make metrics from carbon-cache go via relay config

conf/carbon.conf.example

@@ -274,10 +274,27 @@ WHISPER_FALLOCATE_CREATE = True
 
 # By default, carbon itself will log statistics (such as a count,
 # metricsReceived) with the top level prefix of 'carbon' at an interval of 60
-# seconds. Set CARBON_METRIC_INTERVAL to 0 to disable instrumentation
+# seconds. Set CARBON_METRIC_INTERVAL to 0 to disable instrumentation. See
+# also: RELAY_CACHE_METRICS.
 # CARBON_METRIC_PREFIX = carbon
 # CARBON_METRIC_INTERVAL = 60
 
+# By default (RELAY_CACHE_METRICS = False), each cache will keep its internal
+# metrics locally. If you run multiple instances, this is likely to violate
+# your RELAY_METHOD, which means the webapp will query the wrong cache to find
+# recent data, and if the caches don't share storage, it will become impossible
+# to query many metrics at all. In order to get metrics to the right place,
+# instead configure the cache to forward its internal metrics to an external
+# instance (RELAY_CACHE_METRICS = True). RELAY_METHOD, REPLICATION_FACTOR,
+# DESTINATIONS and DESTINATION_PROTOCOL will be used to determine the endpoint
+# for each datapoint -- either make them consistent with a relay, or set up a
+# relay as the sole destination.
+# RELAY_METHOD = constant
+# REPLICATION_FACTOR = 1
+# DESTINATIONS = 127.0.0.1:2014
+# DESTINATION_PROTOCOL = pickle
+RELAY_CACHE_METRICS = False
+
 # Enable AMQP if you want to receive metrics using an amqp broker
 # ENABLE_AMQP = False
 
@@ -611,6 +628,10 @@ FORWARD_ALL = True
 # Note that if the destinations are all carbon-caches then this should
 # exactly match the webapp's CARBONLINK_HOSTS setting in terms of
 # instances listed (order matters!).
+#
+# Also note that if there are multiple DESTINATIONS, it's likely that
+# RELAY_CACHE_METRICS will need to be set on the caches for correct webapp
+# behaviour.
 DESTINATIONS = 127.0.0.1:2004
 
 # If you want to add redundancy to your data by replicating every

lib/carbon/conf.py

@@ -106,6 +106,7 @@
   USE_WHITELIST=False,
   CARBON_METRIC_PREFIX='carbon',
   CARBON_METRIC_INTERVAL=60,
+  RELAY_CACHE_METRICS=False,
   CACHE_WRITE_STRATEGY='sorted',
   WRITE_BACK_FREQUENCY=None,
   MIN_RESET_STAT_FLOW=1000,

lib/carbon/instrumentation.py

@@ -170,7 +170,10 @@ def cache_record(metric, value):
     else:
       fullMetric = '%s.agents.%s-%s.%s' % (prefix, HOSTNAME, settings.instance, metric)
     datapoint = (time.time(), value)
-    cache.MetricCache().store(fullMetric, datapoint)
+    if settings.RELAY_CACHE_METRICS:
+      state.client_manager.sendDatapoint(fullMetric, datapoint)
+    else:
+      cache.MetricCache().store(fullMetric, datapoint)
 
 
 def relay_record(metric, value):

lib/carbon/routers.py

@@ -31,6 +31,29 @@ def getDestinations(self, key):
     raise NotImplementedError()
 
 
+class ConstantRouter(DatapointRouter):
+  plugin_name = 'constant'
+
+  def __init__(self, settings):
+    self.destinations = set()
+
+  def addDestination(self, destination):
+    self.destinations.add(destination)
+
+  def removeDestination(self, destination):
+    self.destinations.discard(destination)
+
+  def hasDestination(self, destination):
+    return destination in self.destinations
+
+  def countDestinations(self):
+    return len(self.destinations)
+
+  def getDestinations(self, key):
+    for destination in self.destinations:
+      yield destination
+
+
 class RelayRulesRouter(DatapointRouter):
   plugin_name = 'rules'
 

lib/carbon/service.py

@@ -87,6 +87,8 @@ def setupPipeline(pipeline, root_service, settings):
       setupRelayProcessor(root_service, settings)
     elif processor == 'write':
       setupWriterProcessor(root_service, settings)
+      if settings.RELAY_CACHE_METRICS:
+        setupRelayProcessor(root_service, settings)
     else:
       raise ValueError("Invalid pipeline processor '%s'" % processor)
 

lib/carbon/tests/test_routers.py

@@ -54,5 +54,5 @@ def testBasic(self):
                 router.addDestination(parseDestination(destination))
             self.assertEqual(
                 len(list(router.getDestinations('foo.bar'))),
-                settings['REPLICATION_FACTOR']
+                len(DESTINATIONS) if plugin == 'constant' else settings['REPLICATION_FACTOR']
             )