Why Does Heroku Report H15 Errors for Closed WebSocket Connections?

Troubleshooting Web Socket Close Errors on Heroku (H15)

When deploying a WebSocket server on Heroku, it's possible to encounter an H15 error when the client closes the connection. This error is often interpreted as a timeout issue, leaving developers puzzled. However, the root cause of this error is a misconception within the Heroku Router engine.

Upon investigating the behavior in detail, it becomes evident that this error is merely a "false positive." The client typically pings the server every 20 seconds, effectively keeping the connection active. However, when the connection is closed, the Heroku Router mistakenly perceives the request as being idle for the entire duration of the open connection (up to 55 seconds).

Consequently, when the connection is closed by the client, even gracefully, the Heroku Router registers it as having service time exceeding the allowed duration, triggering the H15 error.

To summarize, the H15 error on Heroku when closing WebSocket connections is not a genuine issue but rather a misinterpretation by the Heroku Router. It commonly occurs when users leave the application, leading to the perceived "too much time" error.

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