Access Map State ( item ) in Step functions - amazon-web-services

Question

I am trying to access item properties which I am iterating over using Map state. I keep getting this error:
Value ($['Map'].snapshot_id.S) for parameter snapshotId is invalid. Expected: 'snap-...'. (Service: Ec2, Status Code: 400, Request ID: 6fc02935-c161-49df-8606-bc6f3e2934a6)
I have gone through the docs, which seems to suggest access using $.Map.snapshot_id.S but doesn't seem to work. Meanwhile, an input item to Map is:
{
"snapshot_id": {
"S": "snap-01dd5ee46df84119e"
},
"file_type": {
"S": "bash"
},
"id": {
"S": "64e6893261d94669b7a8ca425233d68b"
},
"script_s3_link": {
"S": "df -h"
}
}
Map state definition:
"Map": {
"Type": "Map",
"ItemProcessor": {
"ProcessorConfig": {
"Mode": "INLINE"
},
"StartAt": "Parallel State",
"States": {
"Parallel State": {
"Comment": "A Parallel state can be used to create parallel branches of execution in your state machine.",
"Type": "Parallel",
"Branches": [
{
"StartAt": "CreateVolume",
"States": {
"CreateVolume": {
"Type": "Task",
"Parameters": {
"AvailabilityZone": "us-east-2b",
"SnapshotId": "$$.Map.snapshot_id.S"
},
"Resource": "arn:aws:states:::aws-sdk:ec2:createVolume",
"Next": "AttachVolume"
},
"AttachVolume": {
"Type": "Task",
"Parameters": {
"Device": "MyData",
"InstanceId": "MyData",
"VolumeId": "MyData"
},
"Resource": "arn:aws:states:::aws-sdk:ec2:attachVolume",
"End": true
}
}
}
],
"End": true
}
}
},
"Next": "Final Result",
"ItemsPath": "$.Items",
"MaxConcurrency": 40
},

Answer 1

TL;DR "SnapshotId.$": "$.snapshot_id"
By default, each inline map iteration's input is one item from the ItemsPath array, accessible simply as $.
Your state machine definition implies that $.Items is an array of objects with a snapshot_id key (and other keys). If so, each iteration's snapshot id is at $.snapshot_id.
Finally, adding .$ to the parameter name (SnapshotId.$) tells Step Functions the value is not a literal, but rather a path value to be substituted.