Golang Uber Fx

**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 sh