Re: /etc and databases

["Colin Walters" <walters verbum org>]

On Wed, Jun 5, 2019, at 4:12 AM, Menny Hamburger via ostree-list wrote:
> Hi,
>
> I am a newcomer to OSTREE and I'm trying to understand whether putting 
> database files (let's say PostgreSQL) in /etc is the right thing to do.

I think you really want this in /var.  /etc is for human-editable files; it's "config" whereas  /var is 
"state".

> I have a piece of software that uses PostgreSQL and implements DB 
> schema versioning and DB migration between versions.
> Today the software is deployed as a tarball and extracted onto a mount 
> point over /var (where both binaries and the DB are deployed).
> The tarball includes an installation script that runs the database 
> migration in case of upgrade and rolls back to the previous DB in case 
> installation fails.

If you're deploying as a tarball outside of the host, I think you'd be better off going to a full container 
system (e.g. podman).

> I am considering the following setup:
> 1) Deploy software as a layered package (

That said obviously rpm-ostree does support this.

> 2) Set up the DB in /etc and run the installation (including DB 
> migration) script in %post.

Not sure where /etc comes in here.  I would avoid doing anything in %post.  If you're going the "on host" 
path, what you really want to do is execute any custom migrations as part of something like ExecStartPre= in 
a systemd unit.  That way the migration code runs with the same privileges/config as the database, its 
failure won't block other services/upgrades of the host, etc.

If you want to roll back if the database upgrade fails, that is a more complex scenario.  Gets a bit into the 
"auto rollbacks" we've been discussing for Fedora CoreOS, see e.g. https://github.com/LorbusChris/greenboot/