Integration of a Go service with systemd

Vincent Bernat

Unlike other programming languages, Go’s runtime doesn’t provide a way to reliably daemonize a service. A system daemon has to supply this functionality. Most distributions ship systemd which would fit the bill. A correct integration with systemd is quite straightforward. There are two interesting aspects: readiness & liveness.

As an example, we will daemonize this service whose goal is to answer requests with nifty 404 errors:

package main

import (
    "log"
    "net"
    "net/http"
)

func main() {
    l, err := net.Listen("tcp", ":8081")
    if err != nil {
        log.Panicf("cannot listen: %s", err)
    }
    http.Serve(l, nil)
}

You can build it with go build 404.go.

Here is the service file, 404.service1:

[Unit]
Description=404 micro-service

[Service]
Type=notify
ExecStart=/usr/bin/404
WatchdogSec=30s
Restart=on-failure

[Install]
WantedBy=multi-user.target

Readiness

The classic way for an Unix daemon to signal its readiness is to daemonize. Technically, this is done by calling fork(2) twice (which also serves other intents). This is a very common task and the BSD systems, as well as some other C libraries, supply a daemon(3) function for this purpose. Services are expected to daemonize only when they are ready (after reading configuration files and setting up a listening socket, for example). Then, a system can reliably initialize its services with a simple linear script:

syslogd
unbound
ntpd -s

Each daemon can rely on the previous one being ready to do its work. The sequence of actions is the following:

  1. syslogd reads its configuration, activates /dev/log, daemonizes.
  2. unbound reads its configuration, listens on 127.0.0.1:53, daemonizes.
  3. ntpd reads its configuration, connects to NTP peers, waits for clock to be synchronized2, daemonizes.

With systemd, we would use Type=fork in the service file. However, Go’s runtime does not support that. Instead, we use Type=notify. In this case, systemd expects the daemon to signal its readiness with a message to an Unix socket. go-systemd package handles the details for us:

package main

import (
    "log"
    "net"
    "net/http"

    "github.com/coreos/go-systemd/daemon"
)

func main() {
    l, err := net.Listen("tcp", ":8081")
    if err != nil {
        log.Panicf("cannot listen: %s", err)
    }
    daemon.SdNotify(false, "READY=1") // ❶
    http.Serve(l, nil)                // ❷
}

It’s important to place the notification after net.Listen() (in ❶): if the notification was sent earlier, a client would get “connection refused” when trying to use the service. When a daemon listens to a socket, connections are queued by the kernel until the daemon is able to accept them (in ❷).

If the service is not run through systemd, the added line is a no-op.

Liveness

Another interesting feature of systemd is to watch the service and restart it if it happens to crash (thanks to the Restart=on-failure directive). It’s also possible to use a watchdog: the service sends watchdog keep-alives at regular interval. If it fails to do so, systemd will restart it.

We could insert the following code just before http.Serve() call:

go func() {
    interval, err := daemon.SdWatchdogEnabled(false)
    if err != nil || interval == 0 {
        return
    }
    for {
        daemon.SdNotify(false, "WATCHDOG=1")
        time.Sleep(interval / 3)
    }
}()

However, this doesn’t add much value: the goroutine is unrelated to the core business of the service. If for some reason, the HTTP part gets stuck, the goroutine will happily continue to send keep-alives to systemd.

In our example, we can just do a HTTP query before sending the keep-alive. The internal loop can be replaced with this code:

for {
    _, err := http.Get("http://127.0.0.1:8081") // ❸
    if err == nil {
        daemon.SdNotify(false, "WATCHDOG=1")
    }
    time.Sleep(interval / 3)
}

In ❸, we connect to the service to check if it’s still working. If we get some kind of answer, we send a watchdog keep-alive. If the service is unavailable or if http.Get() gets stuck, systemd will trigger a restart.

There is no universal recipe. However, checks can be split into two groups:

  • Before sending a keep-alive, you execute an active check on the components of your service. The keep-alive is sent only if all checks are successful. The checks can be internal (like in the above example) or external (for example, check with a query to the database).

  • Each component reports its status, telling if it’s alive or not. Before sending a keep-alive, you check the reported status of all components (passive check). If some components are late or reported fatal errors, don’t send the keep-alive.

If possible, recovery from errors (for example, with a backoff retry) and self-healing (for example, by reestablishing a network connection) is always better, but the watchdog is a good tool to handle the worst cases and avoid too complex recovery logic.

For example, if a component doesn’t know how to recover from an exceptional condition3, instead of using panic(), it could signal its situation before dying. Another dedicated component could try to resolve the situation by restarting the faulty component. If it fails to reach an healthy state in time, the watchdog timer will trigger and the whole service will be restarted.


  1. Depending on the distribution, this should be installed in /lib/systemd/system or /usr/lib/systemd/system. Check with the output of the command pkg-config systemd --variable=systemdsystemunitdir

  2. This highly depends on the NTP daemon used. OpenNTPD doesn’t wait unless you use the -s option. ISC NTP doesn’t either unless you use the --wait-sync option. 

  3. An example of an exceptional condition is to reach the limit on the number of file descriptors. Self-healing from this situation is difficult and it’s easy to get stuck in a loop. 


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