Error when invoking lambda from StepFunction with paypload - amazon-web-services

Question

I have a lambda and it's one of the steps of a StepFunction statemachine, the lambda handler looks like this below :
def lambda_handler(event, context):
year = event['year'] #payload
month = event['month'] #payload
example_function(year, month)
This lambda execution succeeded if f I start a run in Lambda itself with payload:
{
"year": "2019",
"month": "06"
}
However if I kick off a stepfunction run, it will fail with error:
[ERROR] KeyError: 'year'
Traceback (most recent call last):
File "/var/task/xxx.py", line 34, in lambda_handler
year = event['year']
My stepfunction definition is:
{
"Comment": "xxxxxxxxx",
"StartAt": "invoke lambda",
"States": {
"invoke lambda" : {
"Type": "Task",
"Resource": "arn:aws:states:::lambda:invoke",
"Parameters": {
"FunctionName":"xxxxxxxxxx",
"Payload": {
"Input": {
"year": "2019",
"month": "06"
}
}
}
"End": true
}
}
}
I'm not sure how to debug and solve this issue, might someone be able to help please?

Answer 1

There are two ways to call the lambda function from step function.
First Method, directly by giving the Resource name as lambda Arn and passing the entire input to Parameters. This passes given input as is to Lambda
{
"StartAt":"invoke-lambda",
"States":{
"invoke-lambda":{
"End":true,
"Type":"Task",
"Resource":"arn:aws:lambda:us-east-1:660008888333:function:HelloWorld",
"Parameters":{
"year":"2019",
"month":"06"
}
}
}
}
Second Method using Resource lambda:invoke and Payload with Json body(no need to wrap within Input)
{
"StartAt":"invoke-lambda",
"States":{
"invoke-lambda":{
"End":true,
"Type":"Task",
"Resource":"arn:aws:states:::lambda:invoke",
"Parameters":{
"FunctionName":"arn:aws:lambda:us-east-1:660008888333:function:HelloWorld",
"Payload":{
"year":"2019",
"month":"06"
}
}
}
}
}
If input to Lambda is from either previous step or from input to step function input, we can use "Payload.$": "$" or even "Payload.$": "$.subPath"(to pass part of json)
Also it is recommended to add a Retry for exceptions that are triggered because of Exceptions caused by AWS.
{
"StartAt": "invoke-lambda",
"States": {
"invoke-lambda": {
"End": true,
"Retry": [
{
"ErrorEquals": [
"Lambda.ServiceException",
"Lambda.AWSLambdaException",
"Lambda.SdkClientException"
],
"IntervalSeconds": 2,
"MaxAttempts": 6,
"BackoffRate": 2
}
],
"Type": "Task",
"Resource": "arn:aws:states:::lambda:invoke",
"Parameters": {
"FunctionName": "arn:aws:lambda:us-east-1:660008888333:function:HelloWorld",
"Payload.$": "$",
"InvocationType": "RequestResponse"
}
}
}
}