Step Function is defined like that:
{
"StartAt": "Decision_Maker",
"States": {
"Decision_Maker":{
"Type": "Choice",
"Choices": [
{
"Variable": "$.body.MyData",
"StringEquals": "null", //that doesn't work :(
"Next": "Run_Task1"
}],
"Default": "Run_Task2"
},
"Run_Task1": {
"Type": "Task",
"Resource": "url_1",
"Next": "Run_Task2"
},
"Run_Task2": {
"Type": "Task",
"Resource": "url_2",
"End": true
}
}
}
Basically it's a choice between 2 tasks. Input data is like this:
{
"body": {
"prop1": "value1",
"myData": {
"otherProp": "value"
}
}
}
The problem is that sometimes there's no myData in JSON. So input may come like this:
{
"body": {
"prop1": "value1",
"myData": null
}
}
How do I check whether or not myData is null?
As of August 2020, Amazon States Language now has an isNull and isPresent Choice Rule. Using these you can natively check for null or the existence of a key in the state input inside a Choice state.
Example:
{ "Variable": "$.possiblyNullValue", "IsNull": true }
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"
}
]
},
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.