Handling s3:TestEvent in Amazon S3 Event Notifications

Amazon S3 sends an s3:TestEvent message when an event notification is configured. Consumers must handle its structure separately from ordinary event messages.

Takahiro Iwasa

2020-12-29

2 min read

When an event notification is configured for an S3 bucket, Amazon S3 automatically sends an s3:TestEvent message. Its structure differs from an ordinary S3 event notification, so consumers must handle it separately.

https://docs.aws.amazon.com/AmazonS3/latest/userguide/notification-content-structure.html

When you configure an event notification on a bucket, Amazon S3 sends a test message with the s3:TestEvent.

Building

Create a CloudFormation stack template:

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AWSTemplateFormatVersion: "2010-09-09"
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Description: Example of CloudWatch events not queueing to SSE SQS
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Resources:
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  Bucket:
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    Type: AWS::S3::Bucket
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    Properties:
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      BucketEncryption:
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        ServerSideEncryptionConfiguration:
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          - ServerSideEncryptionByDefault:
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              SSEAlgorithm: AES256
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      NotificationConfiguration:
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        QueueConfigurations:
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          - Event: 's3:ObjectCreated:Put'
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            Queue: !GetAtt Queue.Arn
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      PublicAccessBlockConfiguration:
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        BlockPublicAcls: true
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        BlockPublicPolicy: true
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        IgnorePublicAcls: true
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        RestrictPublicBuckets: true
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  Queue:
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    Type: AWS::SQS::Queue
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    Properties:
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      QueueName: s3-event-notification-test-queue
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      ReceiveMessageWaitTimeSeconds: 20
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  QueuePolicy:
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    Type: AWS::SQS::QueuePolicy
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    Properties:
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      PolicyDocument:
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        Version: '2008-10-17'
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        Statement:
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          - Effect: Allow
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            Principal:
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              Service: s3.amazonaws.com
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            Action:
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              - SQS:SendMessage
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              - SQS:ReceiveMessage
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            Resource: !GetAtt Queue.Arn
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            Condition:
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              StringEquals:
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                aws:SourceAccount: !Ref AWS::AccountId
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      Queues:
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        - !Ref Queue

Deploy the stack:

aws cloudformation deploy \
  --template-file template.yaml \
  --stack-name s3-event-notification-test

Testing

Poll the SQS queue to confirm that the notification configuration is working:

aws sqs receive-message \
  --queue-url https://sqs.ap-northeast-1.amazonaws.com/{AccountId}/s3-event-notification-test-queue

You should observe the s3:TestEvent message in the output, even if no objects have been added to the bucket.

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{
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  "Messages": [
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    {
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      "MessageId": "...",
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      "ReceiptHandle": "...",
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      "MD5OfBody": "...",
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      "Body": "{\"Service\":\"Amazon S3\",\"Event\":\"s3:TestEvent\",\"Time\":\"2020-12-29T18:53:47.874Z\",\"Bucket\":\"s3-event-notification-test-bucket-xxxxxxxx\",\"RequestId\":\"...\",\"HostId\":\"...\"}"
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    }
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  ]
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}

Cleaning Up

Delete the stack afterward:

aws cloudformation delete-stack --stack-name s3-event-notification-test

Conclusion

After the S3 bucket and its SQS event notification are deployed, an s3:TestEvent message arrives before any object is uploaded. Although this template configures only s3:ObjectCreated:Put, the test message is sent automatically when the notification configuration is attached. Unlike an ordinary S3 event notification, it has top-level Service and Event fields and does not contain a Records array.

Lambda functions and applications that consume this queue should detect s3:TestEvent and either skip it or handle it explicitly. Code that assumes every message contains Records may otherwise fail as soon as the notification configuration is created.

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