golang-dependency-injection

What this skill does

**Persona:** You are a Go software architect. You guide teams toward testable, loosely coupled designs — you choose the simplest DI approach that solves the problem, and you never over-engineer.

**Modes:**

- **Design mode** (new project, new service, or adding a service to an existing DI setup): assess the existing dependency graph and lifecycle needs; recommend manual injection or a library from the decision table; then generate the wiring code.
- **Refactor mode** (existing coupled code): use up to 3 parallel sub-agents — Agent 1 identifies global variables and `init()` service setup, Agent 2 maps concrete type dependencies that should become interfaces, Agent 3 locates service-locator anti-patterns (container passed as argument) — then consolidate findings and propose a migration plan.

> **Community default.** A company skill that explicitly supersedes `samber/cc-skills-golang@golang-dependency-injection` skill takes precedence.

# Dependency Injection in Go

Dependency injection (DI) means passing dependencies to a component rather than having it create or find them. In Go, this is how you build testable, loosely coupled applications — your services declare what they need, and the caller (or container) provides it.

This skill is not exhaustive. When using a DI library (google/wire, uber-go/dig, uber-go/fx, samber/do), refer to the library's official documentation and code examples for current API signatures.

For interface-based design foundations (accept interfaces, return structs), see the `samber/cc-skills-golang@golang-structs-interfaces` skill.

## Best Practices Summary

1. Dependencies MUST be injected via constructors — NEVER use global variables or `init()` for service setup
2. Small projects (< 10 services) SHOULD use manual constructor injection — no library needed
3. Interfaces MUST be defined where consumed, not where implemented — accept interfaces, return structs
4. NEVER use global registries or package-level service locators
5. The DI container MUST only exist at the composition root (`main()` or app startup) — NEVER pass the container as a dependency
6. **Prefer lazy initialization** — only create services when first requested
7. **Use singletons for stateful services** (DB connections, caches) and transients for stateless ones
8. **Mock at the interface boundary** — DI makes this trivial
9. **Keep the dependency graph shallow** — deep chains signal design problems
10. **Choose the right DI library** for your project size and team — see the decision table below

## Why Dependency Injection?

| Problem without DI | How DI solves it |
| --- | --- |
| Functions create their own dependencies | Dependencies are injected — swap implementations freely |
| Testing requires real databases, APIs | Pass mock implementations in tests |
| Changing one component breaks others | Loose coupling via interfaces — components don't know each other's internals |
| Services initialized everywhere | Centralized container manages lifecycle (singleton, factory, lazy) |
| All services loaded at startup | Lazy loading — services created only when first requested |
| Global state and `init()` functions | Explicit wiring at startup — predictable, debuggable |

DI shines in applications with many interconnected services — HTTP servers, microservices, CLI tools with plugins. For a small script with 2-3 functions, manual wiring is fine. Don't over-engineer.

## Manual Constructor Injection (No Library)

For small projects, pass dependencies through constructors. See [Manual DI examples](./references/manual-di.md) for a complete application example.

```go
// ✓ Good — explicit dependencies, testable
type UserService struct {
    db     UserStore
    mailer Mailer
    logger *slog.Logger
}

func NewUserService(db UserStore, mailer Mailer, logger *slog.Logger) *UserService {
    return &UserService{db: db, mailer: mailer, logger: logger}
}

// main.go — manual wiring
func main() {
    logger := slog.Default()
    db := postgres.NewUserStore(connStr)
    mailer := smtp.NewMailer(smtpAddr)
    userSvc := NewUserService(db, mailer, logger)
    orderSvc := NewOrderService(db, logger)
    api := NewAPI(userSvc, orderSvc, logger)
    api.ListenAndServe(":8080")
}
```

```go
// ✗ Bad — hardcoded dependencies, untestable
type UserService struct {
    db *sql.DB
}

func NewUserService() *UserService {
    db, _ := sql.Open("postgres", os.Getenv("DATABASE_URL")) // hidden dependency
    return &UserService{db: db}
}
```

Manual DI breaks down when:

- You have 15+ services with cross-dependencies
- You need lifecycle management (health checks, graceful shutdown)
- You want lazy initialization or scoped containers
- Wiring order becomes fragile and hard to maintain

## DI Library Comparison

Go has three main approaches to DI libraries:

- [google/wire examples](./references/google-wire.md) — Compile-time code generation
- [uber-go/dig + fx examples](./references/uber-dig-fx.md) — Reflection-based framework
- [samber/do examples](./references/samber-do.md) — Generics-based, no code generation

### Decision Table

| Criteria | Manual | google/wire | uber-go/dig + fx | samber/do |
| --- | --- | --- | --- | --- |
| **Project size** | Small (< 10 services) | Medium-Large | Large | Any size |
| **Type safety** | Compile-time | Compile-time (codegen) | Runtime (reflection) | Compile-time (generics) |
| **Code generation** | None | Required (`wire_gen.go`) | None | None |
| **Reflection** | None | None | Yes | None |
| **API style** | N/A | Provider sets + build tags | Struct tags + decorators | Simple, generic functions |
| **Lazy loading** | Manual | N/A (all eager) | Built-in (fx) | Built-in |
| **Singletons** | Manual | Built-in | Built-in | Built-in |
| **Transient/factory** | Manual | Manual | Built-in | Built-in |
| **Scopes/modules** | Manual | Provider sets | Module system (fx) | Built-in (hierarchical) |
| **Health checks** | Manual | Manual | Manual | Built-in interface |
| **Graceful shutdown** | Manual | Manual | Built-in (fx) | Built-in interface |
| **Container cloning** | N/A | N/A | N/A | Built-in |
| **Debugging** | Print statements | Compile errors | `fx.Visualize()` | `ExplainInjector()`, web interface |
| **Go version** | Any | Any | Any | 1.18+ (generics) |
| **Learning curve** | None | Medium | High | Low |

### Quick Comparison: Same App, Four Ways

The dependency graph: `Config -> Database -> UserStore -> UserService -> API`

**Manual**:

```go
cfg := NewConfig()
db := NewDatabase(cfg)
store := NewUserStore(db)
svc := NewUserService(store)
api := NewAPI(svc)
api.Run()
// No automatic shutdown, health checks, or lazy loading
```

**google/wire**:

```go
// wire.go — then run: wire ./...
func InitializeAPI() (*API, error) {
    wire.Build(NewConfig, NewDatabase, NewUserStore, NewUserService, NewAPI)
    return nil, nil
}
// No lifecycle hooks (OnStart/OnStop) or health checks; cleanup via returned func() from providers
```

**uber-go/fx**:

```go
app := fx.New(
    fx.Provide(NewConfig, NewDatabase, NewUserStore, NewUserService),
    fx.Invoke(func(api *API) { api.Run() }),
)
app.Run() // manages lifecycle, but reflection-based
```

**samber/do**:

```go
i := do.New()
do.Provide(i, NewConfig)
do.Provide(i, NewDatabase)    // auto shutdown + health check
do.Provide(i, NewUserStore)
do.Provide(i, NewUserService)
api := do.MustInvoke[*API](i)
api.Run()
// defer i.Shutdown() — handles all cleanup automatically
```

## Testing with DI

DI makes testing straightforward — inject mocks instead of real implementations:

```go
// Define a mock
type MockUserStore struct {
    users map[string]*User
}

func (m *MockUserStore) FindByID(ctx context.Context, id string) (*User, error) {
    u, ok := m.users[id]
    if !ok {
        return nil, ErrNotFound
    }
    return u, nil
}

// Test with manual injection
func TestUserService_GetUser(t *testing.T) {
    mock := &MockUserStore{
        users: map[string]*User{"1": {ID: "1", Name: "Alice"}},
    }
    svc := NewU