How to ensure that there is a delay before a service is started in systemd?

I am having a service that depends on Cassandra coming up gracefully and the cluster being up and ready.

To ensure that the dependency order is met, I have the following unit file

[Unit]
Requires=cassandra.service
After=cassandra.service

[Service]
Environment=JAVA_HOME=/usr/java/jre
[email protected]@/webapps/bringup-app/bin/bringup
TimeoutStartSec=0
ExecStop=
[email protected]@/logs/bringup.pid
Restart=always

[Install]
WantedBy=multi-user.target

How do I ensure that the bringup-app process waits for 30 seconds before it attempts to start up? Currently although it is started after Cassandra, I have noticed that the Cassandra cluster is not up yet and hence any attempt from the bringup-app to connect to Cassandra as part of startup fails.

I therefore want to add a delay. Is that possible via the unit file?

You can run the sleep command before your ExecStart with ExecStartPre :

[Service]
ExecStartPre=/bin/sleep 30

You can create a .timer systemd unit file to control the execution of your .service unit file.

So for example, to wait for 1 minute after boot-up before starting your foo.service, create a foo.timer file in the same directory with the contents:

[Timer]
OnBootSec=1min

It is important that the service is disabled (so it doesn't start at boot), and the timer enabled, for all this to work (thanks to user tride for this):

systemctl disable foo.service
systemctl enable foo.timer

You can find quite a few more options and all information needed here: https://wiki.archlinux.org/index.php/Systemd/Timers