The delimiter setting was introduced to make cef decoding possible on byte stream inputs like the TCP input.

Without a delimiter it's impossible to guarantee that each write containing a single cef message won't be concatenated together in the multiple buffers along the way.

Therefore, a configuration like input { tcp { port => 3333 codec => cef } } should throw an error letting the user know that no delimiter has been set, requiring the user to do something like input { tcp { port => 3333 codec => cef { delimiter => "\n" } } }