Techniques for Web Telerobotics - Australia's Telerobot on the Web ...

32 The CGI System
and one where a checkbox was present but not checked. Images behave in a similar way; only if
the image is clicked is it present in the values sent with the request. Images act as both an input
and submit element. Once an image is clicked the mouse location is encoded as two seperate name
value pairs and the <strong>for</strong>m is sent. This means that it is not possible to click two images in a <strong>for</strong>m,
as the <strong>for</strong>m is automatically sent after the first one.
3.2.3 State Management
State management using CGI scripts is awkward, due to the stateless request response nature of
HTTP, and the lack of persistence of CGI process. Each request from a browser is serviced by a
new CGI process. For a complex system a large portion of the CGI processes work may involve
re-establishing the state of the previous request. The new request must be matched up with the
previous request from the same user. Some piece of data such as a session identifier must persist
across sequential requests from the same user known as a session. This identifier can then be used
to re-establich the previous state from some <strong>for</strong>m of persisitent storage. Alternatively, all state
in<strong>for</strong>mation can pass between the browser and CGI script with every conversation. This is only
practial where the amount of state in<strong>for</strong>mation is small, is not shared with other users, and does
not change between requests. Some of the more popular techniques <strong>for</strong> identifying sessions are as
follows:
• Remote Address
• URL
• Hidden field in a HTTP <strong>for</strong>m
• Cookies
Using remote address is included because it is often implemented as a first attempt at session
management. However, it is not reliable, as multiple users may have the same IP address, due to
proxies or shared workstations. Some people also seem to have dynamic IP addresses that change
from request to request.
Passing in<strong>for</strong>mation as part of the URL is a widely used solution that works quite reliably. This
can be done using the query string, as used by search engines, or by adding extra path in<strong>for</strong>mation
after the script name. However this can cause problems as URLs become very confusing and
might refer to old session identifiers.
A third technique of use when all pages are navigated using HTML <strong>for</strong>ms is to use a hidden field.
This will be included in the variables passed to the CGI script. This works very reliably provided