#MQTT

Last updated    - V6 (03-Oct-2025)

Key Concepts

Topic	SubTopic	Basic ✅	Intermediate ✅	Advanced ✅	Expert ✅
Introduction	What is MQTT, Features, Advantages, Use cases	✅	✅	✅	✅
MQTT vs HTTP	Differences, Performance, Use cases	✅	✅	✅	✅
Architecture	Broker, Clients, Publish/Subscribe model	✅	✅	✅	✅
QoS Levels	QoS 0, QoS 1, QoS 2, Message delivery guarantees	✅	✅	✅	✅
Topics & Subscriptions	Topic structure, Wildcards, Hierarchies	✅	✅	✅	✅
Messages	Payload, Retained messages, Duplicate messages	✅	✅	✅	✅
MQTT Broker	Popular brokers, Configuration, Scalability	✅	✅	✅	✅
MQTT Client	Libraries, Connecting, Subscribing, Publishing	✅	✅	✅	✅
Security	Authentication, Authorization, TLS/SSL, Certificates		✅	✅	✅
MQTT with IoT	Integration with IoT devices, Sensors, Real-time updates	✅	✅	✅	✅
Persistent Session	Session state, Clean session, Durable subscriptions		✅	✅	✅
Last Will & Testament	LWT messages, Configuration, Use cases		✅	✅	✅
Retained Messages	Definition, Use cases, Pros & Cons		✅	✅	✅
Advanced Features	MQTT over WebSockets, Bridge, Clustering			✅	✅
Best Practices	Topic naming, QoS selection, Security, Scalability	✅	✅	✅	✅

Interview question

Basic

1. What is MQTT and why is it used?
2. Explain the publish/subscribe model in MQTT.
3. What are the key features of MQTT?
4. Which transport protocol does MQTT primarily use?
5. What are MQTT clients and brokers?
6. Explain the concept of Topics in MQTT.
7. What is QoS in MQTT?
8. List the different QoS levels in MQTT.
9. Which TCP/IP port is commonly used by MQTT?
10. What is the difference between MQTT and HTTP?
11. What is the use of the CONNECT packet in MQTT?
12. Explain the PUBLISH packet in MQTT.
13. What is the role of the SUBSCRIBE packet in MQTT?
14. What is the role of the UNSUBSCRIBE packet in MQTT?
15. What is the use of the DISCONNECT packet in MQTT?
16. What is the difference between MQTT broker and client?
17. What is retained message in MQTT?
18. What are Last Will and Testament (LWT) messages?
19. What is the difference between QoS 0, QoS 1, and QoS 2?
20. Explain MQTT session persistence.
21. What are clean sessions in MQTT?
22. Why is MQTT considered lightweight?
23. What is the default port number of MQTT over TLS/SSL?
24. What are wildcards in MQTT topics?
25. Explain the difference between single-level (+) and multi-level (#) wildcards in MQTT.

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Intermediate

1. Explain the role of persistent sessions in MQTT.
2. What is the difference between retained messages and QoS in MQTT?
3. How does MQTT handle offline clients?
4. What is the purpose of Keep Alive in MQTT?
5. How does MQTT handle message ordering?
6. Explain the DUP flag in MQTT packets.
7. What is the significance of Packet Identifier in MQTT?
8. How does MQTT handle authentication?
9. What are MQTT v3.1.1 and v5.0 differences?
10. Explain the concept of Shared Subscriptions in MQTT 5.0.
11. What is message expiry in MQTT 5.0?
12. Explain the concept of User Properties in MQTT 5.0.
13. What is Session Expiry in MQTT 5.0?
14. What is Payload Format Indicator in MQTT 5.0?
15. Explain Content Type property in MQTT 5.0.
16. What is Subscription Identifier in MQTT 5.0?
17. What are flow control mechanisms in MQTT?
18. How does MQTT ensure reliability over TCP?
19. What is MQTT-SN and how is it different from MQTT?
20. Explain MQTT over WebSockets.
21. What are common MQTT security mechanisms?
22. How can TLS be used with MQTT?
23. How does MQTT perform load balancing?
24. What is the difference between MQTT and AMQP?
25. What are some popular MQTT broker implementations?

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Advanced

1. How does clustering work in MQTT brokers?
2. What are the scaling challenges in MQTT?
3. How does MQTT ensure high availability?
4. Explain how retained messages can be misused.
5. How do you implement access control in MQTT?
6. What are topic aliasing features in MQTT 5.0?
7. Explain MQTT bridge connections.
8. How does MQTT handle large payloads?
9. What are some strategies to secure IoT devices using MQTT?
10. How does MQTT handle backpressure?
11. Explain the concept of inflight messages.
12. What is the impact of QoS levels on bandwidth usage?
13. Explain MQTT message deduplication.
14. What is the role of a Session State in MQTT?
15. How does MQTT deal with duplicate messages?
16. What is the maximum QoS supported by MQTT?
17. How do MQTT brokers handle topic hierarchy?
18. Explain persistent vs non-persistent storage in MQTT brokers.
19. How do MQTT clients reconnect after failures?
20. What are best practices for MQTT topic naming?
21. Explain MQTT message ordering guarantees.
22. What are retained message pitfalls?
23. Explain MQTT message compression.
24. How does MQTT handle resource-constrained devices?
25. Explain MQTT message delivery semantics.

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Expert

1. Compare MQTT v3.1.1 and v5.0 in detail.
2. How do you scale MQTT for millions of devices?
3. How do you integrate MQTT with cloud providers like AWS IoT or Azure IoT Hub?
4. Explain how MQTT can be integrated with Kafka.
5. How do you monitor MQTT broker performance?
6. What are advanced load-balancing strategies for MQTT?
7. How can you implement multi-tenancy in MQTT?
8. Explain the role of back-end databases with MQTT brokers.
9. How do you secure MQTT at scale?
10. Explain MQTT over QUIC protocol.
11. How do you handle distributed MQTT brokers?
12. Explain MQTT message traceability.
13. How do you implement custom authentication plugins for brokers?
14. How does MQTT interact with REST APIs?
15. What are common challenges in MQTT-based IoT systems?
16. How do you optimize MQTT for high-frequency trading-like systems?
17. Explain QoS 2 message exchange flow with packet sequence.
18. What is the impact of network latency on MQTT?
19. How to benchmark an MQTT broker?
20. Explain MQTT with Edge Computing.
21. How do you debug MQTT message flow in production?
22. How do you achieve end-to-end security in MQTT IoT ecosystems?
23. How to design a fault-tolerant MQTT system?
24. Explain MQTT integration with event-driven microservices.
25. What is the future of MQTT in IoT and real-time messaging?

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