make HASH_TYPE configurable in carbon.conf

conf/carbon.conf.example

@@ -184,6 +184,11 @@ PICKLE_RECEIVER_PORT = 2014
 # RELAY_METHOD = consistent-hashing
 RELAY_METHOD = rules
 
+# If using consistent-hashing, you can choose your hashring function
+# md5 is the default function. crc32 uses less CPU, but will have
+# choose different destinations for a given metric
+# HASH_TYPE = md5
+
 # If you use consistent-hashing you may want to add redundancy
 # of your data by replicating every datapoint to more than
 # one machine.
@@ -243,7 +248,7 @@ PICKLE_RECEIVER_PORT = 2024
 # use multiple carbon-cache instances then it would look like this:
 #
 # DESTINATIONS = 127.0.0.1:2004:a, 127.0.0.1:2104:b
-# 
+#
 # The format is comma-delimited IP:PORT:INSTANCE where the :INSTANCE part is
 # optional and refers to the "None" instance if omitted.
 #

lib/carbon/conf.py

@@ -60,6 +60,7 @@
   MANHOLE_USER="",
   MANHOLE_PUBLIC_KEY="",
   RELAY_METHOD='rules',
+  HASH_TYPE='md5',
   REPLICATION_FACTOR=1,
   DESTINATIONS=[],
   USE_FLOW_CONTROL=True,
@@ -342,7 +343,6 @@ class CarbonAggregatorOptions(CarbonCacheOptions):
     optParameters = [
         ["rules", "", None, "Use the given aggregation rules file."],
         ["rewrite-rules", "", None, "Use the given rewrite rules file."],
-        ["hash-type", "", None, "Type of hashing to use for consistent hashing"],
         ] + CarbonCacheOptions.optParameters
 
     def postOptions(self):
@@ -362,7 +362,6 @@ class CarbonRelayOptions(CarbonCacheOptions):
     optParameters = [
         ["rules", "", None, "Use the given relay rules file."],
         ["aggregation-rules", "", None, "Use the given aggregation rules file."],
-        ["hash-type", "", None, "Type of hashing to use for consistent hashing"],
         ] + CarbonCacheOptions.optParameters
 
     def postOptions(self):
@@ -371,9 +370,10 @@ def postOptions(self):
             self["rules"] = join(settings["CONF_DIR"], "relay-rules.conf")
         settings["relay-rules"] = self["rules"]
 
-        if self["hash-type"] is None:
-            self["hash-type"] = "md5"
-        settings["hash-type"] = self["hash-type"]
+        if settings["HASH_TYPE"] not in ("md5", "crc32"):
+          print ("In carbon.conf, HASH_TYPE must be either 'md5'  'crc32'.  Invalid value: '%s'" %
+            settings.HASH_TYPE)
+          sys.exit(1)
 
         if self["aggregation-rules"] is None:
           self["aggregation-rules"] = join(settings["CONF_DIR"], "aggregation-rules.conf")

lib/carbon/service.py

@@ -177,7 +177,7 @@ def createRelayService(config):
     root_service = createBaseService(config)
 
     # Configure application components
-    hash_type = settings["hash-type"]
+    hash_type = settings.HASH_TYPE
     if settings.RELAY_METHOD == 'rules':
       router = RelayRulesRouter(settings["relay-rules"])
     elif settings.RELAY_METHOD == 'consistent-hashing':