Go-Swagger Tricks. Standard HTTP handler.

Hey, reader! Maybe you had tried go-swagger library so far. If yes, you may notice that sometimes it's not so easy to use. And it may look a bit complicated to start using it.

In this number of small articles, I will share my experience on how to make go-swagger more friendly. Let's start.

Handling requests in go-swagger

By default, go-swagger generates a specific handler type for each endpoint in your Swagger scheme - that is, you will get a code-generated structure with all import parameters kindly parsed for you and a number of responders for each response type (with pre-generated structures as well).

So usually it's ok, because you don't have to parse JSON requests by yourself and can never mind the response data marshaling - the go-swagger framework will handle all that stuff for you. But sometimes you just want to use a standard net/http compatible handler with some endpoint - maybe some library provides that for you (like Prometheus metrics endpoint or Kubernetes probes). Or maybe you have some HTTP libraries written in your organization - e.g. for custom authentication and so on.

So you can't do that in vanilla go-swagger - each generated handler function requires an exact handler to be assigned. Let's say we want to expose a Prometheus metrics endpoint, so for the spec like this

paths:
  /metrics:
    get:
      tags:
        - instruments
      summary: "Prometheus metrics"
      produces:
      - "application/json"
      responses:
        200:
          description: ok
          schema:
            $ref: "#/definitions/Any"

the generated handler function will look like that:

func MetricsHandler(p instruments.GetMetricsParams) middleware.Responder {
    // some logic here
    return instruments.NewGetMetricsOK().
        WithPayload(models.Any(
        // response data here
    ))
}

So is it even possible to use standard handlers? Or should I rewrite all of them in go-swagger-way?

Don't worry, there is no need to do extra work.

Adding wrapper

There is a way to do that by adding a tiny wrapper. We just need to find a reader and writer and put them to our http.HandlerFunc function. The reader is always in the generated request structure, but the reader is provided to the generated responder object. So we have to implement a middleware.Responder interface to get it. That may look like this:

type CustomResponder func(http.ResponseWriter, runtime.Producer)

func (c CustomResponder) WriteResponse(w http.ResponseWriter, p runtime.Producer) {
    c(w, p)
}

And now we just need to wrap our standard handler into this wrapper and provide reader and writer as usual!

func MetricsHandler(p instruments.GetMetricsParams) middleware.Responder {
    return CustomResponder(func(w http.ResponseWriter, _ runtime.Producer) {
        promhttp.Handler().ServeHTTP(w, p.HTTPRequest)
    })
}

Improvements

We can go deeper and add constructor function, which will save us a couple of lines in future:

func NewCustomResponder(r *http.Request, h http.Handler) middleware.Responder {
    return CustomResponder(func(w http.ResponseWriter, _ runtime.Producer) {
        h.ServeHTTP(w, r)
    })
}

and simplify our handler a bit:

func MetricsHandler(p instruments.GetMetricsParams) middleware.Responder {
    return NewCustomResponder(p.HTTPRequest, promhttp.Handler())
}

Sure it may look not so impressive as a distributed database internals, but it will save you some time and help to integrate a go-swagger with ease.

See you in the next "Go-Swagger Tricks" episode!