Core MQTT concepts explained: brokers, topics, QoS, sessions and MQTT 5 features, with hands-on examples on the free public broker broker.connectmq.com.
MQTT was invented in 1999 by Andy Stanford-Clark of IBM and Arlen Nipper of Arcom to monitor oil pipelines over satellite. Follow its path to open source, OASIS, ISO and today's managed cloud brokers.
MQTT can technically carry a 256 MB packet, but brokers, client libraries and devices impose far lower limits. Learn where the real limits are, what happens when you exceed them, and how to send large payloads.
Reconnect loops are one of the most common MQTT problems. Learn the five usual causes, from duplicate client IDs to keep alive timeouts and oversized packets, and follow a diagnosis flow that pinpoints yours.
MQTT and CoAP both target constrained IoT devices but take opposite approaches: brokered pub/sub over TCP versus RESTful request/response over UDP. Here is how they compare and when to use each.
Forty MQTT interview questions with concise answers covering fundamentals, QoS, sessions, retained messages, Last Will, MQTT 5 and security, plus ways to practice each concept on broker.connectmq.com.
An MQTT broker and a message queue both sit between producers and consumers, but they deliver messages in fundamentally different ways. Learn the semantics of each and when a cloud MQTT broker is the right building block.
How many bytes does an MQTT message really cost? Work through connect, keep alive and publish overhead at each QoS level, add TCP and TLS, and build a monthly data estimate for cellular IoT devices.
Getting "Connection Refused: not authorised" or reason code 0x87? Work through this checklist covering credentials, client ID binding, ports and TLS, and per-broker accounts to find the cause quickly.
A practical MQTT 3.1.1 vs 5.0 comparison covering protocol behavior changes, client library support and a step-by-step migration checklist for upgrading a mixed fleet gradually.
MQTT brokers can hold messages for clients that are temporarily offline, but only under specific conditions. Learn the rules, the limits and a reproducible test you can run in five minutes.
Last Will and Testament lets the broker announce that a client vanished without saying goodbye. Learn the will fields, the exact trigger rules and how to test LWT by killing a client on a live broker.
What each MQTT 5 subscription option does, when to use it and how to set it in MQTT.js and paho-mqtt, with runnable examples against a public MQTT 5 broker.
MQTT is publish/subscribe, but devices often need to answer questions. Learn how MQTT 5 response topic and correlation data turn MQTT into a clean request/response channel, with working Python and Node.js code.
Every MQTT 5 reason code in one table, grouped by the packets that carry it, with the usual causes and practical fixes. Includes the mapping from MQTT 3.1.1 return codes and Python code to log reason codes properly.
MQTT keeps messages in order, but only within narrow rules. Learn exactly what the spec guarantees, the real-world situations that reorder messages, and how sequence numbers make your consumers robust.
MQTT 3.1.1 brokers answer every CONNECT with a CONNACK return code from 0 to 5. Learn what each one means, how it looks in Mosquitto, Paho and MQTT.js, and the fastest fix for each refusal.
A stale retained message keeps greeting every new subscriber with old data. Here is how to delete a retained message in MQTT with an empty retained publish, from the command line, Python, JavaScript and a browser client.
A device that drops Wi-Fi for three seconds should not page your on-call engineer. MQTT 5's will delay interval holds the Last Will message back long enough for a quick reconnect.
MQTT runs over TCP, either directly, inside TLS, or inside WebSocket. Learn why the protocol depends on an ordered, reliable stream, where MQTT-SN and QUIC fit, and which ports to open for a ConnectMQ broker.
MQTT-SN adapts MQTT for battery-powered sensors on UDP, Zigbee and other non-TCP networks. Learn topic IDs, sleeping clients and gateways, and how to forward MQTT-SN traffic into a standard MQTT broker.
MQTT 5 lets a publisher say how long a message stays valid. Learn how the message expiry interval works with persistent sessions and retained messages, and how to stop late devices from executing stale commands.
Where does MQTT latency come from? This guide breaks down network, TLS, QoS and broker processing delays and gives you a Python script to measure round-trip time against broker.connectmq.com.
The QoS you ask for is not always the QoS you get. Learn how effective QoS is calculated, how brokers advertise a maximum QoS and grant lower subscription levels, and how to check what your connection really uses.
MQTT does not care what is inside a payload, so the choice is yours. Compare JSON, Protobuf and CBOR on size, schemas, debugging and tooling, using byte counts for the same sensor reading.
Fifteen practical MQTT use cases across manufacturing, energy, automotive, smart buildings, agriculture and logistics, each with a sample topic design, the MQTT features it relies on and the broker plan that fits its scale.
Topic names are the API of an MQTT system and are hard to change later. These 12 naming rules, with good and bad examples, keep your topic tree readable, secure and wildcard-friendly.
From ACL to Will Delay Interval, this MQTT glossary defines 60 protocol, broker and Sparkplug B terms in plain English, with examples you can try on broker.connectmq.com.
The retain flag lets an MQTT broker hand every new subscriber the last known value on a topic immediately. Learn how it works, where it shines and where it causes stale-data bugs.
MQTT bridging forwards selected topics between brokers so each site keeps a local broker while the cloud sees everything. Learn the patterns, a Mosquitto bridge config and a Node-RED alternative pointing at ConnectMQ.
MQTT 5 is more than a version bump. Learn the features that change how you build IoT systems, from reason codes and expiry to shared subscriptions and request/response, with hands-on examples.
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