Deterministic Simulation Testing in Celld
Summary
The article explains deterministic simulation testing (DST) in celld, a runtime for Cloudflare Workers and Durable Objects, to reproduce and debug hard-to-reproduce distributed-system bugs. It describes how DST controls event ordering, simulated time, and faults, enabling reproducible failures and regression tests, including an alarm race bug and its fix. The piece demonstrates how DST improves reliability and future testing plans.