---
type: report
date: 2026-10-12
---
# Mosquitto: the bus on the car Orin

The message bus (project-yukart decision 4), installed on the car unit.

## Install

```bash
sudo apt-get install mosquitto mosquitto-clients     # 2.0.11-1ubuntu1.2 on Ubuntu 22.04
```

`/etc/mosquitto/conf.d/yukart.conf`:

```
# Yukart bus. Local only: displays reach it through svc-gw, never directly.
listener 1883 127.0.0.1
allow_anonymous true
set_tcp_nodelay true
```

`systemctl enable --now mosquitto`. It listens on `127.0.0.1:1883` only, so
anonymous access is local access; nothing on the LAN, the tailnet or LTE can
reach it.

## Behaviour worth knowing

- **Retained messages survive a reboot.** Ubuntu's `mosquitto.conf` sets
  `persistence true` (`/var/lib/mosquitto/`), so after a reboot the broker
  hands out the last values from before it. svc-gw therefore treats a value
  known only from a retained copy as stale until a live one arrives.
- **`set_tcp_nodelay true`** turns Nagle's algorithm off for the broker's
  sockets. Without it, small messages to subscribers wait for the previous
  one's ACK: a 40 ms tail at raw CAN rates (svc-core `BUS-BENCH.md`).
- Units that use the broker order themselves `After=mosquitto.service` and
  nothing else, and reconnect by themselves when it restarts.
- Startup tier 1 (project-yukart `docs/ORIN-RUNTIME.md`).

## First use

`orin-status` (svc-core) publishes the Orin's health on `yukart/host/<signal>`
once a second; `svc-gw` subscribes and serves it to the tablet. Watching it:

```bash
mosquitto_sub -v -t 'yukart/#'
```

## Throughput

Raw CAN rates are fine: a saturated 500 kbit/s bus (~4,400 frames/s) lost no
frames, per frame or batched; batched it costs the broker about 1 % of a core.
Measured 2026-10-12, svc-core `docs/impl/reports/BUS-BENCH.md`.