Iterating over results from previous state in AWS Step Functions - amazon-web-services

Question

I need to develop a 'State Machine' using 'AWS Step Functions' that accomplishes the following:
Call a Lambda function that will connect to DynamoDb & retrieve a list of rows. (I know how to do this.)
For each row from previous step, I need to call another Lambda function until all rows are read.
How do I do step #2 above in AWS Step Functions? In other words, how do I iterate over the results from the previous step.

Answer 1

It's not pretty, but this can be accomplished out of the box / without an iterator lambda by using JSONPath's slice operator and a Sentinel value.
Here's an example state machine:
{
"Comment": "Example of how to iterate over an arrray of items in Step Functions",
"StartAt": "PrepareSentinel",
"States": {
"PrepareSentinel": {
"Comment": "First, prepare a temporary array-of-arrays, where the last value has a special SENTINEL value.",
"Type": "Pass",
"Result": [
[
],
[
"SENTINEL"
]
],
"ResultPath": "$.workList",
"Next": "GetRealWork"
},
"GetRealWork": {
"Comment": "Next, we'll populate the first array in the temporary array-of-arrays with our actual work. Change this from a Pass state to a Task/Activity that returns your real work.",
"Type": "Pass",
"Result": [
"this",
"stage",
"should",
"return",
"your",
"actual",
"work",
"array"
],
"ResultPath": "$.workList[0]",
"Next": "FlattenArrayOfArrays"
},
"FlattenArrayOfArrays": {
"Comment": "Now, flatten the temporary array-of-arrays into our real work list. The SENTINEL value will be at the end.",
"Type": "Pass",
"InputPath": "$.workList[*][*]",
"ResultPath": "$.workList",
"Next": "GetNextWorkItem"
},
"GetNextWorkItem": {
"Comment": "Extract the first work item from the workList into currentWorkItem.",
"Type": "Pass",
"InputPath": "$.workList[0]",
"ResultPath": "$.currentWorkItem",
"Next": "HasSentinelBeenReached"
},
"HasSentinelBeenReached": {
"Comment": "Check if the currentWorkItem is the SENTINEL. If so, we're done. Otherwise, do something.",
"Type": "Choice",
"Choices": [
{
"Variable": "$.currentWorkItem",
"StringEquals": "SENTINEL",
"Next": "Done"
}
],
"Default": "DoWork"
},
"DoWork": {
"Comment": "Do real work using the currentWorkItem. Change this to be an activity/task.",
"Type": "Pass",
"Next": "RemoveFirstWorkItem"
},
"RemoveFirstWorkItem": {
"Comment": "Use the slice operator to remove the first item from the list.",
"Type": "Pass",
"InputPath": "$.workList[1:]",
"ResultPath": "$.workList",
"Next": "GetNextWorkItem"
},
"Done": {
"Type": "Succeed"
}
}
}