Support server node weights.

Support setting a weighting value for each node that will determine
the proportion of load balancing requests that it receives in relation
to the other defined servers and their weights.

Also adds a file describing the worker JSON message versioning.

Fixes bug 1117349

Change-Id: Ie1460c165f03b0523ee28d864b74ea9e5858b256

libra/worker/controller.py

@@ -75,7 +75,7 @@ class LBaaSController(object):
         version this worker supports.
         """
         # Version of the JSON message format that this worker understands.
-        msg_fmt_version = "1.0"
+        msg_fmt_version = "1.1"
         self.msg['version'] = msg_fmt_version
         self.msg[self.RESPONSE_FIELD] = self.RESPONSE_SUCCESS
         return self.msg

libra/worker/drivers/base.py

@@ -48,7 +48,7 @@ class LoadBalancerDriver(object):
         """ Add a supported protocol and listening port for the instance. """
         raise NotImplementedError()
 
-    def add_server(self, protocol, host, port):
+    def add_server(self, protocol, host, port, weight):
         """ Add a server for the protocol for which we will proxy. """
         raise NotImplementedError()
 

libra/worker/drivers/haproxy/driver.py

@@ -92,9 +92,9 @@ class HAProxyDriver(LoadBalancerDriver):
             if proto == 'http':
                 output.append('    cookie SERVERID rewrite')
 
-            for (addr, port) in protocfg['servers']:
-                output.append('    server server%d %s:%s' %
-                              (serv_num, addr, port))
+            for (addr, port, weight) in protocfg['servers']:
+                output.append('    server server%d %s:%s weight %d' %
+                              (serv_num, addr, port, weight))
                 serv_num += 1
 
         return '\n'.join(output) + '\n'
@@ -123,11 +123,20 @@ class HAProxyDriver(LoadBalancerDriver):
         else:
             self._bind(proto, '0.0.0.0', port)
 
-    def add_server(self, protocol, host, port):
+    def add_server(self, protocol, host, port, weight=1):
         proto = protocol.lower()
+
+        try:
+            weight = int(weight)
+        except ValueError:
+            raise Exception("Non-integer 'weight' value: '%s'" % weight)
+
+        if weight > 256:
+            raise Exception("Server 'weight' %d exceeds max of 256" % weight)
+
         if 'servers' not in self._config[proto]:
             self._config[proto]['servers'] = []
-        self._config[proto]['servers'].append((host, port))
+        self._config[proto]['servers'].append((host, port, weight))
 
     def set_algorithm(self, protocol, algo):
         proto = protocol.lower()