Why is branching and merging easier in Mercurial than in Subversion?

In Subversion (and CVS), the repository is first and foremost. In git and mercurial there is not really the concept of a repository in the same way; here changes are the central theme.

+1

The hassle in CVS/SVN comes from the fact that these systems do not remember the parenthood of changes. In Git and Mercurial, not only can a commit have multiple children, it can also have multiple parents!

That can easily observed using one of the graphical tools, gitk or hg view. In the following example, branch #2 was forked from #1 at commit A, and has since been merged once (at M, merged with commit B):

o---A---o---B---o---C         (branch #1)
     \       \
      o---o---M---X---?       (branch #2)

Note how A and B have two children, whereas M has two parents. These relationships are recorded in the repository. Let's say the maintainer of branch #2 now wants to merge the latest changes from branch #1, they can issue a command such as:

$ git merge branch-1

and the tool will automatically know that the base is B--because it was recorded in commit M, an ancestor of the tip of #2--and that it has to merge whatever happened between B and C. CVS does not record this information, nor did SVN prior to version 1.5. In these systems, the graph would look like:

o---A---o---B---o---C         (branch #1)
     \    
      o---o---M---X---?       (branch #2)

where M is just a gigantic "squashed" commit of everything that happened between A and B, applied on top of M. Note that after the deed is done, there is no trace left (except potentially in human-readable comments) of where M did originate from, nor of how many commits were collapsed together--making history much more impenetrable.

Worse still, performing a second merge becomes a nightmare: one has to figure out what the merge base was at the time of the first merge (and one has to know that there has been a merge in the first place!), then present that information to the tool so that it does not try to replay A..B on top of M. All of this is difficult enough when working in close collaboration, but is simply impossible in a distributed environment.

A (related) problem is that there is no way to answer the question: "does X contain B?" where B is a potentially important bug fix. So, why not just record that information in the commit, since it is known at merge time!

P.-S. -- I have no experience with SVN 1.5+ merge recording abilities, but the workflow seems to be much more contrived than in the distributed systems. If that is indeed the case, it's probably because--as mentioned in the above comment--the focus is put on repository organization rather than on the changes themselves.

Because Subversion (at least version 1.4 and below) doesn't keep track of what have been merged. For Subversion, merging is basically the same as any commit while on other version control like Git, what have been merged are remembered.

Untouched by any of the already provided answers, Hg offered superior merge capabilities because it uses more information when merging changes (hginit.com):

For example, if I change a function a little bit, and then move it somewhere else, Subversion doesn’t really remember those steps, so when it comes time to merge, it might think that a new function just showed up out of the blue. Whereas Mercurial will remember those things separately: function changed, function moved, which means that if you also changed that function a little bit, it is much more likely that Mercurial will successfully merge our changes.

Of course, remembering what was last merged (the point addressed by most of the answers provided here) is also a huge win.

Both improvements, however, are questionable since subversion 1.5+ stores additional merge information in the form of subversion properties: that information available, there's no obvious reason why subversion merge couldn't implement merge as successfully as Hg or Git. I don't know if it does, though, but it certainly sounds like subversion developers are on their way to get around this issue.

I suppose this might partially be because Subversion has the idea of a central server along with an absolute time line of revisions. Mercurial is truly distributed and has no such reference to an absolute time line. This does allow Mercurial projects to form more complicated hierarchies of branches for adding features and testing cycles by sub project however teams now need to much more actively keep on top of merges to stay current as they can't just hit update and be done with it.

In Subversion (and CVS), the repository is first and foremost. In git and mercurial there is not really the concept of a repository in the same way; here changes are the central theme.

I've not thought much about how you'd implement either but my impression (based on bitter experience and lots of reading) is that this difference is what makes merging and branching so much easier in non-repository based systems.