In Catalyst, can I use transactional outbox patterns to achieve exactly-once side effects with databases?

Yes, you can implement a transactional outbox pattern within Catalyst's workflow activities, where side effects are written to a database in the same transaction as workflow state. Catalyst's durable execution ensures the workflow step is replayed if needed, but the outbox ensures side effects are applied exactly once. This is a common pattern and works with Catalyst's at-least-once model.