SOA-C03 practice question: Event-driven automation

Deployment, Provisioning, and Automation A free question from our original SOA-C03 practice exam. Try it, then open the answer to see why each option is right or wrong.

A ski resort's guests upload photos to an S3 bucket. An S3 Event Notification for all object-created events invokes a Lambda function that writes a resized copy of each photo back into the same bucket. After the function was deployed, Lambda invocations and S3 request costs rose sharply, and resized copies of resized copies started to appear. What should a CloudOps engineer do to fix this?

Choose one answer.

A. Set the function's reserved concurrency to 10 so that the number of copies the function can create at once is limited.
B. Turn on S3 Versioning so that each resized copy is stored as a new version of the original rather than as a new object.
C. Replace the S3 Event Notification with an EventBridge rule that matches Object Created events from the bucket.
D. Upload to an uploads/ prefix, filter the notification on it, and write resized copies to resized/.

Show the answer and explanation

Answer: D

A. Wrong: Reserved concurrency slows the loop down but does not stop it. Each resized copy still triggers another invocation. Learn more: Configuring reserved concurrency for a function (opens in a new tab)
B. Wrong: A new version still creates an object-created event, so the function would keep triggering itself. Versioning also keeps every copy and adds storage. Learn more: Retaining multiple versions of objects with S3 Versioning (opens in a new tab)
C. Wrong: An EventBridge rule matching every Object Created event from the bucket still matches the function's own writes. The loop continues unless the output is separated from the input. Learn more: Using EventBridge with Amazon S3 (opens in a new tab)
D. Right: Correct. Prefix filtering means only original uploads trigger the function. Writing output under a different prefix, or to a different bucket, stops the function from triggering itself. Learn more: Configuring event notifications using object key name filtering (opens in a new tab)

Explanation

A function that writes back to the same location that triggers it creates a recursive loop. Separate input and output with key name prefix filters or a second bucket so that the function's writes do not match the trigger. Lambda can detect and stop some recursive loops, but the design should not rely on that.

SOA-C03 practice question: Event-driven automation

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