EDIT: I just realized that it is an old question. Not sure, why this popped up on my feed. Anyway, it might help someone in the future.

I believe this is happening when someone keeps the page open without any activity on the channel and when the connection stays open but idle for 55 seconds, Heroku closes the connection with the error Idle connection. You have to keep the connection alive, by implementing some sort of ping system which periodically sends a message to the server and the server simply sends an ok in response. A simple example is:

app/assets/javascripts/channels/notifications.js:

App.notifications = App.cable.subscriptions.create('NotificationsChannel', {    received: function(data) {    // ignore the data when the type is 'pong' as it is just server's response to our 'ping'    if(data.type !== 'pong'){      return this.showMessage(data);    }  },  showMessage: function(data) {    showNotice(data.message);  },  ping: function() {    return this.perform('ping', {      message: {"event": "ping"}    });  }});document.addEventListener("DOMContentLoaded", function(event) {  // this will send a ping event every 30 seconds  setInterval(function(){    App.notifications.ping()  }, 30000);});

On the server-side, you need to send a response message e.g 'pong' to the 'ping' event. You can send that response to the same user, or broadcast to all users, depending on your channels setup and requirement.

class NotificationsChannel < ApplicationCable::Channel    def subscribed    stream_from "notifications_#{current_user.id}" if current_user  end  # data is not required, you can choose to ignore it or not send it at all from the client-side.  def ping(data)    NotificationsChannel.broadcast_to("notifications_#{current_user.id}", { message: 'pong' })  endend

This should keep the connection alive even if the user is idle.

I have not worked with ActionCable since it's first release. Let me know if something does not work or any syntax error, I will try to fix.

Which plan of heroku do you use? If free - don't forget that it sleeps after 30 minutes of inactivity, what might cause some H15 errors (idle connection...)

This probably happens because either

1. HTTP connection isn't ended "normally" since the server returns the 101 response (Switching Protocols - to establish the websocket connection) - or
2. at some point the underlying TCP connection breaks down (user moves to new wifi, closes laptop etc)

Being pragmatic - the resulting websocket seems to be functioning correctly on the underlying TCP connection. Any issues with the websocket connection trigger the retry logic in the browser which cleans things up from the user's perspective - so the app is functioning normally for my users despite this log message.

For my app I felt that it was safe to just ignore the H15 error code from Heroku but that may not be safe for you if you have other situations where HTTP requests could likely become hung up.

Check out the Heroku documentation for reference:

https://devcenter.heroku.com/articles/error-codes#h15-idle-connection

H15 - Idle connection

The dyno did not send a full response and was terminated due to 55 seconds of inactivity. For example, the response indicated a Content-Length of 50 bytes which were not sent in time.

2010-10-06T21:51:37-07:00 heroku[router]: at=error code=H15 desc="Idle connection" method=GET path="/" host=myapp.herokuapp.com fwd=17.17.17.17 dyno=web.1 connect=1ms service=55449ms status=503 bytes=18