title: How to create a simple cache using RxJs
body: <p>I would like to create a cache srvice using RxJs. Idea is simple, I have a class that will fetch some data from server initialy and a websocket that will push notification to this service (add / edit / delete of entity).</p>
  <p>The following code is a simplification to make me anderstand how to achieve what I need.</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>const source = of(['World', 'test', 'azerty', 'uiop']);
  const add$ = new Subject&lt;string&gt;();
  const update$ = new Subject&lt;string&gt;();
  const delete$ = new Subject&lt;string&gt;();</code></pre>
  </div>
  </div>
  </p>
  <p>So, the real question is: How can I create an high order observable that will react on any update from add$, update$ and delete$ ?</p>
  <p>I would like to have one variable 'cache$' that is an observable composed from source, add update and delete. And initialy filled from data streamed by source (that will be an http call observable).</p>