Generic single-database configuration.

Migration scripts are created each time modifications are made on the database model or data.

For example if you want to add a new field to a table, add the column in the SQLAlchemy model (models.py).

Then autogenerate the migration script with:

.build/venv/bin/alembic revision --autogenerate -m 'Add column x'

A new migration script is created in alembic/versions/. Make sure the script looks correct, adjust if necessary.

To add the new column to the database(s), run the migration (see below) and make sure the database is updated correctly.

Commit the changes.

A migration should be run each time the application code is updated or if you just created a migration script.

.build/venv/bin/alembic upgrade head

To make sure that migrations are correctly applied to both databases or none, the migration is by default doing a two-phase commit.

Prepared transactions which are required for the two-phase commit are by default disabled on PostgreSQL. If so you will get an error message like '(psycopg2.OperationalError) prepared transactions are disabled'.

To enable, set max_prepared_transactions in postgresql.conf (the recommend value is max_connections, by default 100). Restart required.

More information here: http://www.postgresql.org/docs/9.4/static/runtime-config-resource.html

To run the migration without using a two-phase commit, the twophase flag can be disabled:

.build/venv/bin/alembic -x twophase=False upgrade head

Please refer to Alembic documentation for more information. http://alembic.readthedocs.org/en/latest/