Durable changes and acknowledgements

Leani delivery is deliberately at-least-once. Exactly-once application effects come from an application transaction, not from pretending an HTTP response and a database commit are atomic.

Leani change ─> begin destination transaction
                ├─ apply idempotently
                ├─ persist opaque cursor + sequence
                └─ commit
              ─> acknowledge Leani

If the acknowledgement response is lost, the same change may arrive again. The destination’s sequence key makes the mutation idempotent, and its committed cursor proves what it already applied. Leani’s acknowledgement separately tells the node what delivery history a required consumer still fences from pruning.

Leases and reset requirements

A durable consumer has a stable ID, a credential, a role, an acknowledgement, and a finite lease. Active SDK iterators renew their lease. If a best-effort consumer falls behind retained history, the stream emits reset_required with coverage hints rather than skipping invisibly.

Query then follow

A consistent snapshot is held across pagination and returns the boundary cursor needed to continue with changes after the snapshot. Release the snapshot when finished. This closes the race between “last row queried” and “first live event received” without freezing ingestion.

See Consume changes durably or the complete PostgreSQL adapter.