Observability

The generated code imports no tracing or metrics library. It gives you two hooks, and you bring the library. The recipes below use OpenTelemetry's otelhttp.

Side	Hook
server	WithOperationMiddleware, see operation middleware
client	WithHTTPClient, see HTTP client

Operation names #

Server and client put the name of the operation, GetPet, on the request's context. runtime.OperationID(ctx) reads it. Use it as a span name, a metric label or a log field.

func operationName(_ string, r *http.Request) string {
	return runtime.OperationID(r.Context())
}

Server #

import (
	"go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp"
	"go.opentelemetry.io/otel/attribute"

	"github.com/mockzilla/mockzilla-codegen/pkg/runtime"
)

func labelOperation(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		labeler, _ := otelhttp.LabelerFromContext(r.Context())
		labeler.Add(attribute.String("operation", runtime.OperationID(r.Context())))
		next.ServeHTTP(w, r)
	})
}

tracing := otelhttp.NewMiddleware("", otelhttp.WithSpanNameFormatter(operationName))
router := NewRouter(svc, WithOperationMiddleware(tracing, labelOperation))

• Each request gets a server span named GetPet.
• The span covers reading the request, the service and writing the response. A 400 for a bad parameter is in it too.
• The service's ctx holds the span, so the spans the service starts are its children.
• labelOperation adds operation="GetPet" to otelhttp's request metrics, such as http.server.request.duration. Leave it out if you only want spans.

otelhttp.NewMiddleware also fits WithMiddleware. There it runs before the route is picked, so its spans cannot be named after the operation.

Client #

httpClient := &http.Client{
	Transport: otelhttp.NewTransport(http.DefaultTransport, otelhttp.WithSpanNameFormatter(operationName)),
}
c, err := NewPetClient(baseURL, WithHTTPClient(httpClient))

Each call gets a client span named GetPet.

One trace across services #

Set a propagator once, in the client's program and in the server's:

otel.SetTextMapPropagator(propagation.TraceContext{})

The client then sends a traceparent header, and the server span joins the caller's trace.