Issue Summary

The AimDB MQTT connector (tokio implementation) has a hard-coded channel capacity of 10, which causes deadlocks when subscribing to more than 10 topics.

Root Cause

File: aimdb-mqtt-connector/src/tokio_client.rs
Line: 221

// Create client and event loop
let (client, event_loop) = AsyncClient::new(mqtt_opts, 10);  // ← HARD-CODED TO 10

The rumqttc::AsyncClient::new() creates an internal channel with capacity 10. When subscribing to 12 topics (6 nodes × 2 sensors), the subscription confirmations from the broker fill this queue, causing the connector to hang during initialization.

Symptoms

Working Configuration (10 records)

# 5 nodes × 2 sensors = 10 records
- vienna (temp + gps)
- salzburg (temp + gps)
- graz (temp + gps)
- innsbruck (temp + gps)
- linz (temp + gps)

Logs:

Collected 10 inbound routes for scheme 'mqtt'
MQTT router has 10 topics
Connector built and spawned successfully: mqtt
Database running, background tasks active

Failing Configuration (12 records)

# 6 nodes × 2 sensors = 12 records
- vienna, salzburg, graz, innsbruck, linz, klagenfurt

Logs:

Collected 12 inbound routes for scheme 'mqtt'
MQTT router has 12 topics
[HANGS HERE - never reaches "Connector built and spawned successfully"]

Technical Details

Subscription Flow

1. MqttConnectorImpl::build_internal() creates MQTT client with capacity 10
2. Loop subscribes to all topics (lines 231-238):

   for topic in &topics {
       client_arc.subscribe(topic.as_ref(), rumqttc::QoS::AtLeastOnce).await?;
   }

3. Each subscription sends a SUBACK message from broker to client
4. SUBACK messages queue up in the internal channel (capacity 10)
5. With 12 subscriptions, 2 SUBACK messages can't be queued → deadlock

Why 10 Works But 12 Doesn't

The rumqttc library's internal event loop processes:

• Outbound subscription requests
• Inbound SUBACK confirmations
• Inbound MQTT messages

With 10 topics, all SUBACKs fit in the queue. With 12, the queue fills up, blocking new SUBACKs, which blocks the event loop, which blocks subscription completion.