Cloud IoT Core supports two protocols for device connection and communication: MQTT and HTTP. Devices communicate with Cloud IoT Core across a "bridge" — either the MQTT bridge or the HTTP bridge. The MQTT/HTTP bridge is a central component of Cloud IoT Core, as shown in the components overview.
When you create a device registry, you select protocols to enable: MQTT, HTTP, or both.
MQTT is a standard publish/subscribe protocol that is frequently used and supported by embedded devices, and is also common in machine-to-machine interactions.
HTTP is a "connectionless" protocol: with the HTTP bridge, devices do not maintain a connection to Cloud IoT Core. Instead, they send requests and receive responses. Cloud IoT Core supports HTTP 1.1 only (not 2.0).
The following table compares how the two protocols work in Cloud IoT Core:
|MQTT bridge||HTTP bridge|
|Device connection is maintained||Connectionless (request/response)|
|Full-duplex TCP connection||Half-duplex TCP connection|
|JWT is sent in the password field of the CONNECT message||JWT is sent in the header of the HTTP request|
|Telemetry events are pushed to Cloud Pub/Sub||Telemetry events are pushed to Cloud Pub/Sub|
|Device connection status is reported||No device connection status reported|
|Device configurations are propagated via subscriptions||Device configurations must be explicitly requested (via polling)|
|Most recent configuration (whether newer or not) is always received by devices on subscription||Devices can specify that only newer configurations should be received|
|Device configurations are acknowledged (ACKed) when using QoS 1||No explicit ACK for device configurations|
|Last device heartbeat time is retained||No device heartbeat data|
You might also want to consider the following general features of each protocol:
Both bridges use public key (asymmetric) device authentication and JSON Web Tokens (JWTs). For details, see the section on device security.
Tip: If you're not sure which protocol is best for your use cases, start with HTTP to get familiar with Cloud IoT Core, and then switch to MQTT if needed.