Gracefully scale down heroku web dynos

Question

Heroku has great utilities to manage web dynos. When we do a small release we use Preboot to gracefully switch over web dynos over to the new release.

However, if you want to scale down your web dynos, there is no graceful way to do so. Ideally we can mark a web dyno so that Heroku Scheduler would stop giving it new requests for 3 minutes before sending SIGTERM.

So if i have 6 dynos running, but outside of office hours I want to ps:scale web=2 , it sends SIGTERM to the other 4. If there was a straggler in slow times routed to one of those 4 (instead of the remaining 2), they'd experience a crash resulting from SIGTERM.

Is there a way to gracefully scale down web dynos?

Answer 1

Correctly handling SIGTERM is the recommended approach for graceful shutdown, see https://devcenter.heroku.com/articles/dynos#graceful-shutdown-with-sigterm . Please note, this the same shutdown process that occurs during preboot too. So when your process receives a SIGTERM, it should stop responding to requests, and finish responding to its current requests, after 30 seconds, if the process hasn't exited, then the process is forcefully SIGKILL'd. There is no builtin way to perform rolling scaledown, you'd have to write your own interface on the Heroku API to achieve this.