Ponderings of a Subversion User: What is a "branch" in Mercurial terms?

7
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I'm a Subversion user, and I think I've got my head mostly around it all now. So of course now we're thinking of switching to Mercurial, and I need to start again.

In our single repository, we have the typical branches, tags, trunk layout. When I want to create a feature branch I:

  • Use the repo browser to copy trunk to branches/Features/[FeatureName].
  • Checkout a new working copy from branches/Features/[FeatureName].
  • Start working on it.
  • Occasionally commit, merge trunk in, resolve conflicts and commit.
  • When complete, one more merge of trunk, then "Reintegrate" the feature branch into trunk.

(Please note this process is simplified as it doesn't take into account release candidate branches etc).

So I have questions about how I'd fulfil the same requirements (i.e. feature branches rather than working on trunk) in Mercurial:

  • In Mercurial, is a branch still within the repository, or is it a whole new local repository?
  • If we each have a copy of the whole repository, does that mean we all have copies of each other's various feature branches (that's a lot of data transfer)?
  • I know Mercurial is a DCVS, but does that mean we push/pull changes from each other directly, rather than via a peer repository on a server?
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3 Answers

7
votes

In Mercurial, is a branch still within the repository, or is it a whole new local repository?

The equivalent of the subversion way of working would be a repository with multiple heads in mercurial. However, this is not the idiomatic way of doing things. Typically you will have only one head in a given repository, so separate repositories for each branch.

If we each have a copy of the whole repository, does that mean we all have copies of each other's various feature branches (that's a lot of data transfer)?

Yes, if you look at the history of the head of your local repository, then you'll be able to see all the feature branches that were merged in. But mercurial repositories are remarkably space efficient. For example, I have done a hg clone https://www.mercurial-scm.org/repo/hg to get the source for mercurial itself, and it is only 34.3 MB on an NTFS file system (compared to the source code download, which is 1.8 MB). Mercurial will also make use of hardlinks if your file system supports it, so there is little overhead if you clone a repository to another location on the same disk.

I know Mercurial is a DCVS, but does that mean we push/pull changes from each other directly, rather than via a peer repository on a server?

One way of working is indeed to have each developer expose a public repository in which he pushes his own changes. All other developers can then pull what they want.

However, typically you'll have one or more "blessed" repositories where all the changes are integrated. All developers then only need to pull from the blessed repository. Even if you didn't explicitly have such a blessed repository I imagine people would automatically organize themselves like that, e.g. by all pulling from a lead developer.

5
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Steve Losh's article on branching in mercurial linked above is fantastic. I also got into some explaining of branching and how the DAG works in a presentation I gave a couple of months ago on mercurial that's out on slideshare. The pertinent slides start at slide #43.

I think that understanding that all commits to the same repository are stored in a DAG (Directed Acyclic Graph) with some simple rules really helps demystify what's going on.

  • a node with no child nodes is a "head"
  • the root node has no parents
  • regular nodes have a single parent
  • nodes that are the result of a merge have two parents
  • if a merge node's parents are from different branches, the child node's branch is inherited from the first parent

Named branches are really just metadata labels on commits, but really aren't any different than the anonymous branches that happen when you merge someone elses work into your repository, or if you go back to an earlier version and then make a commit there to make a new head (which you can later merge).