RQL expression</a>, e.g.:</p> |
| <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>http://localhost:8080/api/<1|2>/things?filter=gt(attributes/counter,42) |
| </code></pre></div></div> |
| |
| <h3 id="example-for-sse-on-things">Example for SSE on Things</h3> |
| |
| <p>Assuming we have a Thing (in API version 1) with the following JSON content:</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">"thingId"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto:fancy-thing"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"acl"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"{userId}"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"READ"</span><span class="p">:</span><span class="w"> </span><span class="kc">true</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"WRITE"</span><span class="p">:</span><span class="w"> </span><span class="kc">true</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"ADMINISTRATE"</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="p">},</span><span class="w"> |
| </span><span class="s2">"attributes"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"manufacturer"</span><span class="p">:</span><span class="w"> </span><span class="s2">"ACME corp"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"complex"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"some"</span><span class="p">:</span><span class="w"> </span><span class="kc">false</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"serialNo"</span><span class="p">:</span><span class="w"> </span><span class="mi">4711</span><span class="w"> |
| </span><span class="p">}</span><span class="w"> |
| </span><span class="p">},</span><span class="w"> |
| </span><span class="s2">"features"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"lamp"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"properties"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"on"</span><span class="p">:</span><span class="w"> </span><span class="kc">false</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"color"</span><span class="p">:</span><span class="w"> </span><span class="s2">"blue"</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><span class="p">}</span><span class="w"> |
| </span></code></pre></div></div> |
| |
| <p>From within JavaScript we can now create an <code class="highlighter-rouge">EventSource</code> in order to open up a SSE stream in Ditto and simply print each |
| event to the console. This one tracks only changes to the Thing with ID <code class="highlighter-rouge">org.eclipse.ditto:fancy-thing</code> and only watches |
| for changes on the feature <code class="highlighter-rouge">lamp</code>:</p> |
| <div class="language-javascript highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// the javascript must be served from the same domain as Ditto is running in order to avoid CORS problems</span> |
| <span class="kd">let</span> <span class="nx">source</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">EventSource</span><span class="p">(</span><span class="s1">'/api/1/things?ids=org.eclipse.ditto:fancy-thing&fields=thingId,features/lamp'</span><span class="p">);</span> |
| <span class="nx">source</span><span class="p">.</span><span class="nx">onmessage</span> <span class="o">=</span> <span class="kd">function</span> <span class="p">(</span><span class="nx">event</span><span class="p">)</span> <span class="p">{</span> |
| <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">event</span><span class="p">.</span><span class="nx">data</span><span class="p">);</span> |
| <span class="p">};</span> |
| </code></pre></div></div> |
| |
| <p>This would print for each <code class="highlighter-rouge">Thing</code> the authenticated user is allowed to <code class="highlighter-rouge">READ</code> (see <a href="basic-acl.html">acl</a>) the changed content of the Thing.</p> |
| |
| <p>So when the <code class="highlighter-rouge">on</code> property of the <code class="highlighter-rouge">lamp</code> feature is changed to <code class="highlighter-rouge">true</code> via such an HTTP API call:</p> |
| <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>PUT /api/1/things/org.eclipse.ditto:fancy-thing/features/lamp/properties/on |
| payload: true |
| </code></pre></div></div> |
| |
| <p>The JavaScript snippet would log to console:</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">"thingId"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto:fancy-thing"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"features"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"lamp"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"properties"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"on"</span><span class="p">:</span><span class="w"> </span><span class="kc">false</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><span class="p">}</span><span class="w"> |
| </span></code></pre></div></div> |
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