[SOLVED] Systemd unit dependency on slotted service

[fabalthazar]

Hello everybody,

I would like to load a systemd service after a slotted/versioned service, but being version-agnostic.

Specifically:
My application can use a PostgreSQL database: its service should start at boot time after the PostgreSQL service (think `After=` in systemd). In Gentoo, PostgreSQL is shipped with a systemd `postgresql-<version>.service`.
I want to make my service resistant to PostgreSQL updates, i.e. avoiding `After=postgresql-16.service`

For instance, the following thread creates a dependency over `postgresql-12.service` which is to avoid:
Requires=postgresql-12.service

Is there an elegant way of having a generic `postgresql.service` active whatever the underlying `postgresql-<version>.service`?

Do any of Gentoo virtual packages or `PartOf=` systemd [Unit] parameter be possible solutions?

Here is the systemd service I'm working on:

[logrusx]

I'm a programmer, so first thing that comes to mind is abstraction. Unfortunately I'm not a systemd expert and what I suggest might not be possible or a good idea:

Create a target for your application and make it depend on the target in which postgresql is started.

Best Regards,
Georgi

[fabalthazar]

Abstraction looks interesting.

I imagine this abstracted Unit delivered by the postgresql implementation, or by some kind of virtual package https://wiki.gentoo.org/wiki/Virtual_packages

Not sure if virtual packages are designed for it and if they can ship files and not only dependency relations.

[logrusx]

[fabalthazar]

Let's sum up my initial need:

What is expected by the end user is :
- emerge postgresql
- emerge trytond (via an overlay)
- systemctl enable trytond.service
And that's all.

- The application (Tryton server here), delivered through an existing overlay (https://foss.heptapod.net/tryton/gentoo-overlay), provides a system service. Actually on OpenRC service only but I'm working on a systemd one.
- This app makes use of a postgresql database.
- PostgreSQL only provides versioned services (e.g. postgresql-16.service).

The problematic is:

How to provide these two system services, automatically via Portage without configuration, in order to make the app (Tryton here) system service start after the postgresql service whichever its version.
Then no problem with PostgreSQL updates.

Hence my trytond.service systemd unit draft:

[logrusx]

Add a warning in the ebuild that postgresql service needs to be enabled and let the administrator deal with it. What the ebuild of your application delivers should be only related to your application. No other ebuild should be in any way engaged into delivering something your application needs, neither should your ebuild deliver something pertaining to other applications.

Best Regards,
Georgi

[fabalthazar]

[logrusx]

[fabalthazar]

I see. As is, PostgreSQL is slotted certainly because apps should stick to specific compatible versions/slots.

In effect, the admins already have to manually enable specific versions. Currently: postgresql-16.service for the last stable version. Furthermore, one has to manually migrate databases from one major version to another.

So I abandon the idea of PostgreSQL providing a unique way or declaring its availability. It is against its design.

If I focus on the idea of making the app depend on PostgreSQL, then it could simply specify in its ebuild the exact PostgreSQL version it depends on (and is compatible with). My example of Tryton app already has a "postgres" USE flag.
Let add one line to the ebuild:

[logrusx]

[fabalthazar]

Thanks. Let's mark it as solved.