Rule: Support native `--rule-file` auto-reload in `thanos rule`

[yosshi825]

Is your proposal related to a problem?

thanos rule supports reloading rule files via SIGHUP or POST /-/reload, but it does not automatically detect updates to files specified by --rule-file.

This is particularly inconvenient in Kubernetes environments where rule files are managed via ConfigMap or Secret. Even when a mounted file is updated automatically, thanos rule does not pick up the change on its own, so users still need an external reloader sidecar just to detect the file change and call /-/reload.

The --rule-file flag description itself also notes this current limitation:

Note that rules are not automatically detected, use SIGHUP or do HTTP POST /-/reload to re-read them.

Describe the solution you'd like

It would be very helpful if thanos rule had a native auto-reload mechanism for files specified by --rule-file.

For example, an optional mechanism that:

• watches rule files on disk
• detects updates automatically
• reloads rules without requiring an external sidecar
• keeps the current behavior if the updated rule files are invalid

A feature similar in spirit to Prometheus --enable-feature=auto-reload-config would be ideal.

Describe alternatives you've considered

The current workaround is to use an external reloader sidecar or watcher process that detects file changes and then triggers SIGHUP or POST /-/reload.

This works, but it adds operational complexity, especially in Kubernetes deployments where mounted ConfigMap / Secret updates are common.

I also looked at existing behavior in other Thanos components. Notably, thanos receive already supports automatic reload for --receive.hashrings-file, so there is already a precedent in Thanos for native file-watching and dynamic reload behavior.

Additional context

This would be especially useful for Kubernetes deployments where rule files are updated through mounted ConfigMaps or Secrets.

It would also make thanos rule more consistent with other systems that already support native config/rule auto-reload.

Reference for the Prometheus feature mentioned above:
Prometheus: Auto Reload Config

[yosshi825]

@GiedriusS
Thanks for adding the good first issue and help wanted labels.
I'd like to work on this issue myself, if that sounds good to you.

My plan is to investigate how thanos receive handles file watching/reload today, and also look into Prometheus --enable-feature=auto-reload-config as a similar reference, to see what approach could be applied to thanos rule for --rule-file.

Please let me know if you have any concerns or guidance before I start.

[GiedriusS]

It seems to me that we could just make --rule-file= automatic without any parameter changes. We should also reuse the logic that Receive uses. So, if a change is detected through inotify (or through somewhere else), just call reloadRules() and that's all?

[yosshi825]

Thanks for the guidance!

That makes sense. I'll proceed in that direction: reusing the file-watching logic from thanos receive and calling reloadRules() on change detection, without introducing new parameters.

I'll start by exploring the existing Receive implementation and the current thanos rule reload flow to see how cleanly they can be connected.

[yosshi825]

@GiedriusS
Hi, I’ve opened a PR related to this issue.

When you have time, I would greatly appreciate it if you could review it and share any feedback.