INTERACTIVE WEB SERVICES WITH JAVA

HTTP - HyperText Transfer Protocol

The communication protocol of the WWW:

1991 - the original HTTP protocol (v0.9) - read Tim Berners-Lee's design issues
1996 - HTTP/1.0
1999 - HTTP/1.1

HTTP is stateless - interactions follow a request-response pattern with no protocol support for sessions consisting of multiple interactions between the same client and server.

A HTTP URL (Uniform Resource Locator) identifies a Web resource:
protocol://host:port/path?query
(Example: http://www.google.com/search?q=interactive+web+services)

`protocol://host:port/path?query`

protocol - http, https, ...
host - server name or IP number (e.g. freewig.brics.dk or 130.225.2.179)
port - server local port (default: 80 for http, 443 for https)
path - path on server to file or program (server decides interpretation)
query - arguments to program (program decides interpretation, usually encoded name-value pairs)

A request from a client to a server is a TCP packet. Example:
GET /index.html HTTP/1.0 Accept: text/html, text/plain User-Agent: Mozilla/4.76 Host: www.brics.dk If-Modified-Since: Friday, 01-Mar-02 12:09:31 GMT

GET /index.html HTTP/1.0 Accept: text/html, text/plain User-Agent: Mozilla/4.76 Host: www.brics.dk If-Modified-Since: Friday, 01-Mar-02 12:09:31 GMT

The response from the server to the client has the form:
HTTP/1.1 OK 200 Date: Mon, 08 Apr 2002 13:19:36 GMT Server: Apache/1.3.23 (Unix) mod_bigwig/2.0 mod_perl/1.26 mod_ssl/2.8.7 OpenSSL/0.9.6c Last-Modified: Tue, 05 Mar 2002 09:33:33 GMT Expires: Fri, 05 Apr 2002 09:33:33 GMT Content-Length: 3682 Content-Type: text/html <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html><head><title>BRICS - Basic Research in Computer Science</title></head> <body bgcolor=white> ... </body></html>

HTTP/1.1 OK 200 Date: Mon, 08 Apr 2002 13:19:36 GMT Server: Apache/1.3.23 (Unix) mod_bigwig/2.0 mod_perl/1.26 mod_ssl/2.8.7 OpenSSL/0.9.6c Last-Modified: Tue, 05 Mar 2002 09:33:33 GMT Expires: Fri, 05 Apr 2002 09:33:33 GMT Content-Length: 3682 Content-Type: text/html <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html><head><title>BRICS - Basic Research in Computer Science</title></head> <body bgcolor=white> ... </body></html>

Request methods:

GET: simple request, arguments (if any) are in query - response may be cached
POST: bigger request, arguments follow request header - response should not be cached
HEAD: as GET, but client only wants response header
...

Most common response codes:

Successful 2xx:
- 200 OK - requested resource follows
Redirection 3xx:
- 301 Moved Permanently - resource has moved, update bookmarks accordingly
- 302 Moved Temporarily - resource has moved, but don't update bookmarks
- 304 Not Modified - resource has not been modified since date given by conditional-GET
Client Error 4xx:
- 400 Bad Request - malformed request
- 401 Unauthorized - proper user authentication not provided
- 403 Forbidden - no permission to access resource
- 404 Not Found - the requested resource does not exist
Server Error 5xx:
- 500 Internal Server Error - bug in server
- 501 Not Implemented - required functionality not supported
- 503 Service Unavailable - due to overloading or maintenance

CGI (Common Gateway Interface) is a standard for making HTTP servers start programs to handle requests.

MIME (Multipurpose Internet Mail Extensions) is used to describe message encodings (e.g. Content-Type).