Why Does My Go WebSocket App on Heroku Throw an H15 Error on Connection Closure?

H15 Error on Heroku Web Socket Termination

When deploying a Go web socket service on Heroku, users often encounter the frustrating "H15 Error" during socket connection termination. This error suggests that the request took an excessive amount of time, despite the client pinging the server every 20 seconds to keep the connection alive.

The underlying cause of this issue lies in the "false positive" nature of Heroku's Router engine. Here's a detailed explanation:

Connection Establishment and Heartbeats

1. Upon establishing a WebSocket connection, the client and server typically engage in an HTTP polling connection before switching to WebSocket.
2. After transitioning to WebSocket, the client sends regular heartbeats to prevent the Heroku Router from terminating the request.
3. As long as these heartbeats are received, the Heroku Router timer remains reset.

Connection Termination and H15 Error

However, when the client closes the connection, the Router detects the request termination and assumes it was idle for the duration from request initiation to closure. This leads to the "service=X milliseconds" error being logged, despite the connection having been active with periodic heartbeats.

Conclusion

The H15 Error on Heroku web socket termination is primarily a false positive caused by the system's inability to differentiate between a client-initiated closure and a prolonged inactivity. It commonly occurs when users leave or close their application, leading to a harmless but annoying error message.

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