Developing Web Services with Go

Golang facilitates implementing powerful web applications and services using a very small amount of code. It can be used to implement both HTML rendering webapps as well as XML/JSON rendering web APIs. In this post, I'm going to demonstrate how easy it is to implement a simple JSON-based web service using Go. We are going to implement a simple addition service, that takes two integers as the input, and returns their sum as the output.

package main

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

type addReq struct {
        Arg1,Arg2 int
}

type addResp struct {
        Sum int
}

func addHandler(w http.ResponseWriter, r *http.Request) {
        decoder := json.NewDecoder(r.Body)
        var req addReq
        if err := decoder.Decode(&req); err != nil {
                http.Error(w, err.Error(), http.StatusInternalServerError)
                return
        }
 
        jsonString, err := json.Marshal(addResp{Sum: req.Arg1 + req.Arg2})
        if err != nil {
                http.Error(w, err.Error(), http.StatusInternalServerError)
                return
        }
        w.Header().Set("Content-Type", "application/json")
        w.Write(jsonString)
}

func main() {
        http.HandleFunc("/add", addHandler)
        http.ListenAndServe(":8080", nil)
}

Lets review the code from top to bottom. First we need to import the JSON and HTTP packages into our code. The JSON package provides the functions for parsing and marshaling JSON messages. The HTTP package enables processing HTTP requests. Then we define two data types (addReq and addResp) to represent the incoming JSON request and the outgoing JSON response. Note how addReq contains two integers (Arg1, Arg2) for the two input values, and addResp contains only one integer (Sum) for holding the total.

Next we define what is called a HTTP handler function which implements the logic of our web service. This function simply parses the incoming request, and populates an instance of the addReq struct. Then it creates an instance of the addResp struct, and serializes it into JSON. The resulting JSON string is then written out using the http.ResponseWriter object.

Finally, we have a main function that ties everything together, and starts executing the web service. This main function, simply registers our HTTP handler with the "/add" URL context, and starts an HTTP server on port 8080. This means any requests sent to the "/add" URL will be dispatched to the addHandler function for processing.

That's all there's to it. You may compile and run the program to try it out. Use Curl as follows to send a test request.

curl -v -X POST -d '{"Arg1":5, "Arg2":4}' http://localhost:8080/add

You will get a JSON response back with the total.