Web Overview

A web server is just a machine connected to the Internet, running a computer program that listens to requests for content, such as HTML files. Those requests come from programs, such as browsers, running on other computers also connected to the Internet. For this to work, there has to be an agreed language for both the requests and what's sent back. That language is the HyperText Transfer Protocol (HTTP).

Rather than repeat what's been written many times, I refer you to

• How the Internet Works for a basic description of many aspects of how the web works.
• HTTP Made Really Easy by James Marshall, which focusses on the basics of HTTP in particular.

Web Servers with Lisp

The nice thing about the web is that all communication between programs is done using HTTP, which uses simple commands in plain text. That simplicity means that you can write a web server or browser in any language you like, as long as it lets you read and write to the network.

Writing a server or browser from scratch would be pretty tedious, though. Fortunately, many languages now have code libraries to take care of such details as reading and writing characters to the network, parsing HTTP commands, creating threads (lightweight parallel processes) for each request to support simultaneous users, and so on.

For Common Lisp there are two major code libraries for web services:

• Franz, Inc.'s AServe library for Allegro Common Lisp for PC's running Windows and Unix.
• John Mallory's CL-HTTP, which runs on many Common Lisp's, including Macintosh Common Lisp, LispWorks, and older versions of Allegro.

In this course, we use Franz, Inc.'s AServe library, for a number of reasons:

• Most students are using Allegro, so they already have AServe available and ready to load.
• AServe is very similar to HTTP libraries for other languages. For example, AServe's responses and entities are much like Java's requests and responses.
• AServe's HTML macro is simple to explain, easy to implement, and extends automatically to new HTML elements and XML.
• AServe has better (or at least more easily found) documentation than CL-HTTP.
• It's a lot easier to implement AServe on top of CL-HTTP than CL-HTTP on top of AServe.

For those students not using Allegro, there is a portable AServe library that runs in a number of Common Lisp's. I use it in LispWorks Personal Edition.

How to Use AServe

Switch to the cs325-user package in your Listener window:

(in-package :cs325-user)

You make URL's available to users with functions like publish, publish-file, and publish-directory. These functions link specific URL's to Lisp functions or HTML files. (This corresponds to servlet mappings in Java web applications.)

To test AServe on your system,

• load the AllegroServe code:
  • Allegro Users: just type (require :aserve)
  • Others: load the file portableaserve/INSTALL.lisp
• load the example AServe code in test-aserve.lisp
• start the server by evaluating (net.aserve:start :port 8000)
• point a browser to http://localhost:8000/

8000 is a port. Ports are simply numbers operating systems use as a kind of address to determine which program should process a network message. Low numbers are pre-assigned to standard services, e.g., 80 is the default for a web server. For development, use a safe high number. In the unlikely case that your machine is already using port 8000 for some service, try some other number over 1024. Use that number in your localhost URL.

If you have a firewall running on your machine, and you should, the first time you start the server, you should get an alert saying that your Lisp is trying to access the network. Tell your firewall this is OK.

The URL http://localhost:8000/ should take you to a page published by the code in test-aserve.lisp. If it doesn't, look at the error message you got.

• If the error says a connection could not be made, then make sure AServe started without error, that you gave the right port number, and that your firewall, if any, isn't rejecting messages on that port.
• If the errors says the page could not be found, then the server is running. Try loading test-aserve.lisp. again and make sure there are no errors.

The home page published by test-aserve.lisp. has links to several examples of things you can do with the AServe package:

• http://localhost:8000/apropos should take you to a page that lets you look up all symbols defined in your Lisp
• http://localhost:8000/base/fields.html you to a simple example of an HTML form that can be filled in and sent to Lisp for processing by Lisp code
• http://localhost:8000/critic should take you to a page where you can submit Lisp code to the Lisp Critic in a web page

These are all very simple examples. Use the code in test-aserve.lisp to guide you when creating other Lisp-based web pages.

While your server is running, any machine on the Internet can access your pages. To access your server from another machine, replace localhost with your server's Internet name or IP address. For example, if your machine's IP address was 555.100.10.100, then you could critique Lisp by typing this URL in your browser:

http://555.100.10.100:8000/cs325/critic

To avoid hackers attacking your system, and to reduce system load, shut down AServe when testing is done:

(net.aserve:shutdown)

For more examples, see:

• the AServe tutorial
• the AServe reference
• Web Programming with AllegroServe, a chapter from Peter Seibel's upcoming Practical Common Lisp

Comments? Send mail to Chris Riesbeck.