title: ActionCable periodic function called on delay * number of users - is this correct?
body: <p>I have an ActionCable implementation where I want to broadcast the number of users in the channel periodically.</p>
  <p>We have the <code>periodically</code> function allow this to happen so in my channel definition I have:</p>
  <pre><code>periodically :broadcast_users, every: 20.seconds
  </code></pre>
  <p>but when I view as a subscriber to the channel I appear to get the notification every 20 seconds for each subscribed user, e.g. with 8 users subscribed I get the update 8 times when I log the message on the console:</p>
  <pre><code>[Log] {active_users: 8, timestamp: &quot;2020-07-24T19:41:31+01:00&quot;} (application-f27a340538c9c0804ff3897eddd5ce90b3846e28f4b2a488dc5e9767b07764bf.js, line 1)
  [Log] {active_users: 8, timestamp: &quot;2020-07-24T19:41:31+01:00&quot;} (application-f27a340538c9c0804ff3897eddd5ce90b3846e28f4b2a488dc5e9767b07764bf.js, line 1)
  [Log] {active_users: 8, timestamp: &quot;2020-07-24T19:41:32+01:00&quot;} (application-f27a340538c9c0804ff3897eddd5ce90b3846e28f4b2a488dc5e9767b07764bf.js, line 1)
  [Log] {active_users: 8, timestamp: &quot;2020-07-24T19:41:32+01:00&quot;} (application-f27a340538c9c0804ff3897eddd5ce90b3846e28f4b2a488dc5e9767b07764bf.js, line 1)
  [Log] {active_users: 8, timestamp: &quot;2020-07-24T19:41:32+01:00&quot;} (application-f27a340538c9c0804ff3897eddd5ce90b3846e28f4b2a488dc5e9767b07764bf.js, line 1)
  [Log] {active_users: 8, timestamp: &quot;2020-07-24T19:41:33+01:00&quot;} (application-f27a340538c9c0804ff3897eddd5ce90b3846e28f4b2a488dc5e9767b07764bf.js, line 1)
  [Log] {active_users: 8, timestamp: &quot;2020-07-24T19:41:33+01:00&quot;} (application-f27a340538c9c0804ff3897eddd5ce90b3846e28f4b2a488dc5e9767b07764bf.js, line 1)
  [Log] {active_users: 8, timestamp: &quot;2020-07-24T19:41:33+01:00&quot;} (application-f27a340538c9c0804ff3897eddd5ce90b3846e28f4b2a488dc5e9767b07764bf.js, line 1)
  </code></pre>
  <p>The documentation doesn't state this very clearly but is it correct it will broadcast this periodically for every user? e.g. every 20.seconds, once per subscriber?</p>
  <p>Just checking if this is expected behaviour and if so is there a way to set a periodic function that doesn't call per subscribed user so only happens once.</p>