[Rails 3] Failed to get session options?

Question

Hi everybody, I'm trying to switch the session store in Rails 3 by changing the config/application.rb as following:

config/application.rb 
#----------------------------------- 
memcache_options = { 
  :compression => true, 
  :debug => false, 
  :namespace => "xx-cache", 
  :readonly => false, 
  :urlencode => false 
} 
CACHE = MemCache.new(memcache_options) 
CACHE.servers = ['127.0.0.1:17898'] 
#check if CACHE is connected 
#puts CACHE 
config.action_dispatch.session = { 
  :session_key => '_xx_session', 
  :secret => 'xx', 
  :cache => CACHE, 
  :expires => 900 
} 
config.action_dispatch.session_store = :mem_cache_store 
#----------------------------------- 

Memcache server is running. However, when run rails s, i got this message:

=> Booting WEBrick 
=> Rails 3.0.0.beta3 application starting in development on http://0.0.0.0:3000 
=> Call with -d to detach 
=> Ctrl-C to shutdown server 
Exiting 
/usr/lib/ruby/gems/1.9.1/gems/actionpack-3.0.0.beta3/lib/ 
action_dispatch/middleware/session/mem_cache_store.rb:19:in 
`initialize': #<ActionDispatch::Session::MemCacheStore:0xa302950> 
unable to find server during initialization. (RuntimeError)

It seems the session options was not passed correctly. But i'm not sure what's wrong here cause I'm new to Rails.

Any help will be appreciated.

Thanks