Creator of DevOps & Node.js contractor in London. •

Just over two weeks ago, Heroku announced (the long-awaited) experimental support for WebSockets. Just like you would expect from Heroku, getting set up is painless and there’s good documentation and good sample code to help you.

I was curious to see what kind of performance one could expect, so I did a quick performance test to find out. (I used Heavy Artillery to generate the load)

Results - updated (28/10)

  1. You can maintain around 6,000 open connections on a single dyno
  2. Creating more than 160 connections/sec will cause H11 errors (backlog too deep)

For comparison, an entry-level box from Digital Ocean (512MB RAM, Ubuntu 13.10) can handle 350+ new connections/sec, and seems to max out at around 7,200 open connections.

Here is the code for the server:

As you can see, it does nothing apart from accepting connections.

The client sessions were very simple: connect and do nothing until disconnected by the server.

Draw your own conclusions whether the numbers above are good or not.

Test Runs

I ran 7 tests in total.

The first one created 1000 connections/second for 30 seconds - that obviously didn’t go well. The other 5 were me doing a binary search to find the number of connections/second one could make without seeing H11 errors pop up. I got bored between 160 (okay) and 175 (H11’s start popping up).


If you think I may have messed something up or should’ve done something differently, please share your thoughts on Hacker News or tweet at me - @hhv1. You can also send me an email at

I will be updating this post with more numbers as I run more benchmarks. Scaling WebSocket-based apps is a subject dear to my heart.