What is the difference between a shared module and a global module in NestJS?
A shared module makes providers available only to modules that explicitly import it — dependencies are visible and traceable. A global module decorated with @Global() makes providers available everywhere without any import statement — convenient but hides dependencies and makes modules harder to reason about in isolation.
Shared module vs global module
When to use each:
Shared module — default choice for all feature-level reuse; explicit imports keep the dependency graph readable.
Global module — only for infrastructure that every single module legitimately needs: config, logging, database.
Register global modules exactly once in AppModule — never in feature modules.
Overusing @Global() leads to hidden coupling and makes unit testing harder.