SipEngine is a go-based take on the traditional SIP proxy (FreeSwitch, OpenSIPS). As opposed to creating a lexed configuration language, this library can be imported and implemented however you choose.
The engine itself takes a slice of SIPStep functions, which have a signature of
func (message *Message) error. You simply build your steps, put them into a slice
and they will execute against every message that comes in
There are scenarios where we want to abandon processing at an earlier stage. If you
return a MessageTerminationError, the engine will stop further processing. This does
mean, however, that you will still have to put your message onto the egress channel
for processing before returning that message.