What is the API Gateway pattern and how does it complement Go microservices?

An API Gateway is a single entry point for all client requests. It handles cross-cutting concerns — auth, rate limiting, routing, SSL termination, response aggregation — so individual services don't need to implement them.

// Lightweight Go API gateway (simplified)
type Gateway struct {
    routes map[string]RouteConfig
    auth   *JWTValidator
    limit  *RateLimiter
    log    *slog.Logger
}

type RouteConfig struct {
    Target      string   // upstream URL
    RequireAuth bool
    RateLimit   int      // requests per second
    Methods     []string // allowed HTTP methods
    StripPrefix string
}

func (g *Gateway) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    route, ok := g.matchRoute(r.URL.Path)
    if !ok {
        http.Error(w, "not found", 404); return
    }

    // Auth (before rate limiting to save rate limit quota for auth users)
    if route.RequireAuth {
        if _, err := g.auth.Validate(r); err != nil {
            http.Error(w, "unauthorized", 401); return
        }
    }

    // Rate limiting
    if !g.limit.Allow(clientIP(r)) {
        http.Error(w, "rate limited", 429); return
    }

    // Proxy to upstream
    proxy := httputil.NewSingleHostReverseProxy(mustParseURL(route.Target))
    if route.StripPrefix != "" {
        r.URL.Path = strings.TrimPrefix(r.URL.Path, route.StripPrefix)
    }
    proxy.ServeHTTP(w, r)
}

// In practice: use Kong, Envoy, or AWS API Gateway
// Go excels at writing custom gateway logic as plugins:
// - Kong plugins in Go (go-pdk)
// - Envoy WASM filters in Go (tetratelabs/proxy-wasm-go-sdk)
// - Custom gateway in Go using httputil.ReverseProxy

BFF (Backend for Frontend) pattern: a specialised API gateway for each type of client — mobile BFF, web BFF. Each BFF aggregates calls to multiple services and returns exactly the data its client needs (graph-like queries without GraphQL's complexity). Often written in Go for performance.

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