Exit after configurable idle period for servers with limited RAM or are off/suspended when unused

[tim-seoss]

Output of rest-server --version

rest-server version rest-server 0.14.0-dev compiled with go1.24.4 on linux/amd64

What should rest-server do differently? Which functionality do you think we should add?

Optionally exit after a user-configurable idle period (i.e. no open client connections).

What are you trying to do? What problem would this solve?

Allow a backup server running the restic rest server to automatically sleep to conserve energy when not in use.

When backups are not needed, my backup server is in a low power sleep state (currently ACPI S3 "suspend to RAM", but S4 "hibernate" or S5 "standby/off" are also possible). Overall, this saves approx 90% of the energy which would otherwise be consumed by the backup server if it was left running continuously.

The proposed enhancement allows this to be done more reliably, with greater energy savings, and with less manual monitoring and intervention.

Current arrangement

The system wakes by RTC alarm shortly before a "rendezvous time" that coincides with the backup clients' scheduled backup time.

Arranging wake-up for a "normally-asleep" server is straightforward e.g. by:

• A Linux systemd timer which uses a cron-like OnCalendar trigger with WakeSystem=true
• Similar wake-on-RTC functionality on non-systemd servers.
• "Wake on LAN" (also allows easy remote wake for restores)

(I use a combination of wake-on-RTC for scheduled backups and wake on LAN for manual restores).

Deficiencies

It's not currently trivial to put the server back to sleep once scheduled backups have completed. I currently use a manually configured timer to sleep the system at a fixed time-of-day, but this is prone to both:

• Sleeping too soon: unexpected large changesets can mean that the backups "overrun" and fail to complete with knock-on implications such as stale locks as well as reduced security.
• Staying awake unnecessarily: The safety margin necessary to cover nearly all backups results in +30% unnecessary power usage in my case.

Proposed Solution

Do not run the rest-server when there are no active backups in progress. Do not sleep the system when rest-server is running.

I'm already using systemd socket activation to automatically start the rest server on client connect.

If the server exits once it becomes idle (i.e. no active connections) then systemd-inhibit can be used to postpone a suspend whilst the rest server remains running.

Other mechanisms (e.g. a script with a lock file) could be used to inhibit sleep on non-systemd systems.

Additional benefits

• Servers with extremely limited RAM could benefit by minimising memory usage when the rest server is not in use.
• Potential to limit rest-server RAM usage growth over weeks/months (due to memory fragmentation).
• Mechanisms like systemd-inhibit (which this feature enables) can also be used to postpone scheduled jobs such as automatic security updates (e.g. reboot due to new kernel or upgraded system firmware) so that they don't interrupt in-progress backups.
• Allows the continued use of cheap older hardware which might otherwise have prohibitively high financial and environmental running costs (if running continuously or otherwise for longer periods).
• The continued use (on a "light duty" basis) of older hardware avoids energy and resource expenditure ("embodied energy") due to manufacturing new replacement hardware.

Did rest-server help you today? Did it make you happy in any way?

Thanks for writing and maintaining restic, I find it very useful!

[tim-seoss]

I have implemented this functionality and I'm currently testing it. I'm not very familiar with go, so the code may be a bit rough. Once I've tested for a couple of weeks I plan to tidy the code up and submit a PR. In its current form the changeset:

• Adds a command line flag --exit-after-idle-seconds. If unset the functionality is disabled.
• Adds a module to track the number of open connections.
• The time when the connection count falls to zero is recorded.
• When the idle time is exceeded, the process exits (currently sends SIGINT to itself in order to exit cleanly - there maybe a better way).

[tim-seoss]

TODO:

• Unit test for connection tracking module
• Tidy up changeset
• Document deployment steps (including sample systemd unit files and related configuration changes as well as potential outline methods for non-systemd systems)