HTTP Fundamentals

In this lesson, explore http fundamentals through one small Go program, and connect its output to the language rules behind it.

HTTP requests contain a method, URL, headers and sometimes a body. Responses contain a status code, headers and a body. REST APIs commonly use GET to retrieve, POST to create, PUT to replace, PATCH to change and DELETE to remove resources.

Choose status codes that describe the outcome: 200 for success, 201 for created, 400 for invalid input, 404 for missing resources, and 500 for unexpected server failures.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"fmt"
	"net/http"
	"net/http/httptest"
)

func main() {
	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Content-Type", "text/plain")
		w.WriteHeader(http.StatusCreated)
		fmt.Fprint(w, "created")
	})
	rec := httptest.NewRecorder()
	handler.ServeHTTP(rec, httptest.NewRequest(http.MethodPost, "/tasks", nil))
	fmt.Println("Status:", rec.Code, "Body:", rec.Body.String())
}

Output

Status: 201 Body: created

The handler sets a content type before writing status 201 and the body created. The recorder makes the response inspectable.

• Write headers before calling WriteHeader or writing the body.
• Method choice and status codes are part of the API contract.

Change the handler to return 400 Bad Request with an explanation.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced HTTP Fundamentals in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.