iRule that will limit connections

Make sure I have the logic straight first:

  if cookie exists
      load balance to default pool w/persistence
      (session already exists, so don't increment counter)
  if no cookie exists
      if uristring exists
        redirect to "nocookies" page
      if connection limit not yet reached
        load balance to default pool
        increment counter
      else if connection limit already reached
        redirect to "server busy" page

By re-ordering your original conditions a bit, eliminating one unused array, and incrementing a bit differently, we get:

rule session_limit {
when RULE_INIT {
  set ::total_active_clients 0
  set ::max_active_clients 50
}
when HTTP_REQUEST {
; If persistence cookie already exists,
; allow persistent connection without incrementing counter
  if { [HTTP::cookie exists "BIGipServerhalifax"] } {
      pool halifax
      return
; If no cookie, check for URI link parameter
; & redirect if present
  } else {
    if { [HTTP::uri] contains "linkID=halifax" } {
      HTTP::redirect "http://pleasehold.evenue.net/bigip/nocookies.html" 
      return
    }
; Still no cookie
; Check limit vs. active
; Allow in if there's room, and incr counter for this new cnx
    if { $::total_active_clients < $::max_active_clients } {
      incr ::total_active_clients
      pool halifax
      return
; otherwise redirect
    } else {
      HTTP::redirect "http://www.yahoo.com/"
      return
    }
  }
}
when CLIENT_CLOSED {
  incr ::total_active_clients -1
}
}

This will enforce a connection limit of 50 total concurrent users on the VS. If you use "least connections" load balancing, you will get the most even distribution of connections across the server pool. I'm not sure how nicely node connection limits will play with the rule limits, so I'm going to recommend you leave off the node limits until you have the rule logic working the way you want, and then I'll leave the question of node limits open for your experimentation.

HTH

/d