Amazon S3 target data validation - AWS Database Migration Service
PrerequisitesPermissionsLimitationsValidation only tasks

Amazon S3 target data validation

AWS DMS supports validating replicated data in Amazon S3 targets. Because AWS DMS stores replicated data as flat files in Amazon S3, we use Amazon Athena CREATE TABLE AS SELECT (CTAS) queries to validate data.

Queries on data that is stored in Amazon S3 are computationally intense. Thus, AWS DMS runs validation on Amazon S3 data during change data capture (CDC) only once a day, at midnight (00:00) UTC. Each daily validation that AWS DMS runs is called an interval validation. During an interval validation, AWS DMS validates all of the change records that were migrated to the target Amazon S3 bucket for the previous 24 hours. For more information about limitations for interval validation, see Limitations for using S3 target validation.

Amazon S3 target validation uses Amazon Athena, so additional costs apply. For more information, see Amazon Athena Pricing.

Note

S3 target validation requires AWS DMS version 3.5.0 or later.

S3 target validation prerequisites

Before using S3 target validation, check the following settings and permissions:

  • Set the DataFormat value for your endpoint's S3Settings to parquet. For more information, see Parquet settings for S3.

  • Ensure that the role assigned to the user account that was used to create the migration task has the correct set of permissions. See Permissions following.

For tasks using ongoing replication (CDC), check the following settings:

Permissions for using S3 target validation

To set up access for using S3 target validation, ensure that the role assigned to the user account that was used to create the migration task has the following set of permissions. Replace the sample values with your values.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": [
                "athena:StartQueryExecution",
                "athena:GetQueryExecution",
                "athena:CreateWorkGroup"
            ],
            "Resource": "arn:aws:athena:<endpoint_region_code>:<account_id>:workgroup/dms_validation_workgroup_for_task_*"
        },
        {
            "Effect": "Allow",
            "Action": [
                "glue:CreateDatabase",
                "glue:DeleteDatabase",
                "glue:GetDatabase",
                "glue:GetTables",
                "glue:CreateTable",
                "glue:DeleteTable",
                "glue:GetTable"
            ],
            "Resource": [
                "arn:aws:glue:<endpoint_region_code>:<account_id>:catalog",
                "arn:aws:glue:<endpoint_region_code>:<account_id>:database/aws_dms_s3_validation_*",
                "arn:aws:glue:<endpoint_region_code>:<account_id>:table/aws_dms_s3_validation_*/*",
                "arn:aws:glue:<endpoint_region_code>:<account_id>:userDefinedFunction/aws_dms_s3_validation_*/*"
            ]
        },
        {
            "Effect": "Allow",
            "Action": [
                "s3:GetBucketLocation",
                "s3:GetObject",
                "s3:ListBucketMultipartUploads",
                "s3:AbortMultipartUpload",
                "s3:ListMultipartUploadParts"
            ],
            "Resource": [
                "arn:aws:s3:::<bucket_name>",
                "arn:aws:s3:::<bucket_name>/*"
            ]
        }
    ]
}

Limitations for using S3 target validation

View the following additional limitations that apply when using S3 target validation. For limitations that apply to all validations, see Limitations.

  • Your DatePartitionSequence value needs a Day component. S3 target validation does not support the YYYYMM format.

  • When interval validation is running during CDC, you may see false validation errors in the awsdms_validation_failures_v1 table. These errors occur because AWS DMS migrates changes that arrived during the interval validation into the next day's partition folder. Normally, these changes are written into the current day's partition folder. These false errors are a limitation of validating replication from a dynamic source database to a static target, such as Amazon S3. To investigate these false errors, check for records near the end of the validation window (00:00 UTC), which is when these errors typically appear.

    To minimize the number of false errors, ensure that the CDCLatencySource for the task is low. For information about monitoring latency, see Replication task metrics.

  • Tasks in the failed or stopped state don't validate the previous day's changes. To minimize validation errors because of unexpected failures, create separate validation only tasks with the same table mappings, and source and target endpoints. For more information about validation only tasks, see Using validation only tasks with S3 target validation.

  • The Validation Status column in table statistics reflects the state of the most recent interval validation. As a result, a table which has mismatches might show up as validated after the next day's interval validation. Check the s3_validation_failures folder in the target Amazon S3 bucket for mismatches that occurred more than a day ago.

  • S3 Validation uses the bucketed table feature of Amazon Athena. This allows S3 validation to make a bucketed copy of the target table data. This means that the copy of the table data is divided into subsets that match DMS validation's internal partitioning. Athena bucketed tables have a limit of 100,000 buckets. Any tables that S3 validation attempts to validate that exceed this limit will fail validation. The number of buckets that S3 Validation attempts to create is equal to the following:

    (#records in the table) / (validation partition size setting)

    To work around this limitation, increase the validation partition size setting so that the number of buckets created by S3 Validation is less than 100,000. For more information about bucketing, see Partitioning and bucketing in Athena in the Amazon Athena User Guide.

Using validation only tasks with S3 target validation

A validation only task runs validation on data that is to be migrated without running the migration.

Validation only tasks continue to run, even if the migration task stops, which ensures that AWS DMS doesn't miss the 00:00 UTC interval validation window.

Using validation only tasks with Amazon S3 target endpoints has the following limitations:

  • Amazon S3 Validation for Full-Load Tasks with the Validation-Only setting enabled are supported, but operate differently than Full-Load and Validation-Only tasks for other endpoints. For S3 as a Target, a task of this type validates against only the Full-Load Data in the S3 target, and will not validate against any data migrated as part of a CDC migration. Only use this feature to validate data created by a Full-Load only task. Using this mode to validate data in a target that has an active CDC task running will not produce an effective validation.

  • Validation only tasks only validate changes since the last interval validation window (00:00 UTC). Validation only tasks don't validate full-load data or CDC data from previous days.