module-registry

## Module Registry Context When working with any code related to module resolution, compilation, evaluation, or registration in workerd, you **must** read the relevant reference documents before making changes or providing analysis. These documents capture the design decisions, data flow, thread safety model, and integration points that are not obvious from reading the source alone. ### Reference Documents Read these documents (using the Read tool) before proceeding with any module registry work: 1. **`docs/reference/detail/new-module-registry.md`** — The new `jsg::modules::ModuleRegistry` implementation. Covers the two-layer architecture (shared `ModuleRegistry` + per-isolate `IsolateModuleRegistry`), bundle types, resolution priority, compile cache, evaluation flow, and thread safety model. **Read this first** for any work on the new registry. 2. **`docs/reference/detail/legacy-module-registry.md`** — The legacy `ModuleRegistryImpl<TypeWrapper>` implementation. Covers the per-isolate design, `ModuleInfo` structure, `DynamicImportHandler`, and how it interfaces with V8. **Read this** when working on code that still uses the legacy path, or when understanding the dispatch between legacy and new. 3. **`docs/reference/detail/v8-module-internals.md`** — How V8 handles modules internally. Covers `v8::Module` status transitions, `InstantiateModule`, `Evaluate`, `SyntheticModule`, resolve callbacks, and `import.meta`. **Read this** when debugging V8 module state issues, understanding callback contracts, or working on the boundary between workerd and V8. ### When to Read Which | Task | Documents to read | | ------------------------------------------------- | ----------------------------------------------- | | Modifying new registry resolution or caching | new-module-registry, v8 | | Adding a new built-in module | new-module-registry, legacy-module-registry | | Debugging module-not-found errors | all three | | Modifying legacy registry code | legacy-module-registry, v8 | | Understanding legacy-to-new dispatch | all three | | Working on `import.meta` behavior | new-module-registry, v8 | | Working on CJS/require() interop | new-module-registry, legacy-module-registry | | Debugging module evaluation failures | new-module-registry, legacy-module-registry, v8 | | Reviewing a PR that touches module code | all three | | Working on compile cache or cross-isolate sharing | new-module-registry | | Working on dynamic import | all three | | Working on source phase imports (Wasm) | new-module-registry, v8 | ### Key Source Files These are the primary files involved in module handling: - `src/workerd/jsg/modules-new.h` / `.c++` — New registry implementation - `src/workerd/jsg/modules.h` / `.c++` — Legacy registry implementation - `src/workerd/jsg/resource.h:1913–1922` — Legacy vs new dispatch at context creation - `src/workerd/jsg/jsg.c++:357–395` — `Lock::resolveModule` dispatch - `src/workerd/jsg/setup.h:291–297` — `isUsingNewModuleRegistry()` flag - `src/workerd/io/worker-modules.h` — `newWorkerModuleRegistry` construction - `src/workerd/api/modules.h` — Built-in module registration (both legacy and new) - `src/workerd/jsg/observer.h` — `ResolveObserver` metrics interface ### Procedure 1. **Read** the relevant reference documents listed above for your task. 2. **Identify** whether the code path uses the new or legacy registry (check `isUsingNewModuleRegistry()` usage in the surrounding code). 3. **Proceed** with your work, using the reference documents to inform your understanding of data flow, thread safety, and integration contracts. Do not skip reading the reference documents. The module registry has subtle invariants around thread safety, V8 module status transitions, and evaluation context (IoContext suppression) that are easy to violate without full context. --- > Source: [cloudflare/workerd](https://github.com/cloudflare/workerd) — distributed by [TomeVault](https://tomevault.io). <!-- tomevault:4.0:skill_md:2026-07-20 -->