Web basics: a simple HTTP Server in Ruby - DEV Community

Cover image for Web basics: a simple HTTP Server in Ruby  In the previous article, we learned the very basics of networking and how to write a simple TCP server in Ruby. Here, we'll see a simple implementation of an HTTP server in Ruby. ## Dynamic response As of our example, the TCP server responds with a static message, Hey, client!. What if we wanted a dynamic response, for instance: - client A requests, server responds "Hey, A!" - client B requests, server responds "Hey, B!" ...and so on. Let's change our test: tcp_test.rb ```plain text require 'socket' require 'test/unit' class ServerTest< Test::Unit::TestCase def test_client_a server = TCPSocket.open('localhost', 4242) request = 'HELLO from client=A' server.puts(request) response = server.gets assert_equal "Hey, A!\n", response server.close end end ``` And, as for the server implementation, we'll change a little bit to either: - loop forever. then for each iteration, waits for a client connection and executes the request-response logic - look up for a match in the request. If request matches our lookup pattern (regex), server responds accordingly tcp_server.rb ```plain text require 'socket' socket = TCPServer.new(4242) loop do client = socket.accept request = client.gets if result = request.match(/^HELLO from client=(.*?)$/) client_id = result[1] response = "HEY, #{client_id}!" client.puts(response) end client.close end ``` - server accepts a client connection - server checks if request message matches the regex - in case of match, server responds to the client Tests passing, time to move on. ## Upgrading to HTTP HTTP is an application layer and protocol used to establish a set of rules for Web applications. HTTP is, essentially, the foundation of Web. In order to make our client-server to support HTTP, we should request and respond messages using the HTTP format. ### HTTP Request The format of an HTTP request message should be as follows: ```plain text {verb} {path} HTTP/1.1\r\n {request_header_1}\r\n {request_header_2}\r\n {request_header_n}\r\n \r\n {request_body} ``` Notes: - request headers and request body are optional - carriage return (\r\n) are mandatory Here's the example of an HTTP request message without headers: With headers: With headers and body: ```plain text POST /customers HTTP/1.1 Content-Type: application/x-www-form-urlencoded Content-Length: 38 email=test@acme.com&password=pass123 ``` The format of an HTTP response message isn't much different from the request ```plain text HTTP/1.1 {status}\r\n {response_header_1}\r\n {response_header_2}\r\n {response_header_n}\r\n \r\n {response_body} ``` Notes: - response headers and response body are optional - carriage return (\r\n) are mandatory Example of an HTTP response message without headers: With headers: ```plain text HTTP/1.1 201 Created Location: http://myhost.com/customers/42 ``` With headers and body: ### Testing Let's write a test to send an HTTP request message through the TCP socket at the port 80. It could be on any other port, e.g 4242, but let's use port 80 which is the standard for HTTP applications. http_server_test.rb ```plain text require 'socket' require 'test/unit' class ServerTest < Test::Unit::TestCase def test_http_customer_42 server = TCPSocket.open('localhost', 80) request = """ GET /customers/42 HTTP/1.1\r\n Accept: plain/text\r\n \r\n """ server.puts(request) response = '' while line = server.gets response += line end expected = """ HTTP/1.1 200\r\n \r\n Hey, 42!\n """ assert_equal expected, response server.close end end ``` Then, the server implementation as follows: http_server.rb ```plain text require 'socket' socket = TCPServer.new(80) loop do client = socket.accept first_line = client.gets verb, path, _ = first_line.split if verb == 'GET' if result = path.match(/^\/customers\/(.*?)$/) client_id = result[1] response = "HTTP/1.1 200\r\n\r\nHey, #{client_id}!" client.puts(response) end end client.close end socket.close ``` Test should pass! ### HTTP is stateless by default It's strictly important for us, as developers, to understand one of the main traits of a web server, its stateless nature. Such knowledge will save many hours of debugging in the future. Here are the flow of an HTTP server: - accepts a new client connection - reads the client request - performs some logic based on the client request - sends a response to the client - closes the client connection - repeat the process Note that this loop is important, since the server will forget anything that has been exchanged with a specific client. HTTP is stateless by default. There are ways to make a persistent stateful HTTP connection, but that's a topic for another blogpost. ## HTTP Clients Until now we've been using TDD, so our test plays the role of the "HTTP client". However there are lots of HTTP clients available. The Web browser is one of them. ### Web browser Start the server, open your web browser and visit http://localhost/customers/42. The webpage should display the message in just plain text: Also, check the developer console network of your web browser. You should see something similar to: Alt Text  I'm using Safari in this example. More HTTP clients: ```plain text ubuntu:~ $ curl -v http://localhost/customers/42 * Trying 127.0.0.1:80... * TCP_NODELAY set * Connected to localhost (127.0.0.1) port 80 (#0) > GET /customers/42 HTTP/1.1 > Host: localhost > User-Agent: curl/7.68.0 > Accept: */* > * Mark bundle as not supporting multiuse < HTTP/1.1 200 * no chunk, no close, no size. Assume close to signal end < Hey, 42! * Closing connection 0 ``` Alt Text  ## Summary In this blogpost we've seen what's HTTP and how to implement a simple HTTP server in Ruby. Next post will focus on HTML, CSS and Javascript, which altogether shape the World Wide Web.