SecureDrop uses Alembic for database schema migrations. This guide is not a complete explanation of
alembic is or how it is used, so the original documentation should be read.
securedrop/ directory, the file
alembic.ini contains the configuration needed to run
alembic commands, and the directory
alembic/ contains the Python code that executes
The directory looks like this.
. ├── alembic │ ├── env.py │ ├── script.py.mako │ └── versions │ ├── 15ac9509fc68_init.py │ └── faac8092c123_enable_security_pragmas.py └── alembic.ini
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.
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.
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.
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
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
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.
Checkout the previous SecureDrop release
Build Debian packages locally
Provision staging VMs
vagrant ssh app-staging
sudo -u www-data bash
cd /var/www/securedrop && ./loaddata.py
Checkout the release candidate
Re-provision the staging VMs
Check that nothing went horribly wrong