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<h1 class="title indent">Streaming updates</h1>
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<time datetime="2015-02-24T22:41:27+00:00" pubdate data-updated="true"><i class="icon-calendar"></i> February 24, 2015</time>
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<span class="byline author vcard"><i class='icon-user'></i> Paulus Schoutsen</span>
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<span><i class='icon-time'></i> 1 minute reading time</span>
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<p>Home Assistant has learned a new trick to get the latest information from the server: streaming updates. No longer will the frontend poll every 30 seconds for updates but instead it will keep a connection open and get the latest changes pushed as soon as they happen.</p>
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<p>A new toggle has been added ot the sidebar to turn streaming updates on and off. This preference will be saved on a per-browser basis using local storage. The toggle will also indicate when there is an error setting up a stream after which it will fall back to use polling.</p>
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<p>Streaming updates has been implemented using the HTML5 <code class="highlighter-rouge">EventSource</code> tag. Implementation is pretty straight forward as all the reconnection logic will be handled by the event source tag. The <a href="https://github.com/home-assistant/home-assistant/blob/master/homeassistant/components/api.py#L90">server-side code</a> is 50 lines and the <a href="https://github.com/home-assistant/home-assistant-js/blob/master/src/actions/stream.js">client-side code</a> is 80 lines of code.</p>
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<p>All events that happen on the server will now also be sent to the browser. This turns any browser running the UI into a fully functioning <a href="https://home-assistant.io/developers/architecture/#multiple-connected-instances">slave instance</a> of Home Assistant. This opens up new possibilities for Home Assistant components that live completely client-side.</p>
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<p>Implementing EventSource was not without challenges. Here are some of the issues that had to be solved:</p>
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<p>A connection can go stale in Chrome without any event handler being called. This happens when a device goes into standby. For computers this is rare but for phones this occurs quite often. This has been solved by sending a regular ping from the server. The frontend will assume the connection has gone stale when it hasn’t heard any communication for a while. Sending a ping will also help the server detect broken connections and clean them up.</p>
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<p>Another issue that I encountered is that Safari and Firefox would not fire the <code class="highlighter-rouge">open</code> event when the connection has been opened but when the first message has been received. To work around this the server will now fire a ping when the connection gets opened.</p>
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