Systemd: Autostart service after mongodb

ubuntu16.04.1, systemd

An application depends on mongodb. The application and mongodb both start on boot as services. app.service containts next line:

After=network.target mongod.service

Unfortunately, the application fails (with coection to database error), I guess, because the mongodb needs some time to startup and be ready to coections (if I start the app later, everything is ok).

It looks like Type=notify (in [Service] section of mongod.service) will solve the problem if mongodb sends apropriate notification to systemd. But I did not find any documentation about it.

The queston is, how to autostart the application, after mongodb will be ready for incoming coections.