Data migration
##############

Preparing for data migration
----------------------------

Before starting a migration, there are a few important things to do
first:

1. Take a complete backup.

2. Take some time to purge obsolete branches.

A complete backup gives you a safety net in case anything goes wrong.

Purging obsolete branches reduces the amount of data that needs to
be migrated. See `Finding obsolete branches`_ later for some tips
on doing this.


Introducing the upgrade-related commands
----------------------------------------

There are 3 important commands to be aware of when migrating data.

* **check** - check a repository, branch or tree for data integrity errors

* **reconcile** - fix data integrity errors

* **upgrade** - migrate data to a different format.

**reconcile** is rarely needed but it's good practice to run **check**
before and after running **upgrade**.

For detailed help on these commands, see the `Bazaar User Reference`_.

.. _Bazaar User Reference: ../user-reference/index.html


Communicating with your community
---------------------------------

To enable a smooth transition to the new format, you should:

1. Make one person responsible for migrating the trunk.

2. Test the migration of trunk works successfully.

3. Schedule a time for the trunk migration and notify your community
   in advance.

This advance warning should be long enough for users to have time
to upgrade Bazaar and any required plugins before the migration date.

For larger projects, allow some time for the migration itself.
You should have a good idea of how long the migration will take
after doing the test migration. It may make sense to do the migration
on a weekend or a Friday, giving yourself some breathing space if
things go wrong.

After the trunk is migrated, you'll need to notify your community
accordingly, giving them instructions as to how to migrate their
local branches. Sample instructions are provided later in this
document.


Migrating a standalone branch
-----------------------------

The steps are:

1. Run **bzr check**.

2. If there are errors, try using **bzr reconcile** to fix them.
   If that fails, file a bug so we can help you resolve the issue
   and get your trunk clean. If it works, take a backup copy of
   your now clean trunk.

2. Run **bzr upgrade --format** where *format* is 2a or later.

3. Run **bzr check** to confirm the final result is good.


Migrating branches in a shared repository
-----------------------------------------

Upgrade things in the following order:

1. Upgrade the shared repository.
2. Upgrade the branches.
3. Upgrade any lightweight checkouts.

As in the standalone branch case, be sure to run **check** before
and after the upgrade to check for any existing or introduced issues.


Migrating branches on Launchpad
-------------------------------

You have two options for upgrading your Launchpad branches.  You can either
upgrade them remotely or you can upgrade them locally and push the migrated
branch to Launchpad.  We recommend the latter.  Upgrading remotely currently
requires a fast, rock solid network connection to the Launchpad servers, and
any interruption in that connection can leave you with a partially upgraded
branch.  The instructions below are the safest and often fastest way to
upgrade your Launchpad branches.

To allow isolation between public and private branches, Launchpad
uses stacked branches rather than shared repositories as the core
technology for efficient branch storage. The process for migrating
to a new format for projects using Launchpad code hosting is therefore
different to migrating a personal or in-house project.

In Launchpad, a project can define a *development series* and associate a
branch with that series.  The branch then becomes the *focus of development*
and gets special treatment and a shortcut URL.  By default, if anybody
branches your project's focus of development and pushes changes back to
Launchpad, their branch will be stacked on your development focus branch.
Also, branches can be associated with other Launchpad artifacts such as bugs
and merge proposals.  All of these things mean that upgrading your focus of
development branch is trickier.

Here are the steps to follow:

1. The nominated person grabs a copy of trunk and does the migration locally.

2. On Launchpad, unset the current trunk from being the development focus.
   (This *must* be done or the following step won't work as expected.)

   1. Go to your project's home page on Launchpad

   2. Look for "XXX is the current focus of development"

   3. Click on the edit (pencil) icon

   4. Click on "Change details" in the portlet on the right

   5. Scroll down to where it says "Branch: (Optional)"

   6. Blank out this input field and click "Change"

3. Push the migrated trunk to Launchpad.  See below if you want your
   new migrated development focus branch to have the same name as your old
   pre-migration development focus branch.

4. Set it as the development focus.  Follow the instructions above but at step
   5, enter the name of the newly migrated branch you just pushed.

5. Ask users subscribed to the old trunk to subscribe to the new one.

In summary, these steps mean that the old trunk is still available and
existing branches stacked on it will continue to be so. However, the
development focus has switched to the migrated trunk and any new branches
pushed to Launchpad for your project will now stack on it.

You are now ready to tell your community that the new trunk is available
and to give them instructions on migrating any local branches they have.

If you want your new migrated development focus branch to have the same name
as your old pre-migration branch, you need to do a few extra things before you
establish the new development focus.

1. Rename your old pre-migration branch; use something like
   **foo-obsolete-do-not-use**.  You will really not want to delete this
   because there will be artifacts (bugs, merge proposals, etc.) associated
   with it.

2. Rename the new migrated branch to the pre-migration branch's old name.

3. Re-establish the development focus branch using the new migrated branch's
   new name (i.e. the old pre-migration branch's original name).


Migrating local branches after a central trunk has migrated
-----------------------------------------------------------

To migrate a standalone branch:

1. Grab the latest branch from the central location into a
   new directory.

2. Pull or merge any changes you've made in your existing branch
   into the new branch.

To migrate branches in a shared repository:

1. Create a fresh shared repository in the new format (2a or later).

2. Grab the latest branch from the central location into a
   new directory inside the shared repository.

3. Decide which of your local branches you want to migrate. (If you
   haven't already, now's a good time for `Finding obsolete branches`_
   and purging them, after backing up first of course.)

4. To migrate each local branch of interest, there are 2 options:

 * **init** an empty branch in the new repository and **pull** the
   revisions from the branch in the old repository across.

 * In the new repository, **branch** from trunk to the new branch
   name then **merge** your changes from the matching branch in the
   old repository.

The first method will give you a branch which is identical (in terms of
revision history) to the old branch, but it's parent branch will be set to the
old branch, not your new trunk. If you use this method, you'll probably update
the ``parent_location`` configuration variable in the ``branch.conf`` file
with::

    bzr config parent_location=XXX

``XXX`` being the URL to your new trunk.

In contrast, the second approach sets up the parent branch correctly.
However, it isn't ideal if you're not ready to include all the latest
revisions from trunk into that branch yet.
