iRule help - Routing traffic

It'd be something like this:

when HTTP_REQUEST { 
    if { rand() > 0.25 } { 
        pool pool_1
    } else { 
        pool pool_2
    }
}

Please note that unless you use connection multiplexing this rule would establish a new connection server-side every fourth request (more or less). If I were you I'd change the event to CLIENT_ACCEPTED because:

1. Then the connections would not have to be re-established saving server performance.
2. The iRule is only executed once per connection instead of once per request saving LTM performance.

Good luck!

/Patrik