AWS Step Function: check for null - amazon-web-services

Question

Step Function is defined like that:
{
"StartAt": "Decision_Maker",
"States": {
"Decision_Maker":{
"Type": "Choice",
"Choices": [
{
"Variable": "$.body.MyData",
"StringEquals": "null", //that doesn't work :(
"Next": "Run_Task1"
}],
"Default": "Run_Task2"
},
"Run_Task1": {
"Type": "Task",
"Resource": "url_1",
"Next": "Run_Task2"
},
"Run_Task2": {
"Type": "Task",
"Resource": "url_2",
"End": true
}
}
}
Basically it's a choice between 2 tasks.
Input data is like this:
{
"body": {
"prop1": "value1",
"myData": {
"otherProp": "value"
}
}
}
The problem is that sometimes there's no myData in JSON. So input may come like this:
{
"body": {
"prop1": "value1",
"myData": null
}
}
How do I check whether or not myData is null?

Answer 1

As of August 2020, Amazon States Language now has an isNull and isPresent Choice Rule. Using these you can natively check for null or the existence of a key in the state input inside a Choice state.
Example:
{ "Variable": "$.possiblyNullValue", "IsNull": true }
https://docs.aws.amazon.com/step-functions/latest/dg/amazon-states-language-choice-state.html#amazon-states-language-choice-state-rules

Answer 2

The order matters. Set the "IsPresent": false first, then the "IsNull": true, finally the scalar comparison last.
"Check MyValue": {
"Comment": "Check MyValue",
"Type": "Choice",
"Default": "ContinueWithMyValue",
"Choices": [
{
"Or": [
{
"Variable": "$.MyValue",
"IsPresent": false
},
{
"Variable": "$.MyValue",
"IsNull": true
},
{
"Variable": "$.MyValue",
"BooleanEquals": false
}
],
"Next": "HaltProcessing"
},
{
"Variable": "$.MyValue",
"BooleanEquals": true,
"Next": "ContinueWithMyValue"
}
]
},

Answer 3

As per my experience, Choice Type can't deal nulls. Best way could be pre-processing your input using a lambda in the very first state and return the event formatting it as "null". Below code snippet might help.
def lambda_handler(event, context):
if event['body']['MyData']:
return event
else:
event['body']['MyData']="null"
return event
Note: This also handles empty string.