[systemd-devel] Virtual dependencies

[Lennart Poettering]

On Tue, 21.08.12 14:31, David.Yates at continental-corporation.com (David.Yates at continental-corporation.com) wrote:

> Hi All,
> 
> I am hoping someone can help me with a dependency problem that I am 
> currently having and I assume there is already a simple solution available 
> that I am missing.
> 
> I was looking for some handling inside of systemd that would work similar 
> to the concept of virtual packages in the Debian package management 
> situation.
> 
> Whereby, if I install an application that requires a mail transport agent 
> then that application can be set as dependent on a virtual class of 
> service that can be fulfilled by one of many applications that can be 
> installed.
> 
> This would be something like a "Provides" tag in the unit file. I have 
> seen the Alias tag and assume that that could be used in a similar way but 
> I am not sure what happens if you have multiple applications that alias to 
> the same virtual name. This would need some way to tell systemd which app 
> is actually the currently selected app for that virtual class.
> 
> The goal of this all being obviously to be able to handle runtime 
> dependencies and automatically starting the required underlying services 
> when needed.
> 
> Thanks for any help and sorry if I have missed a really easy way that this 
> should be handled.

The way we currently handle this mostly is indeed by using Alias=. For
example the various implementations of a Display Manager all have an
Alias=display-manager.service, and that link then decides which
implementation is pulled in.

"systemctl enable" will fail if a symlink it is supposed to create
already exists. (This can be override with --force, which results in the
symlink being replaced).

This is suboptimal for other use cases though I guess, we could add some
other logic there to make it unnecessary to always maintain a symlink
for this in FS. I am open to ideas there, but the Alias= solution
definitely is something that works right now for many uses.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.