Inserting State Transitions in Postgres

When managing state changes in a PostgreSQL database, using an append-only table for storing status transitions can provide a complete history of changes. However, this approach introduces a challenge when multiple transactions attempt to modify the same user's status simultaneously. In a standard column-based design, the database serializes updates so that only one transaction's change is applied. But with an append-only model, both transactions would succeed, resulting in two conflicting status changes for the user.

To address this issue, a locking mechanism is needed to ensure that the second transaction waits until the first one finishes. In PostgreSQL, the `SELECT ... FOR UPDATE` statement can be used to lock a row for the duration of the transaction. By applying this lock to the parent user row, the second transaction would acquire the lock after the first one commits, allowing it to read the updated status and make an informed decision based on the allowed transitions.

In this scenario, if two admins concurrently change Alice's status from pending to approved and then denied, the first transaction would lock the row, and the second transaction would block until the first one releases the lock. Upon acquiring the lock, the second transaction would see Alice's status as approved and prevent the denied transition, maintaining data consistency.

While this approach requires application logic to implement the transition rules and handle invalid transitions, it ensures that only one valid state change occurs at a time. The append-only model itself does not solve the problem of concurrent state transitions; it merely makes the concurrency requirement explicit. The lock either way is necessary to prevent contradictory state changes, and the append-only table provides additional benefits, such as full history tracking, which may be valuable for auditing or compliance purposes.

In summary, when using an append-only table for state transitions in PostgreSQL, a locking mechanism like `SELECT ... FOR UPDATE` is essential to handle concurrent modifications and maintain data integrity. This approach ensures that only one valid transition occurs at a time, preventing contradictory changes and preserving the integrity of the application's state.