Internet of Things (IoT) & Embedded Systems Architecture
MQTT Cluster Scale Architecture
MQTT is the default protocol for constrained devices because it is lightweight and supports publish-subscribe semantics. Scaling to millions of connections is a broker and infrastructure problem: session state, topic design, and backpressure decide whether the cluster stays stable. EMQX and Mosquitto sit at different points on the scale curve.
Points clés
- Design topic hierarchies around access control and subscription patterns, not device IDs alone.
- Use persistent sessions only where offline delivery is genuinely required.
- Size connection handling and message throughput separately, since they stress different resources.
- Apply rate limits and backpressure per client to protect the broker.
- Terminate TLS at the broker or a load balancer and measure the CPU cost.
- Monitor queue depth, dropped messages, and reconnect storms.
- Test failover with real device reconnect behavior, which can amplify load.
- Keep payloads small and schema-versioned.
Questions fréquentes
How many clients can one broker handle?
It depends on message rate and hardware. Clustering is required well before theoretical connection limits.
Should I use QoS 2 everywhere?
No. It adds overhead and is rarely necessary for telemetry.
How do I handle reconnect storms?
Add jitter, backoff, and broker-side throttling.
Is Mosquitto enough at scale?
For moderate fleets yes. Very large deployments usually need a clustered broker.
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