GoKawiilCloudflare's celld runtime adds deterministic simulation testing to catch distributed bugs
Cloudflare engineers describe building a deterministic simulation testing (DST) tool for celld, a runtime for running Workers and Durable Objects applications outside Cloudflare's infrastructure using only an S3-compatible object store as an external dependency. The simulator, still in development and not yet public, controls event ordering, timing, and storage responses so that failures involving delayed messages, failed writes, or node restarts can be reliably reproduced and debugged.
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Distributed systems bugs are notoriously hard to reproduce because failures often depend on precise, non-deterministic timing of network and storage events. By building a tool that lets engineers replay exact failure conditions, Cloudflare suggests it can find and fix subtle bugs before they affect production, which could improve the reliability of self-hosted Workers deployments built on celld.
- celld is a runtime letting Cloudflare Workers and Durable Objects run on external infrastructure with just an S3-compatible store.
- Cloudflare built a deterministic simulation testing tool to reproduce hard-to-catch distributed systems bugs.
- The simulator can control event ordering, delay or fail storage writes, and manipulate simulated time to test retries and alarms.
Source: celld.dev — Yusuke Tanaka, 2026-10-09
Published there as: “Deterministic Simulation Testing in Celld”
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