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:
docs/reference/detail/new-module-registry.md— The newjsg::modules::ModuleRegistryimplementation. Covers the two-layer architecture (sharedModuleRegistry+ per-isolateIsolateModuleRegistry), bundle types, resolution priority, compile cache, evaluation flow, and thread safety model. Read this first for any work on the new registry.docs/reference/detail/legacy-module-registry.md— The legacyModuleRegistryImpl<TypeWrapper>implementation. Covers the per-isolate design,ModuleInfostructure,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.docs/reference/detail/v8-module-internals.md— How V8 handles modules internally. Coversv8::Modulestatus transitions,InstantiateModule,Evaluate,SyntheticModule, resolve callbacks, andimport.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 implementationsrc/workerd/jsg/modules.h/.c++— Legacy registry implementationsrc/workerd/jsg/resource.h:1913–1922— Legacy vs new dispatch at context creationsrc/workerd/jsg/jsg.c++:357–395—Lock::resolveModuledispatchsrc/workerd/jsg/setup.h:291–297—isUsingNewModuleRegistry()flagsrc/workerd/io/worker-modules.h—newWorkerModuleRegistryconstructionsrc/workerd/api/modules.h— Built-in module registration (both legacy and new)src/workerd/jsg/observer.h—ResolveObservermetrics interface
Procedure
- Read the relevant reference documents listed above for your task.
- Identify whether the code path uses the new or legacy registry (check
isUsingNewModuleRegistry()usage in the surrounding code). - 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.