Dependency Injection — Notes

When DI pays off

• Testing — swap real for fake / mock.
• Multiple deployments — same code, different config (cloud A vs B).
• Plug-in architectures — load implementations at startup.
• Cross-cutting concerns — inject logging / metrics / tracing.

When it's overkill

• Trivial CLI tools.
• Pure functions without external state.

Useful rules

1. Constructor injection by default.
2. Don't pass the container around; pass the things.
3. One composition root.
4. Avoid runtime reflection magic when compile-time injection works (Dagger > Spring runtime for performance).
5. Lifecycle scopes match logical boundaries (request, transaction, batch).

DI vs IoC

• Inversion of Control (IoC): a framework calls you, you don't call the framework.
• Dependency Injection: a specific IoC technique focusing on how dependencies arrive.
• All DI is IoC; not all IoC is DI (e.g. event loops are IoC without DI).

Refs

• Martin Fowler: "Inversion of Control Containers and the Dependency Injection pattern" (2004).
• Mark Seemann: Dependency Injection in .NET (book, but principles are language-agnostic).
• Google Dagger user guide (compile-time DI).