Adding "add" support to the cache API

[crazyal]

The Django system has a "set" function for the cache backend which works with memcached which also has a "set" function.
But memcached also has an "add" function which the Django cache backend doesn't seem to support.

The "add" function becomes important with high traffic sites. If alot of requests are coming to the server and wanting to rebuild the cache, using "set" means each request will effectively try and set the cache which becomes a problem. The "add" function means it will only add the new cache if it hasn't already been added.

[Matt McClanahan <cardinal@…>]

One thing to note about this patch is that it exposes a bug in version 1.36 of the python-memcached client from tummy.com. The add method is missing a required argument that will need to be added if people wish to use add() with that memcached backend. Example patch:

--- /usr/src/python-memcached-1.36/build/lib/memcache.py        2007-06-07 03:30:39.000000000 -0700
+++ memcache.py 2007-07-11 03:32:16.000000000 -0700
@@ -365,7 +365,7 @@
             server.mark_dead(msg[1])
             return None
 
-    def add(self, key, val, time=0):
+    def add(self, key, val, time=0, min_compress_len=0):
         '''
         Add new key with value.
 
@@ -374,7 +374,7 @@
         @return: Nonzero on success.
         @rtype: int
         '''
-        return self._set("add", key, val, time)
+        return self._set("add", key, val, time, min_compress_len)
     def replace(self, key, val, time=0, min_compress_len=0):
         '''Replace existing key with value.

[Matt McClanahan <cardinal@…>]

Replying to Matt McClanahan <cardinal@dodds.net>:

One thing to note about this patch is that it exposes a bug in version 1.36 of the python-memcached client from tummy.com. The add method is missing a required argument that will need to be added if people wish to use add() with that memcached backend. Example patch:

This has been fixed in 1.37.

[Malcolm Tredinnick]

(In [6572]) Fixed #4831 -- Added an "add" cache key method, for parity with memcached's
API. This works for all cache backends. Patch from Matt McClanahan.