When adding custom host status that is namespaced under HostStatus module it's easy to get loading issues in test and production env if user defines his or her status like this

module HostStatus
  class MyStatus < HostStatus::Status
  end
end

Rails loads engine files first so it thinks that HostStatus is defined in custom status file and it fails to find HostStatus.status_registry.

A solution is to load HostStatus module in core initializer (downside is that it won't be auto reloaded but it shouldn't matter in this case), so all engines can rely on this module being loaded.

A workaround for plugins is to define their statuses like class HostStatus::MyStatus < HostStatus::Status which would enforce the module loading.

To reproduce, either use production/test env or turn on config.cache_classes in config/environments/development.rb, define custom status from an engine and try starting server. Since host statuses were introduced in 1.10, setting this also as target version.