I think this might actually be fine since `@@sse_clients` is `Hash` that is only ever written into during an app boot, am I right @dahakawang? However why use a global hash at all? Why not use a plain ivar, by changing `self.extended` (and other methods) like this:

```ruby
def self.extended(base)
  base.class_exec do
    # the entry point for a Controller level SSE source
    def sse_source
      start_serve self.class.client_list
    end

    @sse_clients = []
  end
end
```