Current Web Architecture
Thursday, November 27, 2008
This section of the Internet Tool Survey describes the current architecture of the World Wide Web (WWW). The NCSA Glossary is a useful starting point for Web terms. Another is the ILC glossary of Internet Terms.
The following sections describe
- the basic two-tier architecture of the web in which static web pages (documents) are transferred from information servers to browser clients world-wide,
- extensions that permit three-tiered architectures where content pages can be constructed dynamically and where programs as well as data can be transferred,
- other information transfer protocols, and
- related standards.
The basic web architecture is two-tiered and characterized by a web client that displays information content and a web server that transfers information to the client. This architecture depends on three key standards: HTML for encoding document content, URLs for naming remote information objects in a global namespace, and HTTP for staging the transfer.
- HyperText Markup Language (HTML) - the common representation language for hypertext documents on the Web. HTML had a first public release as HTML 0.0 in 1990, was Internet draft HTML 1.0 in 1993, and HTML 2.0 in 1994. The September 22 1995 draft of the HTML 2.0 specification has been approved as a standard by the IETF Application Area HTML Working Group. HTML 3.0 and Netscape HTML are competing next generations of HTML 2.0. Proposed features in HTML 3.0 include: forms, style sheets, mathematical markup, and text flow around figures. For more detailed information, see the HTML Reference Manual.
HTML is an application of the Standard Generalized Markup Language (SGML ISO-8879), an international standard approved in 1986, which specifies a formal meta-language for defining document markup systems (more here and here). An SGML Document Type Definition (DTD) specifies valid tag names and element attributes. HTML consists of embedded content separated by hierarchical case sensitive start and end tag names which may contain embedded element attributes in the start tag. These attributes may be required, optional, or empty. In addition, documents can be inter or intra linked by establishing source and target anchor points. Many HTML documents are the result of manual authoring or word processing HTML converters, but now several WYSIWYG editors support HTML styles -- see listing at W3C and the Internet Tools Survey section on Authoring HTML.
HTML files are viewed using a WWW client browser (software), the primary user interface to the Web. HTML allows for embedding of images, sounds, video streams, form fields and simple text formatting. References, called hyperlinks, to other objects are embedded using URLs (see below). When an object is selected by a hyperlink, the browser takes an action based on the URL's type, e.g., retrieve a file, connect to another Web site and display a HTML file stored there, or launch an application such as an E-mail or newsgroup reader.
- Universal Resource Identifier (URI) - an IETF addressing protocol for objects in the WWW ("if it's out there, we can point at it"). There are two types of URIs, Universal Resource Names (URN) and the Universal Resource Locators (URL). The current IETF URI spec is here and the URL spec is here.
URLs are location dependent and contain four distinct parts: the protocol type, the machine name, the directory path and the file name. There are several kinds of URLs: file URLs, FTP URLs, Gopher URLs, News URLs, and HTTP URLs. URLs may be relative to a directory or offsets into a document. Arguments to CGI programs (see below) may be embedded in URLs after the ? character.
- HyperText Transfer Protocol (HTTP) - an application-level network protocol for the WWW. Tim Berners-Lee, father of the Web, describes it as a "generic stateless object-oriented protocol." Stateless means neither the client nor the server store information about the state of the other side of an ongoing connection. Statelessness is a scalability property but is not necessarily efficient since HTTP sets up a new connection for each request, which is not desirable for situations requiring sessions or transactions.
- In HTTP, commands (request methods) can be associated with particular types of network objects (files, documents, network services). Commands are provided for
- establishing a TCP/IP connection to a WWW server,
- sending a request to the server (containing a method to be applied to a specific network object identified by the object's identifier, and the HTTP protocol version, followed by information encoded in a header style),
- returning a response from the server to the client (consisting of three parts: a status line, a response header, and response data), and
- closing the connection.
- HTTP supports dynamic data representation through client-server negotiation. The requesting client specifies it can accept certain MIME content types (more on this below) and the server responds with one of these. All WWW clients can handle text/plain and text/html.
- HTTP/1.0 Internet Draft 05 (the seventh release of HTTP/1.0) is targeted as an Internet Informational RFC. The next immediate version of HTTP is HTTP/1.1 Internet Draft 01.
Web Architecture Extensibility
- Common Gateway Interface(CGI) - CGI is a standard for interfacing external programs with Web servers (see Figure 1). The server hands client requests encoded in URLs to the appropriate registered CGI program, which executes and returns results encoded as MIME messages back to the server. CGI's openness avoids the need to extend HTTP. The most common CGI applications handle HTML
- CGI programs are executable programs that run on the Web server. They can be written in any scripting language (interpreted) or programming language (must be compiled first) available to be executed on a Web server, including C, C++, Fortran, PERL, TCL, Unix shells, Visual Basic, Applescript, and others. Security precautions typically require that CGI programs be run from a specified directory (e.g, /cgi-bin) under control of the webmaster (Web system administrator), that is, they must be registered with the system.
- Arguments to CGI programs are transmitted from client to server via environment variables encoded in URLs. The CGI program typically returns HTML pages that it constructs on the fly.
- Some problems with CGI are:
- the CGI interface requires the server to execute a program
- the CGI interface does not provide a way to share data and communications resources so if a program must access an external resource, it must open and close that resource. It is difficult to construct transactional interactions using CGI.
- The current version is CGI/1.1. W3C and others are experimenting with next generation object-oriented APIs based on OMG IDL; Netscape provides Netscape Server API (NSAPI) and Progress Software and Microsoft provide Internet Server API (ISAPI).
- Helpers/Plug-ins - When a client browser retrieves a file, it launches an installed helper application or plug-in to process the file based on the file's MIME-type (see below). For example, it may launch a Postscript or Acrobat reader, or MPEG or QuickTime player. A helper application runs external to the browser while a plug-in runs within the browser. For information on how to create new Netscape Navigator plug-ins, see The Plug-in Developer's Guide.
- Common Client Gateway (CCI) - this gateway allows a third-party application to remotely control the Web browser client. Netscape Client APIs 2.0 (NCAPIs) depends on platform specific native methods of interprocess communication (IPC). They plan to support DDE and OLE2 for Windows clients, X properties for UNIX clients, and Apple Events for Macintosh clients.
- Extensions to HTTP. W3C and IETF Application Area HTTP Working Group are working together on current and future versions of HTTP. The HTTP-NG project is assessing two implementation approaches to HTTP "replacements":
- Spero's approach - allows many requests per connection, the requests can be asynchronous and the server can respond in any order, allowing several transfers in parallel. A "session layer" divides the connection into numerous channels. Control messages (GET requests, meta information) are returned in a control channel; each object is returned in its own channel.
- W3C approach - Jim Gettys at W3C is using Xerox ILU (a CORBA variant) to implement an ILU transport similar to Spero's session protocol. The advantages of this approach are openness with respect to pluggable transport protocols, support for multiple language environments, and a step towards viewing the "web of objects." Related to this approach, Netscape recently announced future support for OMG Internet Inter-ORB Protocol (IIOP) standard on both client and server. This will provide a uniform and language neutral object interchange format making it easier to construct distributed object applications.
Posted bySumedh at 11:10 PM