Write documentation for the cache stores.

1 files changed, 14 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/cache/memory_store.rb b/activesupport/lib/active_support/cache/memory_store.rb
index c1a713b4c5..1b30d49155 100644
--- a/activesupport/lib/active_support/cache/memory_store.rb
+++ b/activesupport/lib/active_support/cache/memory_store.rb
@@ -1,5 +1,19 @@
 module ActiveSupport
   module Cache
+    # A cache store implementation which stores everything into memory in the
+    # same process. If you're running multiple Ruby on Rails server processes
+    # (which is the case if you're using mongrel_cluster or Phusion Passenger),
+    # then this means that your Rails server process instances won't be able
+    # to share cache data with each other. If your application never performs
+    # manual cache item expiry (e.g. when you're using generational cache keys),
+    # then using MemoryStore is ok. Otherwise, consider carefully whether you
+    # should be using this cache store.
+    #
+    # MemoryStore is not only able to store strings, but also arbitrary Ruby
+    # objects.
+    #
+    # MemoryStore is not thread-safe. Use SynchronizedMemoryStore instead
+    # if you need thread-safety.
     class MemoryStore < Store
       def initialize
         @data = {}