I recently booted up my rails c and was greeted with a pleasant warning message:

/Users/Bodah/repos/backupify/backupify/lib/backupify/redis/server.rb:6: warning: already initialized constant Backupify::Redis::Server::REDIS_SHARED_ROLE
/Users/Bodah/repos/backupify/backupify/lib/backupify/redis/server.rb:6: warning: previous definition of REDIS_SHARED_ROLE was here
/Users/Bodah/repos/backupify/backupify/lib/backupify/redis/server.rb:9: warning: already initialized constant Backupify::Redis::Server::MULTI_REDIS_ROLE
/Users/Bodah/repos/backupify/backupify/lib/backupify/redis/server.rb:9: warning: previous definition of MULTI_REDIS_ROLE was here
/Users/Bodah/repos/backupify/backupify/lib/backupify/redis/server.rb:12: warning: already initialized constant Backupify::Redis::Server::DEFAULT_REDIS_CONFIGURATION
/Users/Bodah/repos/backupify/backupify/lib/backupify/redis/server.rb:12: warning: previous definition of DEFAULT_REDIS_CONFIGURATION was here

Well that's a fun error. This type of thing is usually benign, but it's still annoying and not really what you want to happen. What the error is telling us is that we're loading the file lib/backupify/redis/server.rb twice.

Turns out that the culprit was one of our initializers which was calling require 'backupify/redis/server'. We also have eager loading enabled in our development environment. Rails runs the initializers first and then eager loads as documented here.

Rails does some monkey patching shenanigans to require and sets up some non-trivial global state. I'm not entirely sure why it's so complex, but it winds up handling whether or not a piece of code should be loaded with native load or native require. Regardless, it's pretty obvious that calling require on code that will eventually be eager loaded before the eager loading kicks in causes some issues.

If you follow the trail of crumbs for eager loading you'll eventually come to Rails::Engine::eager_load! which calls require_dependency under the hood. This seems to setup the global state such that eager loading knows to not try and call load on the code again.

So just change those require 'backupify/redis/server' statements to require_dependency 'backupify/redis/server' and ahhh... no more warnings.