how to publish mqtt messages to greengrass core devices from cloud via SDK for JavaScript?

Question

Hi, I have a Greengrass core device publishing mqtt messages to AWS IoT Core and I want to send a confirmation to the same device but i couldn't find a way to do that. My idea was via Lambda process a message and send automatically a response to the device. Unfortunately I found many solutions to publish messages to IoT Core but not to the opossite, i mean, from IoT Core to the devices.

Thanks for any help, greetings!

Jan_B · Accepted Answer

Your Greengrass component needs to subscribe to messages from AWS IoT Core on a device specific topic, e.g `fromcloud/device001`, as described [here](https://docs.amazonaws.cn/en_us/greengrass/v2/developerguide/ipc-iot-core-mqtt.html#ipc-operation-subscribetoiotcore).

If you just want to echo a message back to the device, you can use the [AWS IoT Republishing action](https://docs.aws.amazon.com/iot/latest/developerguide/republish-rule-action.html) in the AWS IoT Rule Engine that triggers when a message arrives in AWS IoT on a topic e.g `tocloud/device001` and republish it to a topic e.g `fromcloud/device001` .
You can use the AWS IoT Rule engine SQL function [topic(2)](https://docs.aws.amazon.com/iot/latest/developerguide/iot-sql-functions.html#iot-function-topic) to extract the device id from the topic name to construct the return topic.

If you trigger a Lambda function from the AWS IoT Rule Engine, you can use the [iotdata.publish()](https://docs.aws.amazon.com/iot/latest/apireference/API_iotdata_Publish.html) REST method from one of our AWS SDKs to publish a message to the device specific topic `fromcloud/device001`

Hope that helps

how to publish mqtt messages to greengrass core devices from cloud via SDK for JavaScript?

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