hl7-mllp

Transport-agnostic MLLP framing for HL7 v2 messages.

MLLP (Minimal Lower Layer Protocol) is the standard transport envelope for HL7 v2 messages over TCP/IP. This crate provides pure frame encoding and decoding — with no opinion on async runtimes, I/O libraries, or transports.

Design Philosophy

Most existing Rust MLLP crates are tightly coupled to tokio. This crate is not.

No tokio dependency — bring your own transport
No async-std dependency — works in sync, async, and no_std contexts
[MllpTransport] trait — implement for TCP, serial, UART, in-memory, or anything else
Pure framing logic — encode, decode, find frame boundaries

MLLP Frame Format

[VT 0x0B] [HL7 message bytes ...] [FS 0x1C] [CR 0x0D]

Usage

use hl7_mllp::{MllpFrame, MllpError};

// Encode a raw HL7 payload into an MLLP frame
let payload = b"MSH|^~\\&|SendApp|SendFac|...";
let framed = MllpFrame::encode(payload);

// Decode an MLLP frame back to the raw HL7 payload
let decoded = MllpFrame::decode(&framed)?;
assert_eq!(decoded, payload);

// Find frame boundaries in a streaming buffer
if let Some(frame_len) = MllpFrame::find_frame_end(&buffer) {
    let frame = &buffer[..frame_len];
    let payload = MllpFrame::decode(frame)?;
    // process payload...
}

Implementing a custom transport

use hl7_mllp::MllpTransport;

struct MyTcpTransport { /* ... */ }

impl MllpTransport for MyTcpTransport {
    type Error = std::io::Error;

    fn read_frame(&mut self) -> Result<Vec<u8>, Self::Error> {
        // accumulate bytes until MllpFrame::find_frame_end returns Some
        todo!()
    }

    fn write_frame(&mut self, frame: &[u8]) -> Result<(), Self::Error> {
        // write frame bytes to the stream
        todo!()
    }
}

Ecosystem

This crate is part of a family of transport-agnostic HL7 and FHIR crates:

Crate	Purpose
`hl7-mllp`	MLLP framing (this crate)
`hl7-v2`	HL7 v2 message parser
`hl7-mindray`	Mindray device HL7 field mappings
`fhir-r4`	FHIR R4 resource types
`satusehat`	Indonesian SATUSEHAT FHIR profile

Status

0.1.0 — Spec-complete MLLP implementation (HL7 v2.5.1 Appendix C):

✅ MLLP framing: encode/decode with VT/FS/CR delimiters
✅ Streaming support: MllpFramer for incremental frame accumulation
✅ ACK/NACK generation with optional chrono timestamps
✅ MllpTransport trait for custom transports
✅ Noncompliance feature for tolerant parsing
✅ Comprehensive documentation and examples

See CHANGELOG.md for details.

License

Apache-2.0

hl7-mllp 0.1.0