Using S3 Batch to perform an expedited restore

AWS S3 Batch Operations is a powerful service that can perform large-scale operations on S3 objects, such as copying, restoring, or applying tags. However, as you've noted, the S3InitiateRestoreObject operation currently only supports STANDARD and BULK retrieval tiers, not the EXPEDITED retrieval tier.

The reason for this limitation is not explicitly stated in the AWS documentation, but it could be due to the fact that expedited retrievals are designed for urgent situations where you need your data back in less than 5 minutes. This is a more costly operation, and it may not be suitable for large-scale batch operations where cost-effectiveness is often a priority.

As for your question about using AWS Lambda to perform an EXPEDITED restore, yes, it is technically possible. You can write a Lambda function that uses the AWS SDK to call the restore_object method with the Expedited retrieval option. This Lambda function can then be invoked as part of your S3 Batch Operations job.

Here's a basic example of what the Lambda function could look like in Python:

import boto3

def lambda_handler(event, context):
    s3 = boto3.client('s3')

    for record in event['Records']:
        bucket = record['s3']['bucket']['name']
        key = record['s3']['object']['key']

        s3.restore_object(
            Bucket=bucket,
            Key=key,
            RestoreRequest={
                'Days': 1,
                'GlacierJobParameters': {
                    'Tier': 'Expedited'
                }
            }
        )

This function would be triggered for each object in your S3 Batch Operations job, and it would initiate an expedited restore for that object.

Keep in mind that using Lambda in this way will indeed incur additional costs, both for the Lambda invocations and for the expedited retrievals. Also, expedited retrievals are not always available if AWS is experiencing peak demand, so you should have a fallback plan in place.