fetch message from AWS Iot greengrass component

Hello, I'm not sure I get the whole picture here, but I'm assuming that you are planning to have a video detection app in an edge device (probably raspberry pi) with some ML inference capability deployed on Greengrass. If the user prefer edge computing, where the image captured will be analyzed locally without sending it to cloud services, you can use AWS IoT Core to receive the mqtt message from the edge device as seen in this documentation : [https://docs.aws.amazon.com/iot/latest/developerguide/connecting-to-existing-device.html]. Let say if a particular object detected, your device will send an mqtt message to IoT Core, and trigger a workflow.