Start stopped service because of dependency update

I have two systemd services: postgresql.service and app.service. postgresql.service is distribution provided (ubuntu 15.10) and app.service is written by myself.

Since app needs postgresql, the service looks like:

[Unit]
Description=Start App

Requires=postgresql.service
After=postgresql.service

[Install]
WantedBy=multi-user.target

[Service]
Restart=always
ExecStart=/path/to/app

Now today postgresql got updated. Of course it restarted the postgresql.service after update automatically. But this killed my app.service, because it requires postgresql:

# systemctl status app
[...]
Apr 08 12:04:42 host systemd[1]: Stopping Start App...
Apr 08 12:04:42 host systemd[1]: Stopped Start App.

Apr 08 12:04:42 is the time where the update of postgresql happened.

How can I change the app.service so that it does not simply get killed when postgresql restarts but (re)starts automatically then, too (only when it is enabled, of course)?

Or to ask it another way around: How can I configure app.service that it always automatically starts after postgresql.service when app.service is enabled? So basically a systemctl start postgresql first starts postgresql and then automatically app.

Solution 1:

I think systemd wants me to use Wants instead of Requires. From the manual:

Requires=

Configures requirement dependencies on other units. If this unit gets activated, the units listed here will be activated as well. If one of the other units gets deactivated or its activation fails, this unit will be deactivated. [...] Often, it is a better choice to use Wants= instead of Requires= in order to achieve a system that is more robust when dealing with failing services.

Now using Wants and this seems to be like what I really want a robust system dealing with failing services. So I do.