rockcookies/golang-uber-dig

Persona: You are a Go architect wiring an application graph with dig. You keep the container at the composition root, depend on interfaces not concrete types, and treat constructor errors as first-class failures.

Using uber-go/dig for Dependency Injection in Go

Reflection-based DI toolkit, designed to power application frameworks (it is the engine behind uber-go/fx) and resolve object graphs during startup.

Official Resources:

• pkg.go.dev/go.uber.org/dig
• github.com/uber-go/dig

This skill is not exhaustive. Please refer to library documentation and code examples for more information. Context7 can help as a discoverability platform.

go get go.uber.org/dig

dig vs. fx

fx is built on dig and shares the same container engine — the DI primitives (Provide, Invoke, In/Out structs, named values, value groups) are identical. fx.In/fx.Out are re-exports of dig.In/dig.Out.

What fx adds on top of dig:

| Concern | dig | fx | | --- | --- | --- | | DI container | ✅ dig.New() | ✅ (embedded) | | Lifecycle hooks | ❌ | ✅ fx.Lifecycle OnStart/OnStop | | Module system | ❌ | ✅ fx.Module with scoped decorators | | Signal-aware run loop | ❌ | ✅ app.Run() blocks on SIGINT/SIGTERM | | Structured event logging | ❌ | ✅ fx.WithLogger / fxevent | | Startup/shutdown timeout | ❌ | ✅ fx.StartTimeout / fx.StopTimeout |

Choose dig when you need the wiring graph only: CLI tools, libraries exposing a container to callers, test harnesses, or embedding DI into an existing app that manages its own lifecycle.

Choose fx for long-running services (HTTP servers, workers, daemons) — lifecycle and signal handling are non-negotiable there. See golang-uber-fx skill.

Container

import "go.uber.org/dig"

c := dig.New()

Useful options: dig.DeferAcyclicVerification() (faster startup), dig.RecoverFromPanics() (turn panics into dig.PanicError), dig.DryRun(true) (validate without invoking).

Provide and Invoke

// Register a constructor — lazy, only runs when its output is needed
err := c.Provide(func(cfg *Config) (*sql.DB, error) {
    return sql.Open("postgres", cfg.DSN)
})

// Pull a service out of the container by asking for it as a function parameter
err = c.Invoke(func(db *sql.DB) error {
    return db.Ping()
})

Constructors are lazy and memoized: each output type is built once and shared (singleton per container). Provide errors at registration if the constructor is malformed; Invoke returns the constructor's error wrapped with the dependency path that triggered it.

A dig constructor is any function. Inputs are dependencies, outputs are provided types. error (last return) signals construction failure. Follow "accept interfaces, return structs".

Parameter Objects with dig.In

Once a constructor has 4+ dependencies, embed dig.In to group them as struct fields and tag fields:

type HandlerParams struct {
    dig.In

    Logger *zap.Logger
    DB     *sql.DB
    Cache  *redis.Client `optional:"true"`           // zero value if not provided
    DBRO   *sql.DB       `name:"readonly"`           // named dependency
    Routes []http.Handler `group:"routes"`           // value group
}

func NewHandler(p HandlerParams) *Handler { /* ... */ }

Tags: name:"...", optional:"true", group:"...".

Result Objects with dig.Out

Return several values from one constructor and attach name/group tags to results:

type ConnResult struct {
    dig.Out

    ReadWrite *sql.DB `name:"primary"`
    ReadOnly  *sql.DB `name:"readonly"`
}

func NewConnections(cfg *Config) (ConnResult, error) { /* ... */ }

Named Values

Two providers of the same type collide. Disambiguate with dig.Name:

c.Provide(NewPrimaryDB,  dig.Name("primary"))
c.Provide(NewReadOnlyDB, dig.Name("readonly"))

Consume by adding name:"primary" / name:"readonly" to a dig.In field.

Value Groups

Many providers, one consumer slice — typical for HTTP handlers, health checks, migrations:

type RouteResult struct {
    dig.Out
    Handler http.Handler `group:"routes"`
}

func NewUserHandler(db *sql.DB) RouteResult { /* ... */ }
func NewPostHandler(db *sql.DB) RouteResult { /* ... */ }

type ServerParams struct {
    dig.In
    Routes []http.Handler `group:"routes"`
}

Flatten — append ,flatten (e.g. group:"routes,flatten") to unwrap a slice instead of nesting it. Group order is not guaranteed; if order matters, provide an explicit ordered slice from a single constructor.

Provide as Interface (dig.As)

Register a concrete constructor and expose it under one or more interfaces without a separate adapter:

c.Provide(NewPostgresDB, dig.As(new(Database), new(io.Closer)))
// Consumers ask for Database or io.Closer; *PostgresDB stays hidden.

Full Application Example

func main() {
    c := dig.New()

    must(c.Provide(NewConfig))
    must(c.Provide(NewLogger))
    must(c.Provide(NewDatabase))
    must(c.Provide(NewServer))

    err := c.Invoke(func(srv *http.Server) error {
        return srv.ListenAndServe()
    })
    if err != nil {
        log.Fatal(err)
    }
}

func must(err error) { if err != nil { panic(err) } }

dig has no built-in lifecycle. If you need OnStart/OnStop hooks, signal handling, and graceful shutdown, use fx — see golang-uber-fx skill.

For Decorate, Scopes, optional deps, error helpers, and Visualize, see advanced.md.

Best Practices

1. Keep the container at the composition root — never pass *dig.Container as a parameter; treat it like a plumbing detail of main(). Service-locator patterns defeat the testability gains of DI.
2. Depend on interfaces, not concrete types — lets you swap implementations in tests without touching production code, and lets you use dig.As to expose narrow interfaces from wide structs.
3. Prefer parameter objects (dig.In structs) once a constructor has 4+ dependencies — call sites stay readable and adding a new dependency is a one-line change instead of a signature break.
4. Group registration by module (one file per module that calls c.Provide for its types) — review and refactoring become a per-module concern, and you can extract a module into a fx.Module later without rewriting wiring.
5. Validate the graph eagerly in tests — call c.Invoke against the composition root in CI to surface missing providers at boot time, not at first request. DryRun(true) skips constructor execution.
6. Return errors from constructors instead of panicking — dig wraps them with the dependency path, which makes the failure point obvious.

Common Mistakes

| Mistake | Fix | | --- | --- | | Passing the container into services | The container belongs to main(). Inject the typed dependencies a service needs; otherwise tests need to build a real container. | | Two providers for the same type without Name | dig errors at Provide time. Either name them, or merge into a single provider that returns a dig.Out result struct. | | Ignoring Provide errors | Wrap each Provide with a must helper. A silent registration error becomes a missing-type error far later. | | Using groups when ordering matters | Groups are unordered. If order matters (middleware chain, migration sequence), provide an explicit ordered slice with one constructor. | | Constructors with side effects on import | Keep init() empty — start work only inside the constructor, after the graph is built. |

Testing

dig containers are cheap — build a fresh one per test, override providers with Decorate, and call Invoke to drive the system. For full patterns (per-test wiring, shared helpers, graph validation in CI, asserting wire-time errors, recovering from constructor panics), see testing.md.

Further Reading

• advanced.md — Decorate, Scopes, optional deps, error helpers, Visualize, full Quick Reference
• recipes.md — end-to-end examples: HTTP server with route group, two databases, request scopes, decorators, dry-run validation
• testing.md — testing patterns and graph validation

Cross-References

• → See golang-uber-fx skill for application lifecycle, modules, and signal-aware Run() built on top of dig
• → See golang-dependency-injection skill for DI concepts and library comparison
• → See golang-samber-do skill for a generics-based alternative without reflection
• → See golang-google-wire skill for compile-time DI (no runtime container)
• → See golang-structs-interfaces skill for interface design patterns
• → See golang-testing skill for general testing patterns

If you encounter a bug or unexpected behavior in uber-go/dig, open an issue at https://github.com/uber-go/dig/issues.