ltm rule command LB select

iRule(1)		      BIG-IP TMSH Manual		      iRule(1)



LB::select
       Forces a load balancing selection and returns the result.

SYNOPSIS
DESCRIPTION
       This command forces the system to make a load balancing selection based
       on current conditions, and returns a string in the form of a pool
       command that can be eval'd to activate that selection.

       This enables an iRule to make LB decision early, and use that
       information to manage the connection.

       Two of the most common scenarios are:

       1. To compare the selection that would be made using the load balancing
       criteria to the actual server that should be picked, and force the
       alternative selection before the serverside flow is established with
       the "wrong" server. If you wait to evaluate the load balancing
       selection until after the serverside flow is established (in
       SERVER_CONNECTED or later), then use LB::detach / LB::reselect, client
       data that has already entered the abandoned serverside flow may be
       lost.

       2. To use information about the selected server to influence the
       characteristics of the serverside flow (ie. specific SNAT based on
       selected server, insertion of custom header based on selected server,
       etc. for other conditions or actions - see 2nd example below.)

       Note: The LB::select command will currently only function correctly on
       connections which use a TCP profile. It will not function correctly on
       connections using a UDP, nor on connections which use a FastL4 profile.
       This issue is tracked by ID421708.

       LB::select
	   Forces the system to make a load balancing selection based on load
       balancing criteria and current conditions. Returns a string in the form
       of a pool command that can be evaluated to activate that selection:

	   pool  member  

	   This is the same string that would be returned by the LB__server command when invoked with no parameters. This string can in turn be eval'd to actually make the indicated load balancing selection. (You can use the Tcl eval command to evaluate the return value and actually make the pick, then parse the output from LB__select or use LB__server to get pool name, IP, or port - see examples below.)

       LB::select transparent_port
	   Same as LB::select but port translation will be disabled for this
       pick.

RETURN VALUE
VALID DURING
       ANY_EVENT

EXAMPLES
	To evaluate the selection early, then use it:
	when HTTP_REQUEST {
	    pool fargo
	    set mypick [LB::select]
	    log local0. "The LB choice is: $mypick"
	    # Execute the pick
	    eval $mypick
	}

	To force an early load balancing selection so the details may be used to modify the serverside request attributes:
	when HTTP_REQUEST {
	    log local0. "lbserveraddr: [LB::server addr]"
	    # check if serverside connection had previously been established
	    if { [LB::server addr] eq "" }{
		# if no serverside connection had previously been established, force one
		# so the iRule has the info required to insert a destination-specific header
		eval [LB::select]
	    }
	    switch [LB::server addr] {
		"172.16.5.80" { HTTP::header replace Host www1.customer.com }
		"172.16.5.81" { HTTP::header replace Host www2.customer.com }
		"172.16.5.82" { HTTP::header replace Host www3.customer.com }
	    }
	}

HINTS
SEE ALSO
       Sample Code: 

CHANGE LOG
       @BIGIP-9.2.4 --First introduced the command.



BIG-IP				  2017-01-31			      iRule(1)