---
type: decision
tags: [parking, decision, devices, transport]
sources: []
updated: 2026-06-15
status: settled
---

# Transport for the Relay Controller: HTTP/UDP now, MQTT parked

**Decision (2026-06-15): use direct HTTP + UDP for the [[dingtian-relay]] controller now. MQTT is
deliberately skipped, but kept on the radar** for when the system scales.

## Options the device supports

The Dingtian relay board speaks several protocols: Dingtian string (UDP/TCP), Dingtian binary
(UDP, optional multicast/password), **HTTP CGI**, **HTTP input-link push** (`input_link_url`),
**Modbus** (RTU/TCP/ASCII), and **MQTT**.

## Why not MQTT (yet)

- **A broker is new infrastructure** on a deliberately **single-purpose hardened appliance**
  ([[disk-os-hardening]]) — another service to install, secure, supervise, and keep alive.
- **Extra failure mode on the critical path.** Today host→UDP→relay. MQTT inserts a broker on both
  control and event paths; if it stalls, the lane stalls — and there are 3 processes to debug, not 2.
- **Doesn't fit [[offline-first]] for this scale.** MQTT earns its keep with *many* devices/consumers
  and intermittent links. Here it's **one host + a few devices on one isolated LAN, metres apart** —
  request/response control + a single input event, no fleet.
- **The device's MQTT input publish is periodic** ("default every 30 s"), so it's not even a clean
  on-press event without relying on unverified on-change behaviour.

## Why HTTP/UDP fits

- **Relay control:** direct **UDP string protocol** (port 60001) — `11`=relay1 on, `21`=off,
  `T1`=toggle, `11*`=jog/pulse. No deps, no broker.
- **Input/button events:** the device's **`input_link_url`** can **HTTP POST to the host backend
  when an input fires** — real push, device calls our existing Fastify server directly, no broker.
  (Polling `00` status over UDP every ~50 ms is the self-contained fallback.)
- Fewest moving parts; matches the local same-origin model already in use.

## When to revisit MQTT

If the system grows to **many lanes / many controllers**, or multiple subsystems (LPR, payment,
signage) all need to share events, a broker becomes a worthwhile central event bus. That aligns
with the [[autonomous-direction|unmanned]] roadmap at multi-lane scale — re-evaluate then. Until
then, direct HTTP/UDP wins on simplicity and reliability.