Interact with client devices in components
You can develop custom Greengrass components that interact with client devices connected to a core device. For example, you can develop components that do the following:
-
Act on MQTT messages from client devices and send data to Amazon Web Services Cloud destinations.
-
Send MQTT messages to client devices to initiate actions.
Client devices connect to and communicate with a core device through the MQTT broker component that runs on the core device. By default, client devices can communicate only with each other over MQTT, and Greengrass components can't receive these MQTT messages or send messages to client devices.
Greengrass components use the local publish/subscribe interface to communicate on a core device. To communicate with client devices in Greengrass components, configure the MQTT bridge component to do the following:
-
Relay MQTT messages from client devices to local publish/subscribe.
-
Relay MQTT messages from local publish/subscribe to client devices.
You can also interact with client device shadows in Greengrass components. For more information, see Interact with and sync client device shadows.
Topics
Configure and deploy the MQTT bridge component
The MQTT bridge component consumes a list of topic mappings that each specify a message source and a message destination. To communicate with client devices, deploy the MQTT bridge component, and specify each source and destination topic in the component configuration.
To deploy the MQTT bridge component to a core device
or group of core devices, create a deployment that
includes the aws.greengrass.clientdevices.mqtt.Bridge component. Specify the topic
mappings, mqttTopicMapping, in the MQTT bridge component configuration in the
deployment.
The following example defines a deployment that configures the MQTT bridge component to
relay the clients/MyClientDevice1/hello/world topic from client devices to local
publish/subscribe broker. The merge configuration update requires a serialized
JSON object. For more information, see Update component configurations.
You can use MQTT topic wildcards to relay messages on topics that match a topic filter. If you use MQTT bridge v2.2.0 or later, you can use MQTT topic wildcards in topic filters when the source broker is local publish/subscribe. For more information, see MQTT bridge component configuration.
Receive MQTT messages from client devices
You can subscribe to the local publish/subscribe topics that you configure for the MQTT bridge component to receive messages from client devices.
To receive MQTT messages from client devices in custom components
-
Configure and deploy the MQTT bridge component to relay messages from an MQTT topic where client devices publish to a local publish/subscribe topic.
-
Use the local publish/subscribe IPC interface to subscribe to the topic where the MQTT bridge relays messages. For more information, see Publish/subscribe local messages and SubscribeToTopic.
The Connect and test client devices tutorial includes a section where you develop a component that subscribes to messages from a client device. For more information, see Step 4: Develop a component that communicates with client devices.
Send MQTT messages to client devices
You can publish to the local publish/subscribe topics that you configure for the MQTT bridge component to send messages to client devices.
To publish MQTT messages to client devices in custom components
-
Configure and deploy the MQTT bridge component to relay messages from a local publish/subscribe topic to an MQTT topic where client devices subscribe.
-
Use the local publish/subscribe IPC interface to publish to the topic where the MQTT bridge relays messages. For more information, see Publish/subscribe local messages and PublishToTopic.