| handler has external effects | replay-safe mode |
| duplicate application | offset/version guard |
| incompatible old schema | upcaster or versioned handler |
| replay never catches live | capacity margin and throttling |
| bad cutover | atomic alias and rollback |
| Monitor replay rate, live arrival rate, convergence ETA, rejects, checksums, and storage growth. Preserve the old projection until the new one survives production validation. | |
| ## Test yourself | |
| 1. When can replay never converge? | |
| 2. How do upcasters preserve old events? | |
| 3. What makes cutover reversible? | |
Continue to professional.md. | |