Announcing the sd-notify crate

This is a quick post announcing the sd-notify crate. sd-notify is a Rust library for interacting with systemd or a compatible service manager.

If you're not running Linux or you don't like systemd, this crate is not for you.

systemd in 3 minutes

Assuming you are running systemd, at some point you might find yourself with a program you want to automatically run on start-up. Historically, this meant doing an intricate dance which includes calling fork() twice and writing your PID to a file, but fortunately, with systemd this is no longer required and is actually discouraged.

You can take your plain console application and make a systemd unit for it:

[Unit]
Description=Monitors room temperature

[Service]
User=stats
ExecStart=/usr/local/bin/monitor-temperature
Restart=on-failure
RestartSec=10

[Install]
WantedBy=default.target

You then drop that file under /etc/systemd/system/monitor-temperature.service, run systemctl daemon-reload, systemctl enable --now monitor-temperature and you're done.

For the effort, you get logs (journalctl -efu monitor-temperature), precise child process tracking (using control groups), CPU, memory and I/O accounting, automatic restarts, and a way to manage your service across all the popular Linux distros.

systemd start-up types

If your daemon has a long start-up sequence, systemd can tell you whether it is ready or not. In order to do that, you can set the Type clause.

The simplest, and also the default, option is called simple. It means that the daemon is marked as ready right after it staarts. Another useful option is forking, which means that the service does the classic double-fork dance. You can read more about these two (any many others) in your systemd.service documentation page.

What the sd-notify crate helps with is the notify start-up type. This lets you do whatever initialization you want, then notify systemd that you are ready by sending a over a Unix socket. Services can also say that they are reloading their settings, stopping, or even set a status message. The protocol is described in detail here.

Using sd-notify

The basic usage is quite simple. You do your initialization steps, then call:

let _ = sd_notify::notify(true, &[NotifyState::Ready]);

That's all and now your daemon only shows as "running" when it finished starting up.

$ systemctl status monitor-temperature
     Active: activating (start) since Wed 2022-01-12 20:36:02 EET; 9s ago
     [snip]
$ systemctl status monitor-temperature
     Active: active (running) since Wed 2022-01-12 20:36:12 EET; 2s ago
     [snip]

You can also include a status message:

let _ = sd_notify::notify(
    true,
    &[
        NotifyState::Ready,
        NotifyState::Status("Monitoring room temperature"),
    ],
);
$ systemctl status monitor-temperature
     Active: active (running) since Wed 2022-01-12 20:41:46 EET; 782ms ago
     Status: "Monitoring room temperature"
     [snip]

There's also one advanced feature: you can retrieve file descriptors passed by the service manager, for socket-activated daemons. You can read more on it here.

Final words

sd-notify is written in pure Rust, has no dependencies and is available under a permissive license (MIT or Apache-2.0). My intention is that it stays lightweight, but I'm not necessarily opposed to adding more features.

If you want more functionality today, try the libsystemd or rust-systemd crates.