Add Mcrouter client class

r2/r2/lib/cache.py

@@ -275,6 +275,61 @@ class CMemcache(CacheUtils):
         return '<%s(%r)>' % (self.__class__.__name__,
                              self.servers)
 
+
+class Mcrouter(CMemcache):
+    """Wrapper class to make mcrouter appear like a regular memcached client.
+
+    Expected behavior (benefits of mcrouter):
+    * get() with a cache unresponsive will return `None` to be interpreted as a
+      cache miss rather than raising MemcachedError.
+    * get_multi() with a cache unresponsive returns only the values that were
+      retrieved.
+
+    Error cases:
+    * set() with a cache unresponsive will raise a ServerError.
+    * set_multi() with a cache unresponsive will raise a ServerError. Some of
+      the writes may have succeeded, which is the same behavior in mcrouter and
+      memcached.
+    * add() same as set()
+    * add_multi() same as set_multi()
+
+    In all cases where mcrouter raises a ServerError memcached would raise a
+    MemcachedError. This behavior is acceptable because ServerError inherits
+    from MemcachedError.
+
+    Special cases:
+    * set() if we are using prefix routing and the key doesn't match any routes
+      mcrouter will return `False`. This is converted to a MemcachedError but
+      it's possibly more correct to depend on the client checking the return
+      value and deciding how to proceed.
+
+    Unhandled cases:
+    * delete() with a cache unresponsive will return `False`, but memcached will
+      raise a MemcachedError. This can't be simply interpreted as the error case
+      because `False` is the correct return when deleting a key that doesn't
+      exist. The caller must check the return value.
+    * delete_multi() with a cache unresponsive will return `False`, but
+      memcached will raise a MemcachedError. Same logic follows as delete().
+    * incr() with a cache unresponsive will raise a NotFound exception, which is
+      the same error as attempting to incr an un-set key.
+    * incr_multi() with a cache unresponsive will raise a NotFound exception,
+      but memcached will raise a MemcachedError. This can't be interpreted as
+      being the error case and replaced with a MemcachedError because NotFound
+      is a valid exception when attempting to incr keys that don't exist.
+
+    """
+
+    def set(self, key, val, time=0):
+        success = CMemcache.set(self, key, val, time)
+
+        if not success:
+            # If we are using prefix routing and the key doesn't match any
+            # routes mcrouter will return `False`.
+            raise MemcachedError("set failed")
+        else:
+            return True
+
+
 class HardCache(CacheUtils):
     backend = None
     permanent = True