Prevent multiple instances of web page

[duncanwill]

I am trying to implement a solution in Javascript that prevents (or more realistically detects) multiple instances of a web application or page and if more than one window exists the latest window(s) redirects the user to a 'Please do not open more than one window for this application' message.

I am thinking about use of cookies to 'monitor' or poll the page, every 'x' seconds - but I am concerned about perceived browser performance (the environment is very restrictive). Better still perhaps I could make use of load and unload events? Does anyone have any ideas or experience of such an odd request?

The task is based on a poor performing application (so it's rotten before I start!) and the solution needs to be robust so that in case of browser crashes or similar the user is not locked out or prevented from opening the application again.

Thanks for any help - sorry the request is a bit vague!

[CloudedVision]

Just use sessions with javascript. Update it every 30 seconds with the timestamp. If the person opens another window and the session is valid (hasn't been more than 30 seconds since refresh) don't show the content.

That shouldn't take up much system resources.

[duncanwill]

Thanks - that pretty much were I was heading (using Cookies rather than sessions) and dabbling with onload and onunload/onbeforeunload events.

Frankly it's a dodgy solution to a rubbish problem - but hey, that's my job!