Publish/subscribe (pub/sub)

Publish/subscribe (pub/sub) is a messaging pattern in which senders, called publishers, send messages to a named channel, the topic, instead of to specific receivers. A broker or messaging service delivers each message to every subscriber registered for that topic, so publishers and subscribers never need to know about each other. MQTTProtocolMQTTThe standard pub/sub protocol of IoTView profile and Google Cloud Pub/Sub are both built on it.

How publish/subscribe works

Publishers produce messages, subscribers declare which topics they want, and the broker matches the two. A temperature sensor publishes to a topic such as plant1/line2/temperature; a dashboard and a historian can each subscribe to it without any change on the sensor.

That decoupling has practical effects:

• The publisher needs only the topic name, not the address of each subscriber.
• One message can reach many subscribers, and you can add new consumers without touching the publisher.
• Communication is asynchronous: the publisher doesn't wait for subscribers to process the message.

Pub/sub vs request/response

In request/response (HTTP, REST, or CoAPProtocolCoAPCoAP (RFC 7252) brings GET/POST/PUT/DELETE to devices with 8 KB of RAM over UDP. How Observe and DTLS work, and when to pick it over MQTT.View profile in its basic form), a client asks a specific server for something and waits for the answer. That fits commands and queries. Pub/sub fits streams of events flowing from many devices to many consumers. Many IoTITermIoT (Internet of Things)The IoT (Internet of Things) is the network of physical objects with sensors, software and connectivity that collect and exchange data and act autonomously.View profile systems use both: devices publish telemetry over MQTT, and applications query the stored data through an HTTP API. Our MQTT vs REST comparison covers the trade-offs.

Pub/sub in MQTT and in the cloud

In MQTT, the broker routes messages by topic. Subscriptions can use wildcards: + matches one level and # matches any number of levels, so factory/+/temperature covers every line. MQTT also attaches a delivery guarantee to each message (QoS 0, 1 or 2).

Google Cloud Pub/Sub applies the same pattern as a managed service: publishers send to topics, and subscribers receive through pull, push or export subscriptions. Delivery is at least once by default, with an exactly-once option.

Pub/sub in Cloud Studio IoT

Devices that connect to a Cloud Studio IoT instance over MQTT use the pattern in both directions: they publish readings to {MQTTUserID}/uplink/{DeviceAddress} and subscribe to {MQTTUserID}/downlink/{DeviceAddress} to receive commands, through the instance's own MQTT server with TLS on port 8883.