Understanding database session management and transactions in Invenio

Transactions are tied to requests

In Invenio, a transaction is tied to a HTTP request. By default, the transaction is automatically rolled back unless you explicitly call db.session.commit().

Exceptions causes rollback

If an exception occurs during request handling, then the entire transaction will be rolled back unless there has been an explicit call to db.session.commit() before the exception was raised. This is because the default behavior is to rollback.

Why are transactions tied to requests?

Transactions are tied to requests, because the outer view, in charge of handling the request, needs full control of when a transaction is committed. If the view was not in charge, you could end up with inconsistent data in the database - for instance persistent identifier may have been committed, but the associated record was not committed.

When are SQL statements sent to the database server.

SQLAlchemy only sends the SQL statements (INSERT, UPDATE, SELECT, …) to the database server when needed, or when explicitly requested via e.g. db.session.flush() or db.session.commit().

This means that in many cases, SQL insert and update statements are not sent to the server until a commit is called.

What about nested transactions?

Nested transactions are using database save points, which allow you to do a partial rollback. Also, nested transactions causes a flush to the database, meaning that the SQL statements are sent to the server.

When are partial rollbacks useful?

Partial rollbacks can be useful for instance if you want to try to insert a user, but the user already exists in the table. Then you can rollback the insert and instead execute an update statement at the application level.

When are flushing useful?

Explicitly forcing the flush of SQL statements to the database can be useful if you need a value from the database (e.g. auto-incrementing primary keys), and the application needs the primary key to continue. Also, they can be useful to force integrity constraints to be checked by the database, which may be needed by the database.

What happens with exceptions in nested transactions?

If an exception occurs in a nested transaction, first the save point will be rolled back, and afterwards the entire transaction will be rolled back unless the exception is caught.

For instance in the following code example, the entire transaction will be rolled backed:

@app.route('/')
def index():
    # db operations 1 ....
    with db.session.begin_nested():
        # db operations 2 ....
        raise Exception()
    db.session.commit()

On the other, in the following example, the propagation of the exception is stopped, and only the db 2 operations are rolled back, while db operations 1 are committed to the database.

@app.route('/')
def index():
    # db operations 1 ....
    try:
        db.session.begin_nested():
        # db operations 2 ....
        raise Exception()
        db.session.commit()
    except Exception:
        db.session.rollback()
    db.session.commit()
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