Migration

Migrating between version control systems can be a complicated process, and Bazaar has extensive documentation on the process at http://doc.bazaar.canonical.com/migration/en and we won’t attempt to repeat that here. We will try to give a few motivating examples for conversion from Mercurial and Subversion.

Fast Import

In many projects wishing to use Bazaar, there is pre-existing history for the codebase that should be taken into consideration. Bazaar leverages an interchange format originally developed for Git called fast-import to provide migration strategies for many other version control systems. To work with fast-import files, Bazaar needs the fastimport plugin. This can be installed as with any Bazaar plugin.

The way that fast-import can be used for migration is to export the existing history into a fast-import file, then use the bzr fast-import command. The fastimport plugin includes exporters for Subversion, CVS, Git, Mercurial and darcs, accessible as the fast-export-from-XXX commands. Note that fast-import should not be used in a branch with existing history.

Assuming that ProjectX was first developed in Mercurial before switching to Bazaar, and that the Mercurial repository is in /srv/hg/projectx, the following commands will import that history into a newly created trunk branch. (Recall that in Further Configuration we created the /srv/bzr/projectx directory as a shared repository.)

$ cd /srv/bzr/projectx
$ bzr fast-export-from-hg ../../hg/projectx projectx.fi
$ bzr init trunk
$ bzr fast-import projectx.fi trunk

Subversion Conversion

As the most common centralized version control system, migration from Subversion is particularly important for any new version control system. Bazaar’s svn plugin provides tools for interaction with Subversion projects. In fact, Bazaar can be used transparently with projects stored in Subversion, but that is beyond the scope of this document. (See http://doc.bazaar.canonical.com/en/migration/foreign/bzr-on-svn-projects.html for more on that subject.) What is relevant here is the svn-import command provided by that plugin. This can import an entire subversion repository including tags and branches, particularly if they are stored in Subversion’s recommended directory structure: /tags/, /branches/ and /trunk/.

This command has flexible ways to specify what paths within the Subversion repository contain branches and which contain tags. For example, the recommended layout for Subversion projects (called trunk by the svn plugin) could be specified in ~/.bazaar/subversion.conf as

[203ae883-c723-44c9-aabd-cb56e4f81c9a]
branches = branches/*
tags = tags/*

This allows substantially complicated Subversion repositories to be converted into a set of separate Bazaar branches. After installing the svn plugin, see bzr help svn-import and bzr help svn-layout.

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