[SR-Users] Workings of the kamailio dispatcher module

[Alex Balashov]

On 08/06/2013 09:00 AM, Grant Bagdasarian wrote:
> Hello,
>
> Consider the following Kamailio script:
>
> route {
>
>                  ….
>
>                  route(DISPATCH);
>
>                  route(RELAY);
>
> }
>
> route[DISPATCH] {
>
> ds_select_domain("1", "8");
>
> return;
>
> }
>
> Dispatcher Table
>
> SetID
>
> 	
>
> Destination
>
> 1
>
> 	
>
> 192.168.1.10
>
> 1
>
> 	
>
> 192.168.1.11
>
> Algorithm 8 uses the first destination, but if it’s not available, it
> uses the next one.
>
> Does the ds_select_domain function send the SIP request to the first
> destination or is it the responsibility of the RELAY route to do this? I
> believe it’s the latter, according to the documentation.

All the ds_select_domain() function does is change the domain part of 
the Request URI ($rd).  It's still up to you to take that modified SIP 
request, perform any additional processing, and relay/forward it as 
desired.  When you do this, t_relay() will consume the domain part of 
the request URI, in the absence of an overriding destination set ($du, 
as would be the case if you used ds_select_dst()).

> When Kamailio can’t reach the first destination, how does it internally
> handle the failover? Because once the SIP request has been relayed to
> the host, the script doesn’t have any instructions anymore.

You must arm a failure_route before t_relay(), and call 
ds_select_domain() again to advance through the gateways in the 
dispatcher set.

Example:

route[DISPATCHER_INITIAL] {
         xlog("L_INFO", "[R-DISPATCHER-INITIAL:$ci] Entering dispatcher 
selection route\n");

         if(!ds_select_domain("1", "4")) {
                 # This should only happen if the route set is empty.

                 sl_send_reply("503", "Out of Gateways");

                 xlog("L_ERR", "[R-DISPATCHER-INITIAL:$ci] !> "
                               "No gateways available!\n");
                 exit;
         }

         xlog("L_INFO", "[R-DISPATCHER-INITIAL:$ci] -> "
                        "Selected gateway: $rd:$rp\n");

         t_on_failure("DISPATCHER_ROLLOVER");

	t_relay() etc blah blah;
}


failure_route[DISPATCHER_ROLLOVER] {
	xlog("L_INFO", "[R-DISPATCHER-ROLLOVER:$ci] "
		       "Relay to dispatcher-selected gateway failed\n");

	if(t_is_expired()) {
		xlog("L_INFO", "[R-DISPATCHER-ROLLOVER:$ci] !> "
			       "Transaction expired - aborting\n");
		exit;
	}

	if(t_is_canceled()) {
		xlog("L_INFO", "[R-DISPATCHER-ROLLOVER:$ci] !> "
			       "Transaction cancelled - aborting\n");
		exit;
	}

	if(!ds_next_domain()) {
		# This should happen when we are out of gateways/have tried
		# the last one in the route set.

		xlog("L_ERR", "[R-DISPATCHER-ROLLOVER:$ci] !> "
			      "No more gateways in route set\n");

		t_reply("503", "Out of gateways");
		exit;
	}

	xlog("L_INFO", "[R-DISPATCHER-ROLLOVER:$ci] -> "
		       "Attempting relay to new gateway: $rd:$rp\n");

	t_on_failure("DISPATCHER_ROLLOVER");
	t_relay();
}

-- Alex
-- 
Alex Balashov - Principal
Evariste Systems LLC
235 E Ponce de Leon Ave
Suite 106
Decatur, GA 30030
United States
Tel: +1-678-954-0670
Web: http://www.evaristesys.com/, http://www.alexbalashov.com/