Years ago I was working on a very large Ruby on Rails codebase that used constants to hold lists of credit card transaction states. For example: # example 1 class Txn ACTIONABLE_STATES = [:authenticated, :to_settle] DONE_STATES = [:settled, :declined] # ... end However, we had a bug in which a settled transaction could pass a txn.state.in? ACTIONABLE_STATES check. After hours of searching, we found the offending code in a completely different part of the codebase. # example 2 all_states = ACTIONABLE_STATES.concat(DONE_STATES) all_states.each do |state| # ... end If you’re already knowledgeable of the Ruby Array class, you’ve probably spotted the offending line already. #concat mutates the original list, even though it doesn’t end with the idiomatic !, and because ACTIONABLE_STATES is held statically in memory, when this code gets executed it changes ACTIONABLE_STATES state for the rest of that Ruby virtual machine’s life. In other words, after example 2 gets executed, ACTIONABLE_STATES becomes[:authenticated, :to_settle, :settled, :declined] until you reboot the server. If you have multiple Ruby boxes running behind a Load Balancer (as we did), this can further obfuscate the problem. Because example 2 might only have been run on certain boxes, you might get cases where one request will not exhibit the bug while another identical one will. The solution? Freeze your constants. # example 3 ACTIONABLE_STATES = [:authenticated, :to_settle].freeze DONE_STATES = [:settled, :declined].freeze This would cause example 2 to produce a can't modify frozen array (RuntimeError), and if you’ve unit tested example 2, you would’ve hopefully caught this before it was ever even deployed.
