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POUsing Yaws and websockets to subscribe to data
 

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    <p>I have a gen_server speaking to several hardware sensors. This setup works fine. Now I want to extend this system with a real time visualization of the data. And I want to deliver this data to a web client over websockets.</p> <p>As I might have several "listeners", e.g. several people visualizing this data on different web browsers, I am thinking of something resembling the Observer Pattern. Each time a web client asks to subscribe to a sensor, it should be added to a list of stakeholders for that sensor. As soon as new sensor data arrives, it should be pushed out to the client without delay.</p> <p>I am using yaws to quickly get websocket functionality. My problem is related to the way that yaws seems to work. On the server side I only "see" the client at connection time through the <code>A#arg.clisock</code> value (e.g. <code>#Port&lt;0.2825&gt;</code>). In the code below I register <code>ws_server</code> to receive the callbacks when new data enters from the client. After this point yaws seems to only allow me to respond to messages entering server side.</p> <pre><code>out(A) -&gt; CallbackMod = ws_server, Opts = [{origin, "http://" ++ (A#arg.headers)#headers.host}], {websocket, CallbackMod, Opts}. </code></pre> <p>This is what the callback module looks like:</p> <pre><code>% handle_message(Incoming) % Incoming :: {text,Msg} | {binary,Msg} | {close, Status, Reason} handle_message({Type,Data}) -&gt; gen_server:cast(?SERVER,{websocket,Data}), noreply. </code></pre> <p>Nowhere, it seems, am I able to react to a message such as <code>subscribe &lt;sensor&gt;</code> and (after connection time) dynamically add this stakeholder to a list of observers.</p> <p>How can I use the yaws server to accomplish asynchronous pushing of data to client <em>and</em> during session add and remove sensors that I want to listen to. Basically the easiest way would be if yaws could call back <code>handle_message/2</code> with first argument being <code>From</code>. Otherwise I need to add a ref to keep a ref on both sides and send that to server each time, it seems backwards that I need to keep that information.</p>
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