Retrieving the connection pool happens a lot so we cache it in @class_to_pool. This makes retrieving the connection pool O(1) once the process is warm. When a connection is established or removed, we invalidate the cache.
Ideally we would use #fetch here, as class_to_pool[klass] may sometimes be nil. However, benchmarking (gist.github.com/jonleighton/3552829) showed that #fetch is significantly slower than #. So in the nil case, no caching will take place, but that’s ok since the nil case is not the common one that we wish to optimise for.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 605 def retrieve_connection_pool(klass) class_to_pool[klass.name] ||= begin until pool = pool_for(klass) klass = klass.superclass break unless klass <= Base end class_to_pool[klass.name] = pool end end