TL;DR

[Service]
Type=notify
NotifyAccess=all
TimeoutSec=an appropriate value for your application

Problem

For whatever reason, you may want to notify systemd that your application is ready.

Solution

Your service can have the Type defined as notify: Then, it is expected that the service sends a notification message via sd_notify(3) or an equivalent call when it has finished starting up.

systemd-notify --ready is a way of letting systemd know your app is ready. There may be a library to do this in your language, as the sd_notify gem for Ruby.

So, the first step is to change the Type of your service to notify.

Then, you should look at what value do you need for NotifyAccess. It’ll depend on how you start your app. If the process configured with ExecStart is the main process of the service, then it should be fine with main or exec - I couldn’t find in the documentation if there’s a difference; it treats it as the same. However, if the command you have in ExecStart starts a new process that is the actual app, then your NotifyAccess should be all. Unless you start the new process with the same PID, which can be achieved by using exec when starting it, for example.

You should also look to how long your app takes to start. If it takes more than the default value for TimeoutStartSec or TimeoutSec, then you must update it. Otherwise, systemd kills your app before it’s able to notify systemd that it’s ready, then systemd tries to launch it again, but it will never succeed since probably it will timeout again. Then your service will be stuck in activating state.