How to send HL7 ACK message as a TCP response?

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I'm receiving HL7 messages via TCP connection. Those messages will always be of type ADT. I'm using Kestrel to listen for thise messages and the NHAPI package to deal with them. I took David Fowler's Kestrel sample code to setup a TCP listener. So based on this sample code

internal class HL7Listener : ConnectionHandler
{
    public override async Task OnConnectedAsync(ConnectionContext connection)
    {
        try
        {
            // handle the incoming message
        }
        catch (Exception exception)
        {
            // handle exceptions
        }
        finally
        {
            ACK acknowledgement = new ACK(); // create an ACK message
            PipeParser pipeParser = new PipeParser();
            string ackMessage = pipeParser.Encode(acknowledgement); // produces => MSH|^~\&|||||||ACK|||2.3
            byte[] ackMessageBytes = Encoding.UTF8.GetBytes(ackMessage);
            
            await connection.Transport.Output.WriteAsync(ackMessageBytes); // send the ACK
        }
    }
}

I'm using the tool 7Edit to send HL7 messages to my application. The echo sample from the repository (link above) works fine. The echo sample code produces a log like this

enter image description here

But when using my code I get this error

enter image description here

So I think I don't convert the ACK message correctly. Any ideas how to fix this?

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There are 1 answers

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Amit Joshi On BEST ANSWER

I doubt you are not implementing MLLP (also called LLP) protocol while sending ACK. I know, 7Edit expects MLLP to be implemented. This way, when you send an ACK to 7Edit (TCP/MLLP client), it looks for Start Block in your incoming data. It never finds it. It just discards your entire message considering garbage and keeps waiting; which causes timeout as you can see.

May be you should look for some setting in 7Edit to disable MLLP; but this will be temporary solution. Better, you implement MLLP block.

Description                 HEX     ASCII   Symbol
Message starting character  0B      11      <VT>
Message ending characters   1C,0D   28,13   <FS>,<CR>

With MLLP implemented, your message (the stuff you are writing on socket) should look something like below:

<VT>MSH|^~\\&|......|ACK|......<FS><CR>

Note the <VT>, <CR> and <FS> are place holders in above message.

You should modify your following line of code:

byte[] ackMessageBytes = Encoding.UTF8.GetBytes(ackMessage);

as below:

ackMessage = ((char) 11).ToString() + ackMessage + ((char) 28).ToString() + ((char) 13).ToString();
byte[] ackMessageBytes = Encoding.UTF8.GetBytes(ackMessage);

For more details, you may refer to this answer.