Why I Put Mirth Connect in Front of FastAPI Instead of Parsing HL7 in Python

When I started building my Maternity HL7-to-FHIR Pipeline , my first instinct was to do everything in Python. Parse the HL7 message, map the fields, validate the FHIR resource, persist it, all in one FastAPI service. It was clean. It was simple. It was wrong. The "Just Parse It in Python" Phase My initial architecture looked like this: Hospital System --MLLP--> Python Script --> HAPI FHIR Server I used python-hl7 to split messages on | and count field positions. For a single ADT^A01 (patient admission) message, it worked fine. I could pull the patient name from PID-5 , the MRN from PID-3 , the gender from PID-8 , and build a FHIR Patient resource from it. Then I tried a real-ish maternity workflow (an admission, an order, and a set of vitals) and things fell apart quickly. Five Problems That Changed My Mind 1. MLLP Is Not HTTP Hospital systems don't send HL7 over HTTP. They send it over MLLP (Minimum Lower Layer Protocol), which is a TCP socket protocol with specific framing bytes ( \x0b at the start, \x1c\x0d at the end). The sender expects an ACK or NACK response in HL7 format, not an HTTP status code. Building an MLLP listener in Python is possible . Libraries like aioml7 exist. But you're now maintaining a custom TCP server alongside your HTTP API server, handling connection pooling, timeouts, and HL7 acknowledgment generation. That's a lot of infrastructure code that has nothing to do with your actual transformation logic. Mirth Connect handles MLLP natively. You point it at a port, it listens, it parses, it ACKs. Done. One config screen, no custom code. 2. HL7 Parsing Is Messier Than It Looks The pipe-delimited format looks simple: PID|1||1234567^^^MRN||TEST^PATIENT^MARY^^MS||19920315|F|||14 SAMPLE ST^^SYDNEY^NSW^2000^AU But consider: Component separators : PID-5 is TEST^PATIENT^MARY^^MS , which is family, given, middle, suffix (empty), prefix. Miss the empty suffix and your prefix ends up as the suffix. Repeating fields : PID-3 can contain multiple identifier