Re: subpackages for mgr modules

[Sage Weil <sage@newdream.net>]

On Fri, 19 May 2017, Tim Serong wrote:
> Hi All,
> 
> At some point, we're going to want to have (some of) mgr's python
> modules shipped not as part of the ceph-mgr package, but as subpackages
> that can be optionally installed (ceph-mgr-foo, ceph-mgr-bar, whatever).
>  That's easy to do.  What I'm wondering about is having installation of
> an mgr module subpackage automatically enable the loading of that module.
> 
> Ordinarily, to enable an mgr module that isn't enabled by default at
> compile time, you'd set "mgr modules = foo bar" in the [mgr] section of
> ceph.conf.  But that will get irritating.
> 
> We can't have mgr just load everything that exists (someone might want
> to disable some modules, without necessarily uninstalling them).
> 
> A couple of approaches I thought of are:
> 
> 1) Add a command line argument (or arguments) to ceph-mgr to list
> modules to enable (or disable).  This could in turn be set by some
> variable(s) in /etc/sysconfig/ceph.  Subpackages then tweak those
> variables at install time.
> 
> 2) Do something with symlinks; set up a directory structure something
> like this:
> 
> /usr/lib64/ceph/mgr/enabled/
> /usr/lib64/ceph/mgr/modules/
> /usr/lib64/ceph/mgr/modules/fsstatus
> /usr/lib64/ceph/mgr/modules/rest
> /usr/lib64/ceph/mgr/modules/...
> 
> Then, any module you want to enable, create a symlink to it from the
> "enabled" directory.  Subpackages could do this at install time.  mgr
> iterates through that directory and loads anything present.
> 
> (Personally I rather prefer approach 2 to approach 1).
> 
> I'm sure there's other possibilities.  Thoughts, anyone?

I like #2, too!  This is similar to the sites-enabled thing apache does.

We can make the config option blank (or something like *?) to indicate 
using the directory, otherwise the specified list?

sage