Back-up and Restore
Backing up Bazaar branches can be done in two different ways. If an existing filesystem-based backup scheme already exists, then it can easily be used where the Bazaar branches reside. Alternately, Bazaar itself can be used to mirror the desired branches to or from another location for backup purposes.
Bazaar transactions are atomic in the sense that the disk format is such that it is in a valid state at any instant in time. However, for a backup process that takes a finite amount of time to complete, it is possible to have inconsistencies between different on-disk structures when backing up a live branch or repository. (Bazaar itself manages this concurrency issue by only reading those structures in a well-defined order.) Tools such as LVM that allow instantaneous snapshots of the contents of a disk can be used to take filesystem backups of live Bazaar branches and repositories.
For other backup methods, it is necessary to take the branch or repository offline while the backup is being done in order to guarantee consistency between the various files that comprise a Bazaar branch’s history. This requirement can be alleviated by using Bazaar as its own backup client, since it follows an order for reading that is designed to manage concurrent access (see the next section for details). Depending on the different access methods that are being used for a branch, there are different ways to take the branch “offline”. For bzr+ssh:// access, it is possible to temporarily change the filesystem permissions to prevent write access from any users. For http:// access, changing permissions, shutting down the HTTP server or switching the server to a separate configuration that disallows access are all possible ways to take a branch offline for backup. Finally, for direct filesystem access, it is necessary to make the branch directories un-writable.
Because this sort of downtime can be very disruptive, we strongly encourage using Bazaar itself as a backup client, where branches are copied and updated using Bazaar directly.
Bazaar as its own backup
The features that make Bazaar a good distributed version control system also make it a good choice for backing itself up. In particular, complete and consistent copies of any branch can easily be obtained with the branch and pull commands. As a result, a backup process can simply run bzr pull on a copy of the main branch to fully update that copy. If this backup process runs periodically, then the backups will be as current as the last time that pull was run. (This is in addition to the fact that revisions are immutable in Bazaar so that a prior revision of a branch is always recoverable from that branch when the revision id is known.)
As an example, consider a separate backup server that stores backups in /var/backup. On that server, we could initially run
$ cd /var/backup $ bzr branch bzr+ssh://server.example.com/srv/bzr/trunk $ bzr branch bzr+ssh://server.example.com/srv/bzr/feature-gui
to create the branches on the backup server. Then, we could regularly (for example from cron) do
$ cd /var/backup/trunk $ bzr pull # the location to pull from is remembered $ cd ../var/backup/feature-gui $ bzr pull # again, the parent location is remembered
The action of pulling from the parent for all branches in some directory is common enough that there is a plugin to do it. The bzrtools plugin contains a multi-pull command that does a pull in all branches under a specified directory.
With this plugin installed, the above periodically run commands would be
$ cd /var/backup $ bzr multi-pull
Note that multi-pull does a pull in every branch in the specified directory (the current directory by default) and care should be taken that this is the desired effect. A simple script could also substitute for the multi-pull command while also offering greater flexibility.
Bound Branch Backups
When bzr pull is run regularly to keep a backup copy up to date, then it is possible that there are new revisions in the original branch that have not yet been pulled into the backup branch. To alleviate this problem, we can set the branches up so that new revisions are pushed to the backup rather than periodically pulling. One way to do this is using Bazaar’s concept of bound branches, where a commit in one branch happens only when the same commit succeeds in the branch to which it is bound. As a push-type technology, it is set up on the server itself rather than on the backup machine. For each branch that should be backed up, you just need to use the bind command to set the URL for the backup branch. In our example, we first need to create the branches on the backup server (we’ll use bzr push, but we could as easily have used bzr branch from the backup server)
$ cd /srv/bzr/projectx/trunk $ bzr push bzr+ssh://backup.example.com/var/backup/trunk $ cd ../feature-gui $ bzr push bzr+ssh://backup.example.com/var/backup/feature-gui
and then we need to bind the main branches to their backups
$ cd ../trunk $ bzr bind bzr+ssh://backup.example.com/var/backup/trunk $ cd ../feature-gui $ bzr bind bzr+ssh://backup.example.com/var/backup/feature-gui
A branch can only be bound to a single location, so multiple backups cannot be created using this method.
Using the automirror plugin mentioned under Hooks and Plugins, one can also make a push-type backup system that more naturally handles mutliple backups. Simply set the post_commit_mirror option to multiple URLs separated by commas. In order to backup to the backup server and a remote location, one could do
$ cd /srv/bzr/trunk $ echo "post_commit_mirror=bzr+ssh://backup.example.com/var/backup/trunk,\ bzr+ssh://offsite.example.org/projectx-corp/backup/trunk" >> .bzr/branch/branch.conf $ cd ../feature-gui $ echo "post_commit_mirror=bzr+ssh://backup.example.com/var/backup/feature-gui,\ bzr+ssh://offsite.example.org/projectx-corp/backup/feature-gui" >> .bzr/branch/branch.conf
As for any push-type backup strategy that maintains consistency, the downside of this method is that all of the backup commits must succeed before the initial commit can succeed. If there is a many mirror branches or if the bound branch has a slow network connection, then the delay in the original commit may be unacceptably long. In this case, pull-type backups, or a mixed system may be preferable.
Restoring from Backups
Checking backup consistency
Many a system administrator has been bitten by having a backup process, but when it came time to restore from backups, finding out that the backups themselves were flawed. As such, it is important to check the quality of the backups periodically. In Bazaar, there are two ways to do this: using the bzr check command and by simply making a new branch from the backup. The bzr check command goes through all of the revisions in a branch and checks them for validity according to Bazaar’s internal invariants. Since it goes through every revision, it can be quite slow for large branches. The other way to ensure that the backups can be restored from is to perform a test restoration. This means performing the restoration process in a temporary directory. After the restoration process, bzr check may again be relevant for testing the validity of the restored branches. The following two sections present two restoration recipes.
Restoring Filesystem Backups
There are many different backup tools with different ways of accessing the backup data, so we can’t cover them all here. What we will say is that restoring the contents of the /srv/bzr directory completely will restore all branches stored there to their state at the time of the backup (see Filesystem Backups for concerns on backing up live branches.) For example, if the backups were mounted at /mnt/backup/bzr then we could restore using simply:
$ cd /srv $ mv bzr bzr.old $ cp -r /mnt/backup/bzr bzr
Of course, to restore only a single branch from backup, it is sufficient to copy only that branch. Until the restored backup has been successfully used in practice, we recommend keeping the original directory available.
Restoring Bazaar-based Backups
In order to restore from backup branches, we can simply branch them into the appropriate location:
$ cd /srv $ mv bzr bzr.old $ cd bzr $ bzr branch bzr+ssh://backup.example.com/var/backup/trunk $ bzr branch bzr+ssh://backup.example.com/var/backup/feature-gui
If there are multiple backups, then change the URL above to restore from the other backups.