This is a question I received. So, here's an example. I can't give a lot of code details, but I think a generic overview and description of a possible solution would help.

First, let's take a look at a possible application design.

1. Network layer

The first layer is the network handling. Of course this is application specific. The actual network handling probably is listening to a specific port on the network, receiving a single connection, receiving multiple and synchronous HL7 messages on this connection. Note: the messages will be send with MLLP.

2. Service layer

The service layer will parse the HL7 message to a business object. In the image above this will be a "Person". First the complete message, still available as plain text, must be parsed to a nHapi message object. Here's an example:

 IMessage hl7Message = null;
PipeParser hl7Parser = new PipeParser();
hl7Message = hl7Parser.Parse(message); 

The IMessage object will give us enough information to determine which version of HL7 is used and what kind of message is received. I've used the factory design pattern twice in this example. First to get the right parser for the right HL7 version. The second is to get the right parser for this message.

 private IParser GetParserOnHl7Message(IMessage hl7Message)
 {
 IParser parser = null;

 switch (hl7Message.Version)
 {
 case "2.4" :
 parser = GetMessageParser(hl7Message);
 break;
 default :
 throw new NotImplementedException("HL7 version not supported.");
 }

 return parser;
 } 

Now that we have the right message for the right version we can interpret the message and create our business object.

ADT_A01 message = (ADT_A01)msg;
Person person = new Person();
person.Name = GetPatientName(message.PID.GetPatientName());
 
private string GetPatientName(XPN name)
{
List names = patientNames.Where(xpn => string.Compare(xpn.NameTypeCode.Value, "D") == 0).ToList();
 if (names.Count > 0)
 name = names[0];
 else
 name = patientNames.First();
return name.FamilyName.Surname.Value;
}

This example shows you how to get the "display name" from the HL7 message. This is mostly a safe bet, but not always the right data. To really interpret the message in a right way you actually have to read the HL7 specs. And if you need to implement something like this example, reading the specs is a very good idea.

3. Other layers

The other layers kinda speak for themselves. Now you have a business object you can perform any business operation needed for you application. The example inserts, updates or deletes a record in/from the database in order to keep the list of patients in sync with the central information system.

Remember: this is a very basic and short example. HL7 is able to do a lot and has quite complex messages. But this is basically what you need to do for every messages. After you processed the message HL7 prescribes that you send an acknowledge message to tell the server that the message was processed correctly or not. See my post about ack/nack messages.

If you have any questions or want to discuss something about this topic, please use this forum.