golang-uber-dig

par samber

Implements dependency injection in Golang using uber-go/dig — reflection-based container, Provide/Invoke, dig.In/dig.Out parameter and result objects, named values, value groups, optional dependencies, scopes, and Decorate. Apply when using or adopting uber-go/dig, when the codebase imports `go.uber.org/dig`, or when wiring an application graph at startup. For higher-level lifecycle and modules, see `samber/cc-skills-golang@golang-uber-fx` skill.

npx skills add https://github.com/samber/cc-skills-golang --skill golang-uber-dig

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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:

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 samber/cc-skills-golang@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`)

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 samber/cc-skills-golang@golang-uber-fx skill.

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

Best Practices

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.
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.
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.
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.
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.
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.

Cross-References

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