add skynet.watch for skynet.call, read http://blog.codingnow.com/2013/12/skynet_monitor.html

service/simplemonitor.lua

@@ -2,13 +2,39 @@ local skynet = require "skynet"
 
 -- It's a simple service exit monitor, you can do something more when a service exit.
 
+local service_map = {}
+
 skynet.register_protocol {
 	name = "client",
 	id = 3,
 	unpack = function() end,
 	dispatch = function(_, address)
+		local w = service_map[address]
+		if w then
+			for watcher in pairs(w) do
+				skynet.send(watcher, "exit", address)
+			end
+			service_map[address] = false
+		end
 		print(string.format("[:%x] exit", address))
 	end
 }
 
-skynet.start(function() end)
+local function monitor(session, watcher, command, service)
+	assert(command, "WATCH")
+	local w = service_map[service]
+	if not w then
+		if w == false then
+			skynet.ret(skynet.pack(false))
+			return
+		end
+		w = {}
+		service_map[service] = w
+	end
+	w[watcher] = true
+	skynet.ret(skynet.pack(true))
+end
+
+skynet.start(function()
+	skynet.dispatch("lua", monitor)
+end)