Re: Marelle: prolog for devops

On 8 November 2013 00:31, Michael Hendricks <[email protected]> wrote:

> On Nov 7, 2013 1:49 AM, "larsyencken" <[email protected]> wrote:
> > At it's core, you define a handful of rules which describe a target, how
> to
> > check if it's been met, and how to meet it. For example, to install
> > supervisor Ubuntu 12.04:
> >
> > pkg(supervisor).
> > met(supervisor, linux(precise)) :-
> >     isfile('/usr/bin/supervisorctl').
> > meet(supervisor, linux(precise)) :-
> >     bash('sudo apt-get install -y supervisor').
>
> That's a cool idea. Is there a convenience for working with package
> versions? For example, to declare that 'supervisor' is the most recent
> version available or has version greater than X?
>
I haven't pre-built many of those kinds of rules, mainly because my main
use case is bootstrapping new systems from a stock base. In that situation,
either the package is there, or it's not.

All of the above example can be shortened to:

pkg(supervisor).
installs_with_apt(supervisor).
installs_with_brew(supervisor).

which in turn can all be shortened to:

managed_pkg(supervisor).

If you want the ubuntu packaged version, whatever that version is.

Since supervisor is a Python package installable with pip, the pip version
of the rule supports a spec, but it will only use it when installing a new
package. To work with Python's packaging system instead, our rule would
look like:

pkg(supervisor).
met(supervisor, linux(precise)) :-
    bash('python -c "import supervisor"').
meet(supervisor, linux(precise)) :-
    bash('sudo pip install supervisor').

This is common enough to be shortened to:

python_pkg(supervisor). % handles pkg/1 and met/2.
installs_with_pip(supervisor). % handles meet/2

This is also common enough to have shorthand:

pip_pkg(supervisor).

We can give pip a more specific target for this package name to get a
version we want:

pip_pkg(supervisor, 'supervisor>=3.0').

I general, you can easily put together these kinds of rules yourself for
new packaging systems, or extend them as you like.

Hope that's helpful :)
Lars
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