How Does VistA HL7 Determine which Code to execute?

On Sun, Jul 8, 2018 at 2:14 PM, Steve  wrote: When an HL7 messages comes into VistA, how does it determine which application will decode/execute it?

Answer: When an HL7 message comes into VistA, the MSH segment names the kind of message it is. The MSH segment will look something like this:

MSH

This is used to lookup an entry in the PROTOCOL File#101 which describes itself as:

This file contains the orderables and methods for accomplishing orders (protocols) within OE/RR.

The fields SENDING APPLICATION(101,770.1), TRANSACTION MESSAGE TYPE (101,770.3), EVENT TYPE(101,770.4) and VERSION ID(101,770.95) fields in the PROTOCOL are all used with the "AHL1" and "AHL21" cross-references to find the entry  in the PROTOCOL File that handles that particular message.

For example, let's assume you have an HL7 message. The first line will be an MSH segment: