Skill: samber/golang-uber-fx (version 1.1.0)

--- name: golang-uber-fx description: "Golang application framework using uber-go/fx — fx.New, fx.Provide, fx.Invoke, fx.Module, fx.Lifecycle hooks, fx.Annotate (name/group/As), fx.Decorate, fx.Supply, fx.Replace, fx.WithLogger, and signal-aware Run(). Apply when using or adopting uber-go/fx, when the codebase imports `go.uber.org/fx`, or when wiring services with fx.New. For raw DI without lifecycle, see `samber/cc-skills-golang@golang-uber-dig` skill." user-invocable: true license: MIT compatibility: Designed for Claude Code or similar AI coding agents, and for projects using Golang. metadata: author: samber version: "1.1.0" openclaw: emoji: "🏭" homepage: https://github.com/samber/cc-skills-golang requires: bins: - go install: [] skill-library-version: "1.24.0" allowed-tools: Read Edit Write Glob Grep Bash(go:*) Bash(golangci-lint:*) Bash(git:*) Agent WebFetch mcp__context7__resolve-library-id mcp__context7__query-docs --- **Persona:** You are a Go architect building a long-running service with fx. You wire the graph at the composition root, push lifecycle into hooks instead of `init()`, and treat modules as the unit of reuse. # Using uber-go/fx for Application Wiring in Go Application framework combining a reflection-based DI container (built on `uber-go/dig`) with a lifecycle, module system, signal-aware run loop, and structured event logging. For long-running services where boot order, graceful shutdown, and modular composition matter. **Official Resources:** - [pkg.go.dev/go.uber.org/fx](https://pkg.go.dev/go.uber.org/fx) - [uber-go.github.io/fx](https://uber-go.github.io/fx/) - [github.com/uber-go/fx](https://github.com/uber-go/fx) This skill is not exhaustive. Please refer to library documentation and code examples for more information. Context7 can help as a discoverability platform. ```bash go get go.uber.org/fx ``` ## fx vs. dig fx is built on top of dig and shares the same reflection-based 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: | 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 fx** for long-running services (HTTP servers, workers, daemons) — lifecycle and signal handling are mandatory there, and modules make large service graphs manageable. **Choose raw dig** when you need wiring without a framework: CLI tools, libraries that expose a container to callers, test harnesses, or embedding DI into an existing app that manages its own lifecycle. See `samber/cc-skills-golang@golang-uber-dig` skill. ## The Application ```go import "go.uber.org/fx" app := fx.New( fx.Provide(NewLogger, NewDatabase, NewServer), fx.Invoke(RegisterRoutes), ) app.Run() // blocks until SIGINT/SIGTERM, then runs OnStop hooks ``` Boot stages: `fx.New` validates types (constructors do not run); `app.Start(ctx)` runs each `fx.Invoke` and fires OnStart hooks in topological order; main blocks on `app.Done()`; `app.Stop(ctx)` fires OnStop hooks in reverse order. Default timeout is **15 seconds** — override with `fx.StartTimeout` / `fx.StopTimeout`. ## Provide and Invoke ```go fx.New( fx.Provide(NewLogger, NewDatabase, NewServer), // lazy fx.Invoke(RegisterRoutes, StartMetricsExporter), // always run during Start ) ``` `fx.Provide` registers constructors; `fx.Invoke` is the trigger — without an Invoke (directly or transitively) referencing a type, its constructor never runs. ## Lifecycle Hooks Inject `fx.Lifecycle` and append hooks. Constructors should return quickly; long-running work belongs in `OnStart`. ```go func NewHTTPServer(lc fx.Lifecycle, log *zap.Logger, cfg *Config) *http.Server { srv := &http.Server{Addr: cfg.Addr} lc.Append(fx.Hook{ OnStart: func(ctx context.Context) error { ln, err := net.Listen("tcp", srv.Addr) if err != nil { return err } go srv.Serve(ln) // blocking work in a goroutine return nil }, OnStop: func(ctx context.Context) error { return srv.Shutdown(ctx) }, }) return srv } ``` Both callbacks receive a context bounded by `StartTimeout`/`StopTimeout` — respect cancellation. **OnStart must return quickly** — spawn a goroutine for blocking work; otherwise startup hangs and dependent hooks never fire. `fx.StartHook` / `fx.StopHook` / `fx.StartStopHook` adapt simpler signatures (no context, no error, or both): ```go lc.Append(fx.StartStopHook(srv.Start, srv.Stop)) // matched pair ``` ## Parameter and Result Objects fx re-exports dig's `dig.In` / `dig.Out` as `fx.In` / `fx.Out`. Use them when a constructor has 4+ dependencies, or when you need `name`/`group`/`optional` tags. ```go type ServerParams struct { fx.In Logger *zap.Logger DB *sql.DB Cache *redis.Client `optional:"true"` Routes []http.Handler `group:"routes"` } func NewServer(p ServerParams) *Server { /* ... */ } ``` ## fx.Annotate `fx.Annotate` wraps a constructor to add tags or interface bindings without a `fx.Out` struct. Prefer it for ergonomic name/group/As bindings: ```go fx.Provide( fx.Annotate(NewPrimaryDB, fx.ResultTags(`name:"primary"`)), fx.Annotate(NewPostgresDB, fx.As(new(Database))), // expose interface fx.Annotate(NewUserHandler, fx.As(new(http.Handler)), fx.ResultTags(`group:"routes"`), ), ) ``` ## Value Groups Many constructors, one consumer slice — typical for routes, health checks, metrics collectors: ```go type RouteResult struct { fx.Out Handler http.Handler `group:"routes"` } type ServerParams struct { fx.In Routes []http.Handler `group:"routes"` } ``` Append `,flatten` (`group:"routes,flatten"`) to unwrap a slice instead of nesting it. Order is **not guaranteed** — provide an explicit ordered slice when sequence matters. ## fx.Module `fx.Module` groups providers, invokes, and decorators under a name. Modules **scope decorators** to themselves and their children — a logger renamed in `fx.Module("db", ...)` only appears renamed for code inside that module. ```go var DatabaseModule = fx.Module("database", fx.Provide(NewConnection, NewUserRepository), fx.Decorate(func(log *zap.Logger) *zap.Logger { return log.Named("db") }), ) func main() { fx.New( fx.Provide(NewConfig, NewLogger), DatabaseModule, HTTPModule, ).Run() } ``` Treat each module as a small library that can be lifted into another app — its public surface is the types it Provides. For `fx.Supply`/`fx.Replace`/`fx.Decorate`, optional deps, custom logging, manual lifecycle, and Quick Reference, see [advanced.md](./references/advanced.md). ## Best Practices 1. Keep `main()` thin — providers, modules, and a single `Run()`. Push real work into modules so each can be tested in isolation. 2. Use lifecycle hooks instead of `init()` or goroutines launched from constructors — Start/Stop ordering depends on graph topology, but `init()` goroutines do not, which leads to races and leaks. 3. OnStart must return promptly — long work goes in a goroutine inside the hook. A blocking OnStart hangs the rest of the boot. 4. Respect `ctx.Done()` in hooks — a hook that ignores cancellation is reported as a timeout failure but its goroutine continues, leaking resources. 5. Group by module, not by layer — a module owns the providers, lifecycle, and decorators for one concern (HTTP, DB, metrics). 6. Use `fx.Annotate` for tags rather than wrapping a constructor in an `fx.Out` struct — keeps the constructor reusable outside fx. 7. Replace `fx.Provide` with `fx.Supply` for pre-built values (config, command-line flags). Shorter, signals intent. 8. Validate the graph in CI by booting under `fx.New(...).Err()` — catches missing providers and cycles before deploy. ## Common Mistakes | Mistake | Fix | | --- | --- | | Long-running work directly in OnStart | Spawn a goroutine inside OnStart; the hook itself must return quickly so dependent hooks can run. | | `fx.Provide` something that should be `fx.Supply` | Pre-built values (config, secrets) belong in `fx.Supply` — clearer and avoids a no-op constructor. | | Module decorator leaking to siblings | Decorate inside `fx.Module(...)` — decorators flow only to descendants. A top-level `fx.Decorate` is global. | | Group order assumed | Groups are unordered. If order matters, provide an ordered slice from one constructor. | | Constructors with side effects | Side effects belong in OnStart — constructors should be cheap and pure-ish, since they may run concurrently and lazily. | | Forgotten `fx.Invoke` | Without an Invoke (or downstream consumer), constructors never run. Add at least one Invoke per app. | ## Testing Use `go.uber.org/fx/fxtest` to integrate fx with `*testing.T` (failures call `t.Fatal`, `RequireStop` registers as `t.Cleanup`). `fx.Populate(&target)` pulls values out of the graph; `fx.Replace` swaps real dependencies for fakes. Full patterns in [testing.md](./references/testing.md). ## Further Reading - [advanced.md](./references/advanced.md) — Supply/Replace/Decorate, optional deps, custom event logging, manual lifecycle, full Quick Reference - [recipes.md](./references/recipes.md) — full HTTP service with database/metrics, background workers with graceful drain, multiple impls of the same interface, manual lifecycle for CLI embedding - [testing.md](./references/testing.md) — fxtest patterns, `fx.Replace`, `fx.Populate`, isolated lifecycle tests, CI graph validation ## Cross-References - → See `samber/cc-skills-golang@golang-uber-dig` skill for the underlying container, `dig.In`/`dig.Out`, and DI without lifecycle - → 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-context` skill for context propagation in OnStart/OnStop hooks - → See `samber/cc-skills-golang@golang-testing` skill for general testing patterns If you encounter a bug or unexpected behavior in uber-go/fx, open an issue at <https://github.com/uber-go/fx/issues>.


Security scan verdicts — ClawScan: clean | static analysis: clean | VirusTotal: clean | SkillSpector: clean (LOW)