title: Scalable Pub/Sub engine for realtime
body: <p>I'm looking for a pub/sub engine, with the following requirements:</p>
  <ol>
  <li>Very low latency &lt; 0.5 sec</li>
  <li>Scalable</li>
  <li>Shardable (based on geo localisation)</li>
  </ol>
  <p>I'd like to be able to have multiple pub/sub servers and be able to publish or subscribe to channels from any server, no matter on which server the channel is declared.</p>
  <p>For example:</p>
  <p>If <strong>user A</strong> is connected to server <strong>SRV1</strong> and <strong>user B</strong> connected to server <strong>SRV2</strong>, If <strong>user B</strong> subscribe to &quot;<em>MyChannel</em>&quot; and <strong>user A</strong> publish something on channel &quot;<em>MyChannel</em>&quot;, <strong>user B</strong> will get the message even if he's not connected to the same server.</p>
  <p>I don't know if Redis is able to do that. I didn't find anything about the subject.</p>