Rails Session ArgumentError: key too long [SOLVED]

Problem

I am migrating an app from one version of Ruby to another. In my rollout I decided to first launch one server using the new version of Ruby 1.9.2 and adjust the load balancer to only move over about 10% of the traffic to it. I noticed in newrelic that within 10 minutes there were tons of exceptions like:

ArgumentError: key too long "rt:BAh7Czozg3OWY4MWRkMjAyYjlkNzljNmUwNWYwZDRhMDQ6F
GFiaW5nb19pZGVudGl0eUkiDzEyMTc2ODUyNzIGOgZFRjoKcnRfYWJJIgZiBjsHRjodY2xpY2tfdHJh
Y2tpbmdfZmlyc3RfdXJsSSJAaHR0cDovL3JlYWx0cmF2ZWwuY29tL2ItMjM1MTM4LWxvbmRvbl9ib
G9nLWxvbmRvbjpfYV90b196ZWQGOwdGOihjbGlja190cmFja2luZ19vcmlnaW5hbF9yZWZlcmV
yX3VybCJgaHR0cDovL3d3dy5nb29nbGUuY29tL3NlYXJjaD9zb3VyY2U9aWcmaGw9ZW4mc
mx6PTFHMUdHTFFfRU5VUzQwMCZxPWxvbmRvbithK3RvK3plZCZhcT1mJm9xPToMcmVm
ZXJlckAK--65814e8bacff8432ac09e15520afb13117d0e5cd"
Stack trace (hide Rails)
/path/shared/bundle/ruby/1.8/gems/memcache-client-1.8.5/lib/memcache.rb:703:in `get_server_for_key'
/path/shared/bundle/ruby/1.8/gems/memcache-client-1.8.5/lib/memcache.rb:920:in `request_setup'
/path/shared/bundle/ruby/1.8/gems/memcache-client-1.8.5/lib/memcache.rb:885:in `with_server'
/path/shared/bundle/ruby/1.8/gems/memcache-client-1.8.5/lib/memcache.rb:246:in `get_MemCache_read'
/path/shared/bundle/ruby/1.8/gems/actionpack-2.3.8/lib/action_controller/session/mem_cache_store.rb:31:in `get_session'

The interesting thing was that none of these errors were coming from the new server, but something on the new ruby server was breaking the other, so here is what I found was the problem.

Solution

The new server was configured to use the cookie store and the others were using memcached store. It sounds stupid and it is, but it wasn’t so easy to figure out at first. When I googled for the error I noticed that none of the issues had solutions, so thats why I decided to write this post.

In short, the session[:session_id] with the default cookie store passes both the session data and the session key into the cookie, and the memcached/db store’s only put the session_id from the cookie as session[:session_id].

May your session keys be truly keys!