MLLPong - A lightweight mock server for HL7 v2 messages over MLLP

I was building an HL7 integration and needed something to test against, a server that would just receive my messages and ACK them back. I searched for an existing MLLP mock server and found nothing that was simple to spin up, Docker-ready, and actually useful for real testing scenarios. Every option was either buried inside a massive enterprise tool, unmaintained, or required writing your own test harness from scratch. I built MLLPong in a weekend out of frustration, and it's been my go-to ever since. Healthcare developers working with HL7 v2 over MLLP have no lightweight, standalone mock server to test against. HL7 integration work typically requires a real EHR or middleware system on the other end, which means you need VPN access, test environments, and coordination with other teams just to verify your message format is correct. MLLPong eliminates that dependency. Spin it up locally or in CI with one Docker command, and your HL7 sender has something real to talk to immediately. Who is it for? Backend engineers, integration developers, and QA engineers who build or test HL7 v2 pipelines, especially those working with EHR systems, clinical data platforms, hospital integrations, or health information exchanges. MLLPong gives you three ports out of the box: one that always ACKs, one that always rejects, and a smart one that responds differently per message type based on a JSON rules file you can swap without rebuilding. No configuration UI, no license, no 200MB download, just a 10MB Docker image.