Using stacked branches


If you are working on a project, and you have read access to whose public repository but do not have write access to it, using stacked branches to backup/publish your work onto the same host of the public repository might be an option for you.

Other scenarios for stacked branch usage include experimental branches and code hosting sites. For these scenarios, stacked branches are ideal because of the benefits it provides.

What is a stacked branch?

A stacked branch is a branch that knows how to find revisions in another branch (the stacked-on branch). Stacked branches store just the unique revisions that are not in the stacked-on branch, making them faster to create and more storage efficient. In these respects, stacked branches are similar to shared repositories. However, stacked branches have additional benefits:

  • The new branch can be in a completely different location to the branch being stacked on.
  • Deleting the stacked branch really deletes the revisions (rather than leaving them in a shared repository).
  • Security is improved over shared repositories, because the stacked-on repository can be physically readonly to developers committing to stacked branches.

Creating a stacked branch

To create a stacked branch, use the stacked option of the branch command. For example:

bzr branch --stacked source-url my-dir

This will create my-dir as a stacked branch with no local revisions. If it is defined, the public branch associated with source-url will be used as the stacked-on location. Otherwise, source-url will be the stacked-on location.

Creating a stacked checkout

Direct creation of a stacked checkout is expected to be supported soon. In the meantime, a two step process is required:

  1. Create a stacked branch as shown above.
  2. Convert the branch into a checkout using either the reconfigure or bind command.

Pushing a stacked branch

Most changes on most projects build on an existing branch such as the development trunk or current stable branch. Creating a new branch stacked on one of these is easy to do using the push command like this:

bzr push --stacked-on reference-url my-url

This creates a new branch at my-url that is stacked on reference-url and only contains the revisions in the current branch that are not already in the branch at reference-url. In particular, my-url and reference-url can be on the same host, and the --stacked-on option can be used additionally to inform push to reference the revisions in reference-url. For example:

bzr push --stacked-on bzr+ssh://host/project bzr+ssh://host/user/stacked-branch

This usage fits the scenario described in the Motivation section.

You can also use the --stacked option without specifying --stacked-on. This will automatically set the stacked-on location to the parent branch of the branch you are pushing (or its public_location if configured). For example:

bzr branch source-url my-dir
cd my-dir
(hack, hack, hack)
bzr commit -m "fix bug"
bzr push --stacked

You can combine bzr branch --stacked and bzr push --stacked to work on a branch without downloading or uploading the whole history:

bzr branch --stacked source-url my-dir
cd my-dir
(hack, hack, hack)
bzr commit -m "fix bug"
bzr push --stacked

Limitations of stacked branches

The important thing to remember about a stacked branch is that the stacked-on branch needs to be accessible for almost all operations. This is not an issue when both branches are local, or when both branches are on the same server and the stacked-on location is a relative path. But clearly a branch hosted on a server with a stacked-on location of file:///... is not going to work for anyone except the user that originally pushed it. It’s a good idea to configure public_location to help prevent that.

Similarly, because most of the history is stored in the stacked-on repository, operations like bzr log can be slower when the stacked-on repository is accessed via a network.

If a stacked branch is in a format older than 2a, you cannot commit to it due to bug 375013.

Changing branch stacking

Stacking of existing branches can be changed using the bzr reconfigure command to either stack on an existing branch, or to turn off stacking. Be aware that when bzr reconfigure --unstacked is used, bzr will copy all the referenced data from the stacked-on repository into the previously stacked repository. For large repositories this may take considerable time and may substantially increase the size of the repository.