Greengrass Lambdas Running on Core Device - Using a Variable in the Topic

Question

I want to create a generic greengrass component that runs a lambda function. I have data that is sent to a topic such as

`plugin/config/"THINGNAME"`

"THINGNAME" is the name of each individual thing. Is it possible to set the event source to the a topic which includes the variable for the thingName?

eg.
plugin/config/{iot:thingName}

Each core device that runs this component will have a difference event source based on the thingName.

Below is an example of what I have set the default configuration to it does not work as intended.

It interprets the variable as a literal string.

```
 {
  "lambdaExecutionParameters": {
    "EnvironmentVariables": {}
  },
  "containerParams": {
    "memorySize": 16384,
    "mountROSysfs": false,
    "volumes": {},
    "devices": {}
  },
  "containerMode": "NoContainer",
  "timeoutInSeconds": 10,
  "maxInstancesCount": 100,
  "inputPayloadEncodingType": "json",
  "maxQueueSize": 1000,
  "pinned": true,
  "maxIdleTimeInSeconds": 60,
  "statusTimeoutInSeconds": 60,
  "pubsubTopics": {
    "0": {
      "topic": "plugin/config/{iot:thingName}",
      "type": "IOT_CORE"
    }
  }
}
```

Jan_B · Answer

Hi Phil, `{iot:thingName}` is a supported [GGv2 recipe variables](https://docs.aws.amazon.com/greengrass/v2/developerguide/component-recipe-reference.html#recipe-variables),  but to use recipe variables in Component Configuration you need to set the Greengrass nucleus Configuration option `interpolateComponentConfiguration` to true as explained [here](https://docs.aws.amazon.com/greengrass/v2/developerguide/greengrass-nucleus-component.html#greengrass-nucleus-component-configuration-interpolate-component-configuration)

As an alternative to a GGv2 Lambda you can also use directly the Greengras IPC SDK (python, C++, Java ) in your component code to publish/subscribe to messages as described in the [AWS IoT Greengrass documentation](https://docs.aws.amazon.com/greengrass/v2/developerguide/ipc-iot-core-mqtt.html) or here in the [5.2 Subscriber Implementation section](https://catalog.us-east-1.prod.workshops.aws/workshops/5ecc2416-f956-4273-b729-d0d30556013f/en-US/chapter5-pubsubipc/20-step2) of the AWS Greengrass V2 workshop.

Phil Begg · Answer

Hi Jan,

Thanks for the quick response. I can confirm this now works after revising the deployment and setting the aws.greengras.Nucleus configuration to merge to:

```
{
	"interpolateComponentConfiguration": "true"
}
```
and redeploying my lambda component  with 
```
  "pubsubTopics": {
    "0": {
      "topic": "plugin/config/{iot:thingName}",
      "type": "IOT_CORE"
    }
```

Why is IPC preferred over lambda's? Lambdas have a a very easy interface to set up event sources and currently my lambda runtime is nodejs14.x and the javascript SDK doesn't support IPC.

Greengrass Lambdas Running on Core Device - Using a Variable in the Topic

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