#MLLP

MLLP frames each message as the byte 0x0B, the message, 0x1C 0x0D. The sender waits for the acknowledgement before it sends the next message. Port 2575 is registered for it.

#Sending

def c as hl7.Connection init hl7.connect("10.0.0.30:2575", hl7.options());
def reply as hl7.Message init hl7.send($c, $m);
if (hl7.get($reply, "MSA-1") != "AA") {
    io.printf("refused: %s\n", hl7.get($reply, "MSA-3"));
}
hl7.close($c);

send needs a control ID in MSH-10. When no acknowledgement comes within timeoutMs (30 s) it sends the message again, up to retries (3) times, then fails. The reply must be an acknowledgement (one MSA) whose MSA-2 is this message's control ID; what its code means is the caller's to judge. A resend can bring two acknowledgements, and one can arrive after send gave up: the next send on the connection skips acknowledgements of the message sent before it, so the connection stays in step.

#Receiving

receive returns the next message, or nothing when none began within the wait; reply sends the answer:

def c as hl7.Connection init hl7.accept(hl7.listen(":2575"), hl7.options());
for (def m in hl7.receive($c, 60000)) {
    hl7.reply($c, hl7.ack($m, "AA", "", "ACK" + hl7.get($m, "MSH-10")));
}

Messages that arrive back to back are read one at a time. Once a message's first byte came, the whole message must arrive within timeoutMs, however slowly it is sent. A byte before a message's 0x0B, a message larger than maxBytes, or one that does not arrive whole in time is an error; close the connection after one.

#Serving

serve accepts every client and gives each its own task; each message goes to the handler and its return value back to the sender:

func onMessage(m as hl7.Message) {
    return hl7.ack($m, "AA", "", "ACK" + hl7.get($m, "MSH-10"));
}

hl7.serve(hl7.listen(":2575"), hl7.options(), onMessage);

A handler that throws, or returns something other than a message, is answered with an AE acknowledgement carrying the error. A connection whose input breaks the framing or does not parse, sends a message without a control ID, or sends nothing for idleMs (an hour) is closed. When the listener is closed with net.close from another task, serve closes every connection still open and returns once their tasks ended. Other accept errors (out of file descriptors, say) pause it for 100 ms and it goes on. serve does not limit how many clients connect at once.

#Options

MLLP has no encryption; run it on a trusted network or through a tunnel.