Go Echo Server on Flex with Endpoints

app.go

// Copyright 2016 Google Inc. All rights reserved.
// Use of this source code is governed by the Apache 2.0
// license that can be found in the LICENSE file.

// Sample endpoints demonstrates a Cloud Endpoints API.
package main

import (
	"encoding/json"
	"fmt"
	"net/http"

	"github.com/gorilla/mux"

	"google.golang.org/appengine"
)

func main() {
	r := mux.NewRouter()

	r.Path("/echo").Methods("POST").
		HandlerFunc(echoHandler)

	http.Handle("/", r)
	appengine.Main()
}

// echoHandler reads a JSON object from the body, and writes it back out.
func echoHandler(w http.ResponseWriter, r *http.Request) {
	var msg interface{}
	if err := json.NewDecoder(r.Body).Decode(&msg); err != nil {
		if _, ok := err.(*json.SyntaxError); ok {
			errorf(w, http.StatusBadRequest, "Body was not valid JSON: %v", err)
			return
		}
		errorf(w, http.StatusInternalServerError, "Could not get body: %v", err)
		return
	}

	b, err := json.Marshal(msg)
	if err != nil {
		errorf(w, http.StatusInternalServerError, "Could not marshal JSON: %v", err)
		return
	}
	w.Write(b)
}

// errorf writes a swagger-compliant error response.
func errorf(w http.ResponseWriter, code int, format string, a ...interface{}) {
	var out struct {
		Code    int    `json:"code"`
		Message string `json:"message"`
	}

	out.Code = code
	out.Message = fmt.Sprintf(format, a...)

	b, err := json.Marshal(out)
	if err != nil {
		http.Error(w, `{"code": 500, "message": "Could not format JSON for original message."}`, 500)
		return
	}

	http.Error(w, string(b), code)
}

app.yaml

runtime: go
env: flex

beta_settings:
  # Enable Google Cloud Endpoints API management.
  use_endpoints_api_management: true
  # Specify the Swagger API specification.
  endpoints_swagger_spec_file: swagger.yaml

swagger.yaml

swagger: "2.0"
info:
  description: "A simple Google Cloud Endpoints API example."
  title: "Endpoints Example"
  version: "1.0.0"
host: "cloud-minute-endpoints.appspot.com"
basePath: "/"
consumes:
- "application/json"
produces:
- "application/json"
schemes:
- "https"
paths:
  "/echo":
    post:
      description: "Echo back a given message."
      operationId: "echo"
      produces:
      - "application/json"
      responses:
        200:
          description: "Echo"
          schema:
            $ref: "#/definitions/echoMessage"
      parameters:
      - description: "Message to echo"
        in: body
        name: message
        required: true
        schema:
          $ref: "#/definitions/echoMessage"
      security:
      - api_key: []
definitions:
  echoMessage:
    properties:
      message:
        type: "string"