Wishful Coding

Didn't you ever wish your
computer understood you?

Writing a web server

A colleague asked what would be an interesting exercise to learn more about Perl. I think a HTTP server is a good thing to build because it’s a small project that helps you understand web development a lot better.

This post serves as a broad outline of how a HTTP server works, and as a collection of resources to get started.

There is of course the HTTP specification itself. It’s good for looking up specific things, but otherwise not very easy reading.

HTTP is a relatively simple text-based protocol on top of TCP. It consists of a request and a response, both of which are made up of a status line, a number of headers, a blank line, and the request/response body.

What I recommend doing is playing with a simple working server to see what happens.

Lets create a file and start a simple server.

$ echo 'Hello, world!' > test
$ python -m SimpleHTTPServer

This will serve the current directory at port 8000. We can now use curl to request the file we created. Use the -v flag to see the HTTP request and response.

$ curl -v http://localhost:8000/test
> GET /test HTTP/1.1
> User-Agent: curl/7.30.0
> Host: localhost:8000
> Accept: */*
< HTTP/1.0 200 OK
< Server: SimpleHTTP/0.6 Python/2.7.6
< Date: Wed, 12 Mar 2014 17:51:26 GMT
< Content-type: application/octet-stream
< Content-Length: 14
< Last-Modified: Wed, 12 Mar 2014 17:51:06 GMT
Hello, world!

Take a while to look up all the headers to see what each one does. Explain what happens to a friend, cat or plant.

Now you can in turn take the role of the client or server. Can you get Python to return you the file using netcat?

$ nc localhost 8000
<enter request here>

Now can you get curl to talk to you? Start listening with

$ nc -l 1234

Now in another terminal run

$ curl http://localhost:1234/test

You’ll see the request in the netcat window. Try writing a response. Remember to set Content-Length correctly.

Now it is time to actually write the server in the language of choice. Whichever one you use, it is probably loosely based on the Unix C API. To find out more about that, run

man socket

You’re looking for an PF_INET(IPv4) socket of the SOCK_STREAM(TCP) type. But other types exist.

Be sure to check out the SEE ALSO section for functions for working with the socket.

The basic flow for the web server is as folows.

  1. Create the socket.
  2. bind it to a port.
  3. Start to listen.
  4. accept an incoming connection. (will block)
  5. read the request.
  6. write the response.
  7. close the connection.
  8. Go back to accept.

Note that what you do after accept is subject to much debate. The simple case outlined above will handle only one request at a time. A few other options.

  • Start a new thread to handle the request.
  • Use a queue and a fixed pool of threads or processes to handle the requests. Apache does this.
  • Handle many requests asynchronously with select, epoll(linux) or kqueue(BSD). Node.js does this.

After you have a basic request/response working, there are many things you could explore.

  • Serve static files.
  • Add compression with gzip.
  • Support streaming requests and responses.
  • Run a CGI script.
  • Implement most of HTTP 1.0
  • Implement some HTTP 1.1 parts.
  • Look into pipelining and Keep-Alive.
  • Look into caching.
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