After some posts on how to use nHapi, I thought the time is right for some explanation on what HL7 and nHapi are.

To begin with HL7. HL7 is originally designed to communicate between health care organizations. The acronym stands for Health Care Level 7. Health Care for the type of information that it is designed for and Level 7 for the ISO layer to designate the type of protocol (think of protocols like HTML and FTP). The protocol is able to contain data about every aspect of health care, like patient data, location data and test requests and results. Hl7 is an international standard, but many countries have their own local flavor with additions on the international standard. The communication is through TCP/IP. To separate HL7 packages MLLP (Minimalistic Lower Level Protocol) is used. Information systems used in health care organizations are mostly monolithic type systems. This is why HL7 is more and more used within a single organization instead of between organizations, this is because the application landscapes become more and more service oriented.

HL7 has two types of integration. Unsolicited and query. Unsolicited is a publish/subscribe pattern of communication. Most of the time there is some kind of communications hub where software is registered as 'interested' in a type of communication stream, for example ADT (patient registration data). The receiving software will send an acknowledge to tell the hub the communication is received in good order, or an error has occurred. The information system is the authoritative source of information, which is synchronized to the registered systems.
The other pattern is a request reply pattern. It starts with a query to the information system, for example to request patient information. The information system will send a reply with the requested information, or an error message.

Now, what is this nHapi. nHapi is the .Net port of a java project called Hapi. It's an implementation of the HL7 protocol and enables you to interpret and generate HL7 messages. nHapi gives you a parser to parse HL7 messages to a usable object model and a terser to go the other way. It supports all, if not most, versions of HL7. In other posts I showed some examples on how to use nHapi to create an application to support HL7.

I hope this will give you some context on HL7 and nHapi. If you have any questions, please let me know!