Web Servers

Clients & Servers

A client is an application making a request for external data - typically a browser.

A server is any program listening for requests on a computer port, that then processes these requests and sends responses accordingly.

For web development purposes, both request and response will usually follow the HTTP protocol.

Building a server from scratch

We can use the built-in node http library to build a server.

createServer()

The .createServer method is used to create the actual server itself, and takes a single argument which is a request/response handler - a callback function that is called each time that the server receives a request.

const server = http.createServer((request, response) => {
  console.log('request received');
});

listen()

In order for a server to be able to receive and handle requests, it must be listening for requests. To make an http server listen, we must invoke the server's .listen method with the port number that the server should be listening on.

const server = http.createServer((request, response) => {
  console.log(request);
});
 
server.listen(9090);

In order to get the server listening, the file must be run using node, e.g. node server.js.

The listen method also takes a second argument, which is a callback function that can be used to handle any errors when setting the server up to listen, e.g.:

server.listen(9090, (err) => {
  if (err) console.log(err);
  else console.log('Server listening on port: 9090');
});

Sending a response

Once the server is listening and receives a request from a client, the createServer request/response callback function will be invoked. And using the response parameter, a response can be sent back to the client by setting the appropriate headers (response.setHeader()), status code (response.statusCode =), and body (response.write(), which must be passed a string value):

const server = http.createServer((request, response) => {
  response.setHeader('Content-Type', 'application/json');
  response.statusCode = 200;
  response.write(JSON.stringify({ msg: 'hello world' }));
  response.end();
});
 
server.listen(9090, (err) => {
  if (err) console.log(err);
  else console.log('Server listening on port: 9090');
});

Once the response has been formed, response.end() needs to be invoked in order to send the response, otherwise the browser/insomnia will still be hanging: i.e. the request won't have ended!

Using the method and url

Within the callback function passed to .createServer we can add conditional logic to change what our server does based on different requests. For example changing the logic based on different request methods, url paths and queries.

const server = http.createServer((request, response) => {
  const { method, url } = request;
  if (url === '/' && method === 'GET') {
    response.setHeader('Content-Type', 'application/json');
    response.statusCode = 200;
    response.write(JSON.stringify({ msg: 'Server up and running' }));
    response.end();
  }
  if (url === '/users' && method === 'GET') {
    // logic to retrieve users data and send back to the client on the response body
  }
  if (url === '/users' && method === 'POST') {
    // logic to add the new user from the request body to the data and send the appropriate response
  }
});
 
server.listen(9090, (err) => {
  if (err) console.log(err);
  else console.log('Server listening on port: 9090');
});

Assembling the request body

When dealing with POST/PATCH/PUT requests that include information on the request body, the body of a request must be pieced together from the request object. This is similar to collecting the packets of data that form the body of a response when making requests using https.request.

let body = '';
request.on('data', (packet) => {
  body += packet.toString();
});
request.on('end', () => {
  // logic to handle the body from the request and update the data
});

The Internet & Data Fetching Express 101