---
type: concept
tags: [parking, architecture, devices, entry-flow]
sources: []
updated: 2026-06-15
---

# Device Input Flow (button → backend → relay)

How a physical button press drives the entry lane. The **backend is the source of truth**: the
device only *reports* the press; the host decides and commands the relay. This is the host-in-the-
loop flow the [[dingtian-relay]] makes possible (and the [[uhppote-controller]] could not).

## The path (no polling)

```
car arrives → driver presses button (input I_N, dry contact to GND)
  → device HTTP-pushes  GET …/api/devices/dingtian/<deviceId>/input/<N>/on
  → backend: emit internal device event (device-events bus)
  → backend entry flow: create + sign an entry event, print the ticket
  → backend: pulseOpen(N) over UDP  → barrier opens
  → (on release) device pushes …/input/<N>/off
```

- **Push, not poll.** The device's `input_link_url` feature is configured (by the driver's
  `configureInputPush()`) to call the backend on each input edge — see [[dingtian-relay]]. The
  driver's poll path remains only as a dev/fallback aid.
- **Per-input path** carries the input number in the URL (`…/input/3/on`), so routing needs no
  body parsing. Both edges (`on`/`off`) are sent.
- **Internal event bus** (`device-events.ts`, a Node `EventEmitter`) decouples the HTTP/transport
  layer from business logic — drivers/pushes emit; the entry flow subscribes. Keeps the app
  [[device-adapter-pattern|device-agnostic]].

## Trust model (important — flat network, no VLAN)

The relay-control direction (host → device) is **unauthenticated UDP**, and the site is a **flat
network with no VLAN** ([[network-isolation]] is not yet enforceable here). So we do **not** trust
the device or the network. Instead:

- **Every barrier open is a host decision, recorded as a signed event BEFORE the relay fires**
  ([[append-only-event-chain]]). If anyone opens the relay out-of-band (which the flat network
  allows), there is **no matching signed event → a detectable anomaly**. The anti-fraud guarantee
  is the **signed log**, not device/network auth.
- The inbound push endpoint is intentionally **not behind the SPA's cookie/CSRF auth** (it's a
  machine call from the device). It is guarded by **HTTP Digest auth** + a **source-IP allowlist**
  (defence-in-depth), but these are *not* the security boundary.
- This sharpens under the [[autonomous-direction|unmanned]] roadmap: with no operator, tamper
  detection via the signed log matters more than perimeter auth.

## Push authentication — Digest (decided by hardware testing)

The secret must not be in the URL (sniffable, logged) and the password must not cross the wire in
the clear. We **empirically tested the device** to pick the strongest achievable option:

| Option | Device result |
| --- | --- |
| HTTPS (self-signed) | ❌ device won't push to a self-signed cert |
| **Digest auth** (`auth=2`) | ✅ **works** — full 401-nonce challenge/response |
| Basic auth | ✅ works (but password base64 on the wire) |
| URL token | rejected by design (visible in URL/logs) |

→ **HTTP Digest** (MD5, qop=auth). The password is never sent (only a nonce-keyed hash); nonces
are **single-use** (replay resistance). Per-device credentials (`pushUser`/`pushPassword`) are
generated by the backend on **device assign**, written to the device's `input_link_url` config,
and stored in `lane_devices` — the admin never types a URL or secret. HTTPS would be stronger but
the device can't do it here; Digest + the signed log is the practical answer on a flat network.
See `apps/server/src/digest-auth.ts`.

## Dingtian config-write gotchas (cost a lot of debugging)

Writing the device's config API (`/api/v2/config_set.cgi`) has two non-obvious traps — both now
handled in the driver:

1. **Content-Length is mandatory.** The device's embedded HTTP server does **not** accept chunked
   request bodies. Node uses chunked encoding when `Content-Length` is absent, so the device
   silently ignores the body and returns `{"status":0}` anyway — the write looks successful but
   nothing changes. Always set `Content-Length`.
2. **The `pass` field caps at 31 chars** (longer is silently truncated → Digest mismatch). The
   generated push password is 24 hex chars (96 bits).
3. (Also: the device reboots on apply, so the driver writes then **polls until the change is
   verified**, retrying — back-to-back writes onto a rebooting device are lost.)

## Status

Input push **verified on hardware** with Digest auth (all 4 inputs, real presses authenticated, no
failures). The entry
flow itself (signed event + ticket print + `pulseOpen`) is the next build — see [[dingtian-relay]].