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Asked: 2026-06-13T11:43:40+00:00

I have a Node.js application on Heroku. I’ve recently switched a long query to

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I have a Node.js application on Heroku. I’ve recently switched a long query to use SSE (EventSource). While it works great and fast on my machine, I keep getting an Error H15 (Idle connection) on my GET request. The H15 description says I went over 55 seconds allowed per transaction. But my entire query doesn’t take more than 4-5 seconds.

Furthermore, after reading the description, I’m returning my first byte (just a number) immediately upon hitting the query, before it starts any heavy work – and it still doesn’t work.

My question/s are:

  1. What do I need to do in order to keep this thing going? An HTTP header parameter? Something on the client JS side? How do I avoid H15?
  2. Am I trying to do something not supported by Heroku? I don’t think so, as I have another app on Heroku that uses EventSource successfully (express-eventsource.herokuapp.com, source on github: https://github.com/TravelingTechGuy/express-eventsource)

As always, thanks for your time.

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1 Answer

  1. Editorial Team

    After trying everything, changing and re-writing my code, profiling the network etc., finally my friend (with no Node knowledge whatsoever) came up with an idea that put me on the right track:

    It seems like this is a domain issue – NOT a code issue at all.
    I set an ANAME mapping and it turns out that since Heroku uses 5 different IPs, mapping to just one of them can screw up an SSE call. Basically, mydomain.com showed an IP different than mydomain.herokuapp.com – meaning the server tried returning a response to a different IP than the one that initiated the call, hence it timed out.

    I ditched the ANAME in favor of a CNAME (basically, giving up the naked mydomain.com in favor of http://www.mydomain.com) and now it seems to be working.

    Conclusions:

    1. Not everything is a code issue – if it works well in one environment and not in another, it’s a configuration issue
    2. If you struggle with something for too long, it helps to bring in a new set of eyes. Even with no subject matter expertise, you’ll get great ideas to fresh approaches you can try
    3. 1and1 DNS control sucks b@lls. Moving out of there
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