MQTT publish and subscribe

Report SHT3X readings to a broker and react to commands coming back on a wildcard subscription — the session keeps itself alive.

Click through the files — it's the whole project

What's going on

• connect() returns immediately — the session completes on a background poll thread that owns the keepalives and acks, which is why the loop checks linked() before publishing instead of waiting on connect.
• The wildcard cmd/# subscription catches every sub-topic under cmd/; the onMessage handler gets (topic, payload) and decides what each one means.
• Publishes are QoS 1 — the client retries until the broker acks. Payloads are strings; String(value) is all the formatting this example needs.
• Like all networking, this needs an ESP32 target. MQTT over TLS is the same constructor with an mqtts:// URI.

Flash and try it

npx @typecad/hal create my-project --target esp32s3_devkitc
cd my-project
npx @typecad/hal build --compile --upload --port COM4

Point it at a broker you can reach — Mosquitto on a LAN machine works, and mosquitto_sub -t 'sensors/#' shows the readings arriving. Publish to cmd/report and the board answers on the serial monitor.

Where to go next

• IoT Networking — the full Mqtt API and TLS
• Networking — the WiFi layer underneath
• Sensors — the SHT3X feeding the readings