Run alembic upgrade migrations in a transaction

Question

Does alembic upgrade head run inside a transaction so that all database alterations succeed, or fail? If not, why was it designed this way?

Answer 1

My understanding is alembic runs inside a transaction for databases that support it, like Postgres. If you're on a database that does not support this (cough MySQL cough), you can't use this functionality.

Answer 2

That's something you can decide within the env.py, which is where you customize the behaviour of the migrations to suit your setup. You can see how to make sure your upgrades happen in a transaction from the template provided as an example for generic databases: https://github.com/zzzeek/alembic/blob/eaaafbca88f85f5432e04affe1f94cbf1ad06080/alembic/templates/generic/env.py#L64

def run_migrations_online():
    # ...
    with context.begin_transaction():
        context.run_migrations()