We change the schema through database patches. These live in branches that go through review and landing, just like code changes.

A schema change needs a review of type "db" from the DBA (stub) or, in his absence, the technical architect (lifeless). But request db reviews from both these people, so that they can both track changes. A database patch also needs an identifying "patch number". This is self allocated (see the end of this page).

Changes to the permissions in database/schema/security.py or the comments in database/schema/comments.sql do not require db review. If you get both code and db reviews, the code reviewer should look at them. (But if nothing else in your branch requires code review, then they're for the db reviewer to inspect).

Procedure

1. Discuss non-trivial or controversial changes.
2. Prepare a branch containing your database patch for review. Feel free to request assistance from or outsource work to the DBA (stub).

3. Submit a merge proposal for your branch into lp:launchpad/db-devel (lp:~launchpad-pqm/launchpad/db-devel), requesting two db reviews from respectively the technical architect (lifeless) and the DBA (stub).

4. If your branch also contains code changes, then request a separate code review as well.
5. Follow your reviewers' instructions. The DBA or Technical Architect will allocate a database patch number as required. The schema change is approved for landing when you have an 'Approved' vote from the DBA - no need to block waiting on the Technical Architect review.
6. Ensure the sampledata is up to date on your branch.
7. Ensure the test suite still passes.

8. Land the branch on lp:launchpad/db-devel (lp:~launchpad-pqm/launchpad/db-devel) unless someone has stated it is being landed it on your behalf.

Making a database patch

You need to run these steps whenever you make a schema change, regardless of whether you intend to add new sample data or not. For example, if you are adding a new column to the Person table, these steps ensure that the new sample data will include this new column.

1. Run make schema to get a pristine database of sample data.

2. Create a SQL file in database/schema/pending/ containing the changes you want.

   1. The first line of your file should be SET client_min_messages=ERROR;.

   2. Don't use the TRUNCATE or DROP TABLE statements as they don't work with Slony-I replication.

      1. To drop a table, rename it into the todrop namespace using a statement like ALTER TABLE FooBar RENAME TO todrop.FooBar.

   3. If you are doing data initialization or data migration, don't use indeterminate values that might give different results on different databases, such as random() or CURRENT_TIMESTAMP. This would create skew in our data between the master and slave databases.
      1. For a new column with an indeterminate DEFAULT, you may need to manually set the initial values to something determinate before adding the DEFAULT to the column. eg. initialize the new date_eviscerated column to a fixed, known timestamp before setting its default to CURRENT_TIMESTAMP AT TIME ZONE 'UTC'.

3. Run your new SQL patch on the development database to ensure that it works. Do this by running psql launchpad_dev -f your-patch.sql

4. (Optional): Interact with launchpad.dev to add any additional sample data you want to demonstrate in the web u/i.

5. Run psql launchpad_ftest_playground -f your-patch.sql as the ftest playground db is used to generate newsampledata.sql in the following step. Also run psql launchpad_dev -f your-patch.sql to update the dev database's sampledata.

6. Run make newsampledata.

7. Review the sample data changes that occured using diff database/sampledata/current.sql database/sampledata/newsampledata.sql. This diff can be hard to review as-is. You might want to use a graphical diff viewer like kompare or meld which will make it easier. Make sure that you understand all the changes you see.

8. In database/sampledata/, move newsampledata.sql to current.sql, replacing the latter.

9. In database/sampledata/, move newsampledata-dev.sql to current-dev.sql, replacing the latter.

10. Claim a patch number in the dbpatches file.

11. Move your pending SQL file into database/schema/ with a name like patch-xx-yy-zz.sql (where xx matches the existing patches), and ending with the line INSERT INTO LaunchpadDatabaseRevision VALUES (xx, yy, zz);.

12. Run make schema again to ensure that it works, and that you now have a pristine database with the new sample data.

13. Comments on new tables and columns need to be added to database/schema/comments.sql.

14. Make any necessary changes to database/schema/fti.py, database/schema/security.cfg, and at least the minimum code changes required to keep the test suite passing.

15. Make any necessary changes to the SQL patch to reflect new default values.

16. Run the full test suite to ensure that your new sample data doesn't break any existing tests by side effect.

17. Go have lunch.

Note that if you make subsequent additional changes, you may be able to skip straight to step 5.

Sample data

Let's say your branch needs to make changes to the database schema. You need to follow the steps on this page to ensure that the sample data is updated to match your schema changes.

In fact, there are now two sets of sampledata that need to be updated.

We use sample data to provide well-known baseline data for the test suite, and to populate a developer's Launchpad instance so that launchpad.dev can display interesting stuff. There are some guidelines and recommendations you should be aware of before you make changes to the test suite sample data, or you may break the tests for yourself or others.

Please note that sample data is for developer's instances only. It would make no sense to use the sample data on production systems!

If your tests require new data, you should strongly consider creating the data in your test's harness instead of adding new sample data. This will often make the tests themselves more readable because you're not relying on magical values in the sample database. Doing it this way also reduces the chance that your changes will break other tests by side-effect. Add the new data in your test's setUp() or in the narrative of your doctest. Because the test suite uses the launchpad_ftest database, there is no chance that running the test suite will accidentally add new sample data.

However, if you interact with the web U/I for launchpad.dev your changes will end up in the launchpad_dev database. This database is used to create the new sample data, so it is imperative that you run make schema to start with a pristine database before generating new sample data. If in fact you do want the effects of your u/i interactions to land in the new sample data, then the general process is to

• run make schema

• interact with launchpad.dev

• follow the make newsampledata steps above.

Be aware though that your generation of new sample data will probably have an effect on tests not related to your changes! For example, if you generate new karma events, you will probably break the karma_sample_data tests because they expect all karma events to be dated prior to the year 2002. If you make changes to the sample data, you must run the full test suite and ensure that you get no failures, otherwise there is a very high likelihood that you will break the trunk.

Resolving schema conflicts

Resolving conflicts in current.sql manually is usually more trouble than it's worth. Instead, first resolve any conflicts in comments.sql, then:

cd database/schema/
mv {patch-in-question}-0.sql comments.sql pending/
cp {parent branch, e.g. rocketfuel}/database/schema/comments.sql ./
cp ../sampledata/current.sql.OTHER ../sampledata/current.sql
make
psql launchpad_dev -f pending/patch-xx-99-0.sql
make newsampledata
mv ../sampledata/newsampledata.sql ../sampledata/current.sql
mv pending/{patch-in-question}-0.sql pending/comments.sql ./
make   # Just to make sure everything works
cd ../..
bzr resolve database/sampledata/current.sql

Notes on Changing security.cfg

It is possible to land changes to security.cfg on lp:~launchpad-pqm/launchpad/devel rather than lp:~launchpad-pqm/launchpad/db-devel. These changes are deployed during nodowntime rollouts.

Note that adding new users requires manual DB reconfiguration, so you need to ask a LOSA to grant access to relevant machines before the nodowntime or downtime deployment that needs them.

Triggers

1. Developer needs a database schema change implemented as part of his/her development activities
2. Developer needs a database schema change implemented as part of his/her bug fixing activities

Inputs

1. Developer originated schema change concept

Outputs

1. SQL Patch File

Participants

1. Developer

2. DBA (in our case, StuartBishop)

3. Technical Architect (in our case, RobertCollins)

Subprocesses

• N/A.

Standard Path Events/Activities

1. When a developer has a change they want to make to the database schema, they write a database patch (see database/schema in the source tree).
2. They can give themselves a patch number on their development tree by claiming it in the dbpatches branch. This becomes the name of the patch file.
3. A merge proposal is submitted, with reviews requested from the Launchpad Technical architect and the DBA.
4. When it has passed review, the DBA confirms the patch number as final.
5. The DBA will also issue further instuctions on how to proceed.

Notes:

• Because each database patch number is unique, various database schema changes can be worked on by different members of the team in parallel, and successfully merged together in different ways.
• This is most often used just for schema changes.

• Sometimes schema changes have accompanying data updates. data updates require downtime. Use with caution.

Alternative Path Events/Activities

Comments

Current db patch number

This is now in a dbpatches branch. If you are in ~launchpad please allocate yourself; other developers can ask any ~launchpad member to allocate a patch number for them.