CL-HTTP Guided Tour of the Common Lisp Hypermedia Server


The server supports a full range of features.

It can serve images like:

Yosemite Valley.

It can serve audio like Sound Bite.

It can serve video like Video Bite.

You can compute and generate HTML dynamically using computed URLs and the HTML generation tools, e.g., View Headers.

It can export whole directories of text, html or multimedia at single directory level or recursively. See CL-HTTP Distribution or standards related to the World Wide Web. Directory exports can be eager or lazy (the default) as needed for start-up time requirements and DNS speed.

Standard abstractions can be adapted to compute pages of interest, such as Basic Icons.

Common Lisp source code directories are also conveniently exported:

  1. Configuration Examples
  2. Portable Server Sources
  3. Portable Client Sources
  4. Portable Proxy Sources
  5. Platform-specific Source

The object-oriented architecture spans URLs, exceptional conditions, server instances, as well as authentication objects. See basic class structure.

Fill-out Forms and search URLs provide a convenient way to accept user input to functions that compute responses to requests. For example, response functions defined in http:examples;exports.lisp allow you to:

Dynamic computation of fill-out forms allows the queries asked to vary according to the data driving an application such as Computed Form.

Input types provide a high-level abstraction for eliciting user input using the full power of the HTML 2.0 fill-out form facility. The Input Type System is a basic presentation system that has is upward compatible with the Common Lisp Interface Manager in the Lisp Machine version. See HTML:ACCEPT-INPUT.

A modular and extensible logging facility allows Server Log Files to be maintained in any of several formats, including common logfile format. The variable http:*log-access-log-class* controls the format used. It possible to have multiple logs on a single port or different log formats on different ports. The log window example illustrates an interesting extension of logging. .

IP Subnet security is available globally for the entire server and locally for specific URLS. See HTTP:EXPORT-URL.

Usernames and passwords are available for user authentication, using both the basic and digest authentication methods. View Realms shows the defined realms, users, groups, URL access groups, but you must know the Webmaster password for the server and your browser must support Digest Authentication. See http:examples;access-control.lisp.

Conditional GET is implement so that caching proxy servers can merely check the headers of a document to see if it has changed rather than reloading it when its expiration time arrives. User control is available via the expiration arguments to HTTP:EXPORT-URL.

The server can export image maps and supports both the NCSA and CERN file formats.

Direct image searches allow a response function to invoke functionality based on the coordinates of a mouse click. See: HTML:IMAGE and try clicking on:

Netscape 1.1 extensions to HTML 2.0 implement Server Push and Client Pull (see discussion). See the implmenetation in http:server;netscape-1-1.lisp. Server push is illustrated for Netscape and Mozilla clients by Choose Color Schemes. The example also demostrates HTML 3.2+ tables, and multipart MIME generation (see http:examples;exports.lisp).

Clients from HTML 3.2 (and Netscape 2.0) include client-side images and frames. CL-HTTP writes client-side image maps from server-side image maps with html:with-client-image-map. Direct control is available with html:with-client-image-map. Either way, the name client-side image maps can be invoked with html:image. Lisp examples that generate client-side image maps and frames are available in http:examples;exports.lisp. For further information, see the full set of HTML operators.

HTML that invokes Java Applets can be synthesized with html:with-java-applet, and http:export-url knows how to export both Java source and byte code. The TwistDown Tree Example illustrates how Lisp can generate dynamic invocations of predefined Java classes.

JavaScript scripts can be embedded in HTML streams for HTML 3.2+ clients using html:with-script (also with ns2.0:with-embedded-script for Netscape 2.0+ Clients. CL-HTTP encapsulates scripts with html:define-script. Scripts can then be called by passing events constructed with HTML:with-event-handlers. The marquee headers page illustrates the use of these facilities. See http:examples;exports.lisp for example code.

A plug-in facility (Latest Plug-ins) is supported by the Netscape 2.0 Client. New export types and mime types are easily defined to allow CL-HTTP to serve data from files or computed on the fly for client-side plug-ins. One example is inline speech synthesis using the Talker plug-in for the Macintosh. See the example in http:examples;exports.lisp for example code.

VRML 1.0 Synthesis Tools generate 3D scenes with the Virtual Reality Modeling Language. VRML scenes can be viewed inline with a variety of plug-ins available for major browsers. The Introduction to VRML Generation works through lisp examples in http:examples;vrml;.

The W3P Presentation System provides a portable and fast subset of functionality in The Common Lisp Interface Manager, 2.0. Now, all CL-HTTP platforms can use presentation types for presenting and accepting data via HTML fill-out forms, and thus, minimize code duplication while simplifying the coding associated with Web interfaces. The Color Mixer in http:examples;exports.lisp illustrates the use of presentation types.

Web-based server configuration allows binary images containing the server to be configured by non-programmers. To use this, you need to set security policy for http:export-web-configuration-interface, which is called near the top of http:examples;exports.lisp.

The W4 Constraint-Guided Web Walker provides a declarative architecture for control and action in a Web robot. After loading the system (see http:w4;), you can run the example. Sunil Mishra has developed a HTML streamed parser for use with the web walker. The current release of W4 is considered beta test, so please report any bugs, successes, or issues to bug-w4@nospam.cl-http.org.

The CL-HTTP Mail HyperArchive exports email message archives for public or personal use just as easily as exporting any file. For an example, see the hyperarchives for WWW-CL@csail.mit.edu. Not only does the hyperarchive include conversation threading and cross references, but it also supports color schemes to make the synthesized pages conform to the look and feel of a site. The hyperarchive dynamically computes pages directly from a message archive created by an SMTP mailer. Because there is no batch processing like more primitive tools, Web pages are always current. Naturally, implementation using CLOS multimethods makes the facility easily extensible through method and class specialization. When you load the hyperarchive facility from http:examples;mail-archive.lisp, it adds the new export type :mail-archive to http:export-url. (This is a good example of how to define complex export types and use direct file access for database purposes.) Full-text search over messages is available by loading LambdaVista.

Virtual Hosts are supported on a single IP address. Separate URL's are exported for virtual host and port. HTTP 1.1 conformant clients are required, e.g. Netscape 2.0.2 or higher.

PUT & DELETE Methods for HTTP 1.0 and 1.1 can now be used with clients other that the basic HTTP client that comes with the server. For example, Mozilla and Netscape support the PUT method. Setting Up Web Directories explains how to get started. Together with Web directories, generic file copying provided by http:copy-file greatly enhances Common Lisp.

Distributed AI applications are possible using HTTP to communicate between a CL-HTTP server and client. The standard HTTP POST method is trivially used for communication between remote Lisp processes. A simple example is available in http:examples;client.lisp.

Client-side cookies allow an application to maintain user state across HTTP transactions or user sessions.

Netscape 4.0 provides a sophisticated layering capability in the browser, which is accessible via ns4.0:with-layer. A client-side form illustrates simple use of layers and JavaScript. See the source code in http:examples;exports.lisp.

Site Indexing & Information Retrieval is provided by LambdaVista, a hybrid classification and retrieval system. It provides full-text retrieval for CL-HTTP sites, including email HyperArchives.

The HTML 4.0 generation facility provides the latest standardize markup capabilities from The Web Consortium, including Cascading Style Sheets for control of document look and feel. HTML 4.0 generation, like all tools for generating earlier HTML version, is backward compatible when using the transitional DTD.

A reference manual generation framework named LispDoc, illustrates use of HTML 4.0 and CCS 2 generation under program control.

An XHTML 1.0 generation facility enables applications to generate pure XML user interfaces.
John C. Mallery -- jcma@nospam.csail.mit.edu
M.I.T. Computer Science & Artificial Intelligence Laboratory