When does it make sense to use subrepositories?

[Matt Mackall]

On Thu, 2013-04-18 at 23:04 +0200, Angel Ezquerra wrote:
> On Thu, Apr 18, 2013 at 8:52 PM, Matt Mackall <mpm at selenic.com> wrote:
> > On Thu, 2013-04-18 at 09:36 -0700, v wrote:
> >> I've just backed out of several hours' work transitioning a set of related
> >> projects from one repository to a group of subrepositories. Having been
> >> through this a few times before, I always seem to end up with one of two
> >> scenarios:
> >>
> >> - If changes to one project are usually associated with changes to other
> >> related projects, put them in the same repository.
> >> - Otherwise, use separate repositories, and handle the relationship with you
> >> favourite dependency management solution. If you really want to do this with
> >> your SCM, then use guestrepos.
> >>
> >> I do acknowledge that subrepos are now declared "a feature of last resort",
> >> but I'm wondering what this situation is. Someone paid for them to be
> >> developed, so they must be useful somewhere!
> >
> > Most people are unclear on the fundamental purpose of version control.
> > They think of VCSes as first and foremost a tool to synchronize code
> > between developers. But this is wrong; it is a description of rsync. The
> > primary purpose of an VCS is to precisely track old states of the
> > project for future reference.
> >
> > With that in mind, ask yourself the question:
> >
> > "Do I need to be able to reproduce an exact combination of independent
> > projects from the past in the future?"
> >
> > In other words, "do I need _version control_ on my combination of
> > projects?"
> >
> > If the answer is yes, your options are:
> >
> > - check everything into one repo and hope you don't have to merge in
> > "upstream" changes often
> > - use subrepos
> 
> I could not have said it better! I think subrepos cover the
> requirement that Matt mentioned pretty well, and that is a very
> important and common requirement.
> 
> I'm very happy to see that Matt did not immediately refer to them as a
> feature of last resort.

It is absolutely a feature of last resort. I do not know why people have
so much trouble parsing the semantics of that.

It is a feature: it does things you can't do with normal Mercurial.
It is a last resort: you do not want to use it if you don't need to.

Given the number of n00bs who have tried to use subrepos when they
shouldn't and get teh sad, we need to advertise the latter fact to scare
them off for their own good.

-- 
Mathematics is the supreme nostalgia of our time.