Parallel effects cover what happens when two or more requests touch the same workflow state, or when asynchronous callbacks arrive out of order. They sit beside sequencing (order of user-facing steps) and state alignment (what each layer believes).
Concurrency#
| Page | Focus |
|---|---|
| TOCTOU in workflows | Check-then-act gaps across requests |
| Double spend | Spending the same balance, coupon, or credit twice |
| Parallel requests | Intentional request fan-out against weak locks |
Integration handoff#
| Page | Focus |
|---|---|
| Webhook replay | Duplicate delivery of signed or unsigned callbacks |
| Out-of-order callback | Payment or provisioning events applied in wrong order |
| Idempotency key misuse | Keys ignored, scoped wrong, or reset on retry |