when to close and reopen socket after HL7 message sent

Question

I am trying to open a basic connection to an HL7 server where I send a request and get the ACK response. This will be done continuously.

1. If this is being done continuously, when do I close the socket? Am I implementing this correctly, in this case?
2. If I close the socket, how do I open it again? The javadocs for ConnectionHub indicates the following:

attach(java.lang.String host, int port, Parser parser, 
       java.lang.Class<? extends LowerLayerProtocol> llpClass) 

Returns a Connection to the given address, opening this Connection if necessary.

However, in real life, it will not open a new connection if it was already closed.

Patient patient = appt.getPatient();
Parser parser = new GenericParser();
Message hl7msg = parser.parse(wlp.getORMString(appt));

//Connect to listening servers
ConnectionHub connectionHub = ConnectionHub.getInstance();
// A connection object represents a socket attached to an HL7 server
Connection connection = connectionHub.attach(serverIP, serverPort, 
                            new PipeParser(), MinLowerLayerProtocol.class);
if (!connection.isOpen()) {
   System.out.println("CONNNECTION is CLOSED");
   connection = connectionHub.attach(serverIP, serverPort, new PipeParser(),         
                                     MinLowerLayerProtocol.class);
  if (!connection.isOpen()) {
    System.out.println("CONNNECTION is still CLOSED");
  }
}
Initiator initiator = connection.getInitiator();
Message response = initiator.sendAndReceive(hl7msg);

String responseString = parser.encode(response);
System.out.println("Received response:\n" + responseString);
connection.close();

Result: The first pass goes through perfectly, with request sent and ACK received. Any subsequent call to this method results in java.net.SocketException: Socket closed" on the client side. If I remove the connection.close() call, then it will run fine for a certain amount of time then the socket will close itself.

Answer 1

If you are communicating via HL7 2.X, the expected behavior on the socket is to never disconnect -- you allocate the connection and keep the socket active. Said another way, an HL7 application does not act like a web browser wherein it connects as needed and disconnects when done. Rather, both ends work to keep the socket continuously connected. Most applications will be annoyed if you disconnect. Further, most integration engines have alerts that will fire if you are disconnected for too long.

Once the socket is connected, you need to use the HL7 Minimum Lower Layer Protocol (MLLP or MLP) to communicate the HL7 2.X content. If you are sending data, you should wait for an HL7 Acknowledgment before you send the next message. If you are receiving data, you should generate the HL7 Ack.

References:

MLP - http://www.hl7standards.com/blog/2007/05/02/hl7-mlp-minimum-layer-protocol-defined

Acks - http://www.corepointhealth.com/resource-center/hl7-resources/hl7-acknowledgement