[Gnu-arch-users] microbranches: prism-merge vs multi-merge

[Aaron Bentley]

Hi all,

Just thought I'd share this insight.  I used to see prism-commit as 
having roughtly the same result as multi-merge, but now I think 
prism-commit is better.

What are microbranches?
focused, feature-oriented branches.  My tlasrc--errors--3 branch or 
tla--backbuilder--1 would be examples.  They permit the maintainer to 
merge a set of changes at once, instead of having to cherry-pick.  They 
are the opposite of a development or integration branch

What's multi-merge?
Multi-merge is strategy for producing microbranches in which one commits 
to the microbranches, then merges from the microbranches into the 
integration branch.  I think of it as pull-merging.  It's been 
implemented in

What's prism-merge?
Prism-merge is a strategy for producing microbranches in which one 
commits to a development branch, then applies those changes to a 
microbranch.

I came up with the idea of prism-merge independently, but I wasn't the 
first:

http://mail.gnu.org/archive/html/gnu-arch-users/2004-01/msg00535.html

After discussion with Tom, I figured that the multi-merge strategy 
looked much simpler to implement, so I did that, and you can find a 
merge command in aba that does it.  It just repeatedly star-merges from 
each of a list of source-branches until some changes are produced. 
Works pretty well.

I also like the fact that developing on microbranches forces me to make 
sure that the microbranches themselves work properly.  But there is a 
downside: External merges.

When you're working a devel branch, it's good to periodically merge with 
the blessed branch.  For tla, that's address@hidden/tla--devo--1.3. 
 That way, you're ready when the maintainer wants to merge from you.

But with multi-merge, you must merge the same changes multiple times, 
and then you get to try combining the differing merges in your 
integration branch.

With prism merge, you probably merge first into the devel branch.  In 
the microbranches, you can use whatever merge strategy makes sense.  In 
some cases, this will mean replaying the devel merge.  In others, it 
will mean merging directly from the blessed branch.

So in short, prism merge moves from the center out, and that gives you 
more flexibility when handling external merges.

Although multi-merge has been implemented for quite some time as an aba 
command, I don't know of any prism-commit implementation.  Ask me again 
on Tuesday.

Aaron