helper lambda function for json to json transforms, useful to derive parameters in aws step function states

this lambda uses the python jsonpath-ng library to allow arbitrary json to json transforms from the input event and the lambda output.

Docker is required to deploy (dockerized pip to build the package by https://www.npmjs.com/package/serverless-python-requirements?activeTab=readme)

npm install

pip install -r requirements.txt

./localInvoke.sh

derive a property using a simple string concatenation. any properties in the input data prefixed with "expression_" will be evaluated as jsonpath expressions and the piece after the underscore will be used as the new property name, so in this case the "expression_d" part will create a new property on the output with name "d" and the value for that property will be computed using the jsonpath expression provided, in this case '$.k + " constant string"'.

Input

{
	"k": "v",
	"expression_d": "$.k + \" constant string\""	
}

Output

{
	"d": "v constant string"	
}

Enable input forwarding

if the property "_forward" is present in the input event, all non expression keys will be appended to the output object*

Input

{
	"_forward" : true,
	"k": "v",
	"expression_d": "$.k + \" constant string\""
}

Output

{
	"d": "v constant string",
	"k": "v"
}

Json unwrapping

when calling a step function from another step function, the json output of the state machine will be returned as a json string, this feature could be used to convert the json string to a property in the output object. In a similar way to the expression handling, using the "unescape_" prefix to instruct the lambda what to do, anything after the underscore will be used as the resulting property name for the parsed data passed input.

Input

{
	"unescape_data": "{ \"key\" : \"value\" }"
}

Output

{
	"data": {
		"key": "value"
	}
}