Author: Alistair Keiller

Telemetry Fan-Out (MQTT + TDengine)

Overview

send_message() serializes a telemetry reading once, attaches a ts field, then fans out to both MQTT and TDengine.

Function: send_message

Workflow:

1. Serialize message to JSON; insert ts with the provided timestamp.
2. Publish JSON to MQTT topic T::topic() at QoS AtMostOnce.
3. Convert to TDengine schemaless line protocol and enqueue for write.

Backpressure / error behavior:

• Serialization failure → log, drop
• MQTT queue full → warn, drop; publish error → log
• Line protocol conversion failure → log, drop
• TDengine queue full → warn, drop; channel error → log

Runtime Singletons

get_mqtt_client — lazily creates an AsyncClient (OnceCell), spawns the MQTT event loop. On poll error: logs and retries after 1 second.

get_tdengine_sender — lazily creates a channel sender (OnceCell), ensures the fsae database exists, and spawns 4 worker tasks sharing one receiver. Workers batch up to 5,000 records per write. After 10 consecutive write failures, drops and recreates the database.

Trait: Reading

Requires Serialize. Implementors define topic() -> &'static str to specify the MQTT/TDengine measurement name.

Function: now_ms

Returns current UNIX time as milliseconds (u64).