Database Migrations

SecureDrop uses Alembic for database schema migrations. This guide is not a complete explanation of what alembic is or how it is used, so the original documentation should be read.

Migration Files

In the securedrop/ directory, the file alembic.ini contains the configuration needed to run alembic commands, and the directory alembic/ contains the Python code that executes migrations.

The directory looks like this.

.
├── alembic
│   ├── env.py
│   ├── script.py.mako
│   └── versions
│       ├── 15ac9509fc68_init.py
│       └── faac8092c123_enable_security_pragmas.py
└── alembic.ini

The subdirectory versions/ individual migrations that are generated by alembic. In the example above, there are two migrations. alembic orders these migrations based off of values in the Python files, not off any sort of lexicographic ordering. The file faac8092c123_enable_security_pragmas.py has a module-level documentation string that specifies that it comes after 15ac9509fc68_init.py as well as variables used by alembic that specify the ordering of migrations.

Deployment

Database migrations are automatically applied to production instances via the command alembic upgrade head in the postinst script in the securedrop-app-code Debian package. You do not need to worry about when or how these migrations are applied.

Developer Workflow

Updating the Models

When you want to modify the database schema, you need to add adjust the models in the file models.py. All indices, constraints, or other metadata about the scheme needs to be in this file. The development server creates tables directly from the subclasses of db.Model so that they are available for manual and automated testing.

Creating Migrations

Once you are satisfied with your new model, alembic can auto-generate migrations using SQLAlchemy metadata and comparing it to the schema of an up-to-date SQLite database. To generate a new migration use the following steps.

cd securedrop/
./bin/dev-shell
source bin/dev-deps
maybe_create_config_py
./bin/new-migration 'my migration message'

This will output a new migration into alembic/versions/. You will need to verify that this migration produced the desired output. While still in the dev-shell, you can run the following command to see an output of the SQL that will be generated.

alembic upgrade head --sql

Unit Testing Migrations

The test suite already comes with a test runner (test_alembic.py) that runs a series of checks to ensure migration’s upgrade and downgrade commands are idempotent and don’t break the database. The test runner uses dynamic module import to iterate through all the migrations. You will need to create a python module in the tests/migrations/ directory. You module MUST be named migration_<revision identifier>.py. For example, if your revision is named 15ac9509fc68_init.py, your test module will be named migration_15ac9509fc68.py. Example modules for the first two revisions are shown below.

tests/migrations/
├── __init__.py
├── migration_15ac9509fc68.py
└── migration_faac8092c123.py

Your module MUST contain the following classes with the following attributes.

class UpgradeTester:

    def __init__(self, config):
        '''This function MUST accept an argument named `config`.
           You will likely want to save a reference to the config in your
           class so you can access the database later.
        '''
        self.config = config

    def load_data(self):
        '''This function loads data into the database and filesystem. It is
           executed before the upgrade.
        '''
        pass

    def check_upgrade(self):
        '''This function is run after the upgrade and verifies the state
           of the database or filesystem. It MUST raise an exception if the
           check fails.
        '''
        pass


class DowngradeTester:

    def __init__(self, config):
        '''This function MUST accept an argument named `config`.
           You will likely want to save a reference to the config in your
           class so you can access the database later.
        '''
        self.config = config

    def load_data(self):
        '''This function loads data into the database and filesystem. It is
           executed before the downgrade.
        '''
        pass

    def check_downgrade(self):
        '''This function is run after the downgrade and verifies the state
           of the database or filesystem. It MUST raise an exception if the
           check fails.
        '''
        pass

Your migration test needs to load data that covers all edge cases such as potentially broken foreign keys or columns with unexpected content.

Additionally, your test MUST NOT import anything from the models module as this will not accurately test your migration, and it will likely break during future code changes. In fact, you should use as few dependencies as possible in your test including other securedrop code as well as external packages. This may be a rather annoying requirement, but it will make the tests more robust aginst future code changes.

Release Testing Migrations

In order to ensure that migrations between from the previous to current version of SecureDrop apply cleanly in production-like instances, we have a helper script that is designed to load semi-randomized data into the database. You will need to modify the script qa_loader.py to include sample data. This sample data should intentionally include edge cases that might behave strangely such as data whose nullability is only enforced by the application or missing files.

During QA, the release manager should follow these steps to test the migrations.

  1. Checkout the previous SecureDrop release
  2. Build Debian packages locally
  3. Provision staging VMs
  4. vagrant ssh app-staging
  5. sudo su
  6. cd /var/www/securedrop && ./qa_loader.py
  7. Checkout the release candidate
  8. Re-provision the staging VMs
  9. Check that nothing went horribly wrong