one of the documented procedures in this installation of the ACS
server_cluster_httpget_from_peers { -timeout 20 } url
What it does:
Schedules an HTTP GET request to be issued immediately to all peer hosts (using ad_schedule_proc -once t -thread t -debug t 0).
Defined in: /web/philip/tcl/ad-server-cluster.tcl

Source code:

arg_parser_for_server_cluster_httpget_from_peers $args

    if { ![string match /* $url] } {
	set url "/$url"
    foreach host [server_cluster_peer_hosts] {
	# Schedule the request. Don't actually issue the request in this thread, since
	# (a) we want to parallelize the requests, and (b) we want this procedure to
	# return immediately.
	# ad_schedule_proc -once t -thread t -debug t 0 server_cluster_do_httpget "http://$host$url" $timeout
	ad_schedule_proc -once t -thread f -debug t 0 server_cluster_do_httpget "http://$host$url" $timeout