Why Does My Go WebSocket App on Heroku Get an H15 Error on Socket Close?

Investigating the Heroku H15 Error on Web Socket Close

WebSockets, an essential protocol for real-time communication, can encounter issues when deployed on hosting platforms like Heroku. One such issue is the dreaded Heroku H15 error, which occurs when the platform terminates a request due to inactivity.

Issue Description

A user reported experiencing the H15 error when using a Go service as a WebSocket server on Heroku. The client regularly pinged the server but encountered the error when the socket connection was closed. Heroku logs indicated the connection had remained open for an extended period, triggering the H15 error as if the request had exceeded the timeout limit.

Debugging and Solution

Upon investigating, the H15 error was found to be a "false positive" in the Heroku Router engine. The debugging process involved:

1. Establishing a WebSocket connection between client and server.
2. Analyzing Heroku Router logs, which revealed polling connections before WebSocket protocol was established.
3. Identifying heartbeat messages that kept the connection alive and reset the Heroku Router timeout timer.
4. Tracing the H15 error to the termination of the connection by the client. Heroku Router logs indicated the duration of the connection rather than the actual inactivity time.

Conclusion

The Heroku H15 error on WebSocket close arises when the client app terminates a WebSocket connection after prolonged use. While the error may initially raise concerns, it often represents users leaving the application rather than any actual errors in the service.

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