strategy/pattern to prevent lost update / race conditions on GAE / memcache

Question

Example:

1. Process A - gets the latest version of an entity from memcache
2. Process B - gets the latest version of an entity from memcache
3. Process A - makes some update to the entity retrieved in (1) and then puts in db/memcache
4. Process B - makes some update to the entity retrieved in (2) and then puts in db/memcache

Update 3. is lost.

What strategy/pattern can be used to prevent this?

Answer 1

You probably want to use cas and gets commands:

https://code.google.com/p/memcached/wiki/NewCommands#cas

EDIT GAE memcache is actually compatible with memcached. Here's the reference from GAE:

https://developers.google.com/appengine/docs/python/memcache/clientclass#Client_cas

and final note:

https://developers.google.com/appengine/docs/python/memcache/clientclass#Memcached_compatibility