Map step function result and extract certain keys

Question

I have the following output coming from a step function task: ListObjectsV2

{
  "Contents": [
    {
      "ETag": "\"86c12c034bc6c30cb89b500b954c188f\"",
      "Key": "55271f52fffe4461a2ee3228ebb97157/input/batch_1.csv",
      "LastModified": "2023-02-09T13:46:20Z",
      "Size": 796014,
      "StorageClass": "STANDARD"
    },
    {
      "ETag": "\"58e4a770e0f66073b00d185df500f07f\"",
      "Key": "55271f52fffe4461a2ee3228ebb97157/input/batch_2.csv",
      "LastModified": "2023-02-09T13:47:20Z",
      "Size": 934038,
      "StorageClass": "STANDARD"
    },
    {
      "ETag": "\"460abd0de64d5cb67e8f0d46878cb1ef\"",
      "Key": "55271f52fffe4461a2ee3228ebb97157/input/batch_3.csv",
      "LastModified": "2023-02-09T13:46:57Z",
      "Size": 794264,
      "StorageClass": "STANDARD"
    },
    {
      "ETag": "\"1bfedc3dc92e4ba8d04e24b9b5a0ed58\"",
      "Key": "55271f52fffe4461a2ee3228ebb97157/input/batch_4.csv",
      "LastModified": "2023-02-09T13:46:24Z",
      "Size": 788756,
      "StorageClass": "STANDARD"
    },
    {
      "ETag": "\"9d6c434ce5ebdf203a790fbcf19338dc\"",
      "Key": "55271f52fffe4461a2ee3228ebb97157/input/batch_5.csv",
      "LastModified": "2023-02-09T13:47:07Z",
      "Size": 831156,
      "StorageClass": "STANDARD"
    }
  ],
  "IsTruncated": false,
  "KeyCount": 5,
  "MaxKeys": 1000,
  "Name": "vita-internal-text-classification-dev-183576513728",
  "Prefix": "55271f52fffe4461a2ee3228ebb97157"
}

I want to have an array containing only the Key key, to pass to the next state, like so:

[
    {
        "Key": "55271f52fffe4461a2ee3228ebb97157/input/batch_1.csv",
    },
    {
        "Key": "55271f52fffe4461a2ee3228ebb97157/input/batch_2.csv",
    },
    {
        "Key": "55271f52fffe4461a2ee3228ebb97157/input/batch_3.csv",
    },
    {
        "Key": "55271f52fffe4461a2ee3228ebb97157/input/batch_4.csv",
    },
    {
        "Key": "55271f52fffe4461a2ee3228ebb97157/input/batch_5.csv",
    }
]

So far I've tried setting the ResultPath to:

$.Contents[*].Key

$.Contents[*].['Key']

What I get is:

  [
    "55271f52fffe4461a2ee3228ebb97157/input/batch_1.csv",
    "55271f52fffe4461a2ee3228ebb97157/input/batch_2.csv",
    "55271f52fffe4461a2ee3228ebb97157/input/batch_3.csv",
    "55271f52fffe4461a2ee3228ebb97157/input/batch_4.csv",
    "55271f52fffe4461a2ee3228ebb97157/input/batch_5.csv",
  ]

But I've gotten bad output from that, any help?

Answer 1

The way I've solved this is to use an Inline Map state with a Pass state to build the necessary format. You can see this pattern in an example here for how to use Step Functions Distributed Map to bulk delete objects from S3. You can see this in the inner Create Object Identifier Array Map state. If you were doing this in Standard Workflows, this could be a cost concern given the number of state transitions involved. But since in the Item Processor I'm using Express Workflows, which are billed by duration (and these are super fast), it works pretty well.

{
  "Comment": "A state machine to bulk delete objects from S3 using Distributed Map",
  "StartAt": "Confirm Bucket Provided",
  "States": {
    "Confirm Bucket Provided": {
      "Type": "Choice",
      "Choices": [
        {
          "Not": {
            "Variable": "$.bucket",
            "IsPresent": true
          },
          "Next": "Fail - No Bucket"
        }
      ],
      "Default": "Check for Prefix"
    },
    "Check for Prefix": {
      "Type": "Choice",
      "Choices": [
        {
          "Not": {
            "Variable": "$.prefix",
            "IsPresent": true
          },
          "Next": "Generate Parameters - Without Prefix"
        }
      ],
      "Default": "Generate Parameters - With Prefix"
    },
    "Generate Parameters - Without Prefix": {
      "Type": "Pass",
      "Parameters": {
        "Bucket.$": "$.bucket",
        "Prefix": ""
      },
      "ResultPath": "$.list_parameters",
      "Next": "Delete Objects from S3 Bucket"
    },
    "Fail - No Bucket": {
      "Type": "Fail",
      "Error": "InsuffcientArguments",
      "Cause": "No Bucket was provided"
    },
    "Generate Parameters - With Prefix": {
      "Type": "Pass",
      "Next": "Delete Objects from S3 Bucket",
      "Parameters": {
        "Bucket.$": "$.bucket",
        "Prefix.$": "$.prefix"
      },
      "ResultPath": "$.list_parameters"
    },
    "Delete Objects from S3 Bucket": {
      "Type": "Map",
      "ItemProcessor": {
        "ProcessorConfig": {
          "Mode": "DISTRIBUTED",
          "ExecutionType": "EXPRESS"
        },
        "StartAt": "Create Object Identifier Array",
        "States": {
          "Create Object Identifier Array": {
            "Type": "Map",
            "ItemProcessor": {
              "ProcessorConfig": {
                "Mode": "INLINE"
              },
              "StartAt": "Create Object Identifier",
              "States": {
                "Create Object Identifier": {
                  "Type": "Pass",
                  "End": true,
                  "Parameters": {
                    "Key.$": "$.Key"
                  }
                }
              }
            },
            "ItemsPath": "$.Items",
            "ResultPath": "$.object_identifiers",
            "Next": "Delete Objects"
          },
          "Delete Objects": {
            "Type": "Task",
            "Next": "Clear Output",
            "Parameters": {
              "Bucket.$": "$.BatchInput.bucket",
              "Delete": {
                "Objects.$": "$.object_identifiers"
              }
            },
            "Resource": "arn:aws:states:::aws-sdk:s3:deleteObjects",
            "Retry": [
              {
                "ErrorEquals": [
                  "States.ALL"
                ],
                "BackoffRate": 2,
                "IntervalSeconds": 1,
                "MaxAttempts": 6
              }
            ],
            "ResultSelector": {
              "Deleted.$": "$.Deleted",
              "RetryCount.$": "$$.State.RetryCount"
            }
          },
          "Clear Output": {
            "Type": "Pass",
            "End": true,
            "Result": {}
          }
        }
      },
      "ItemReader": {
        "Resource": "arn:aws:states:::s3:listObjectsV2",
        "Parameters": {
          "Bucket.$": "$.list_parameters.Bucket",
          "Prefix.$": "$.list_parameters.Prefix"
        }
      },
      "MaxConcurrency": 5,
      "Label": "S3objectkeys",
      "ItemBatcher": {
        "MaxInputBytesPerBatch": 204800,
        "MaxItemsPerBatch": 1000,
        "BatchInput": {
          "bucket.$": "$.list_parameters.Bucket"
        }
      },
      "ResultSelector": {},
      "End": true
    }
  }
}