Questions for java greengrass lambda function

Question

I'm beginner of greengrass core application, and finished the demo setup following greengrass developper guide. but i'm still confusing about how lambda functio works.the bellow is the quesitons I want to ask for help.

I want to run a lambda function in my raspberry pi 3 as greengrass core, which can recieve multiple IoT devices' MQTT messages and do some process according to task tpye(i.e various signal filtering or house-hold machine learning algorithms). After proceesing, I need send the information using MQTT to my own server(not AWS IoT cloud) for higher level processing with some topics.

my questions are as follows( I want to use JAVA language): 1 To receive multiple aws iot devices connected to the GGC, should I need to set up a AWSIoTMQTTClient in aws-iot-device-sdk-java? I also find in aws_greengrass_core_sdk_java, there is “IotDataClient” class,what's it for?and what's the different with AWSIoTMQTTClient. here is really very confusing, even with sdk document description.

2 In GGC, when I deployed my lambda function, will it has an internal MQTT broker to receive messages for AWSIoTMQTTClient ?

3 for lambda functions, after creation and deployment on GGC, will it start to work. I saw there is method to invoke another lambda funciton from a lambda funciton. I don't understand the mechanism how lambda works.

4 Can i have multiple lambda functions for different uage,for instance, one is only to receive MQTT messages, another is to process the received info, other one is to send the processed info out to my own MQTT server? if permitted, how to make the work together to perform all the tasks.

5 I saw there is event input to lambda interface, how can I call a lambda only when some specific topic arriverd to AWSIoTMQTTClient defined in the lambda function?

6 the below is JAVA lambda interface template: outputType handler-name(inputType input, Context context) { ... } i think it should permit user to define input data type as he need. but the quesiton is if I define inputtype is string. how to the lambda handler to receive the string. the development guidence have no clear description.

7 finally, can you please share some demo codes for the above questions?

Thanks for you attention and kind help in advance. your help is highly expected

Answer 1

AWSIoTMQTTClient from the device SDK is not for Greengrass Lambda functions. Instead use IotDataClient from the Greengrass Java SDK, create a publish request, and then invoke the publish method. There is an example of that here - https://github.com/aws-samples/aws-greengrass-lambda-functions/blob/master/foundation/CDDBaselineJava/src/main/java/com/timmattison/greengrass/cdd/communication/GreengrassCommunication.java

AWSIoTMQTTClient is for devices/applications that run outside of Greengrass.

If you'd like to see some example Greengrass Lambda function code in Java check out at least this skeleton example - https://github.com/aws-samples/aws-greengrass-lambda-functions/tree/master/functions/CDDSkeletonJava. Note this function and other other ones in the repo depend on a framework called CDD (Cloud Device Driver). It is shared in the same repo and does most of the heavy lifting (messaging, startup, etc). That combined with the Greengrass provisioner - https://github.com/awslabs/aws-greengrass-provisioner - gives you a quick way to develop Java functions on Greengrass. Let me know if you try it out.

If you want to see the internals of CDD the root of it is here - https://github.com/aws-samples/aws-greengrass-lambda-functions/tree/master/foundation/CDDBaselineJava

As far as Lambda functions and how they run briefly I'll say that they can run on-demand (when they receive a message) or they can run "pinned" (forever). Pinned functions can receive messages too. Pinned functions are good when you need to track some kind of state. On-demand functions are more efficient for stateless data processing.