关于DynamoDB流触发Lambda函数同步调用及输出去向的问询

1. How is the output of an AWS Lambda function triggered by DynamoDB Streams handled? #

When a Lambda function is triggered by DynamoDB Streams, its return value is automatically discarded by default—there’s no built-in mechanism in the DynamoDB Streams trigger to consume or route this output to another AWS service or system.

If you want the Lambda’s output to be used elsewhere, you’ll need to explicitly add logic within your function to send it to a target of your choice. For example:

• Push the output to an Amazon SQS queue or SNS topic for further processing
• Write it to another DynamoDB table, Amazon S3 bucket, or Amazon RDS database
• Invoke another Lambda function directly with the output data

Without this custom logic, the output simply vanishes after the Lambda execution completes.

2. Why does AWS Lambda use synchronous invocation for DynamoDB Streams, and what happens to the function’s output? #

The choice of synchronous invocation ties directly to how DynamoDB Streams manages record processing and consistency. Here’s why it’s the right fit:

• Ordered processing and checkpointing: DynamoDB Streams process records in order per shard. Synchronous invocation ensures that Lambda completes processing a batch of records (and returns a success signal) before the stream advances its "checkpoint"—the marker for how far it’s processed records. If Lambda used asynchronous invocation, there’s no real-time feedback to the stream about whether the batch was handled successfully, which would break the ordered processing guarantee and lead to duplicate or skipped records.
• Controlled error handling: When Lambda is invoked synchronously, a failure (like an unhandled exception) tells the stream to retry processing that batch (per the configured retry policy, default is 2 retries before sending to a dead-letter queue if set up). Asynchronous invocation’s error handling (like retries with backoff) doesn’t align with the stream’s need to reprocess failed batches immediately to maintain consistency.
• Atomic state updates: The stream’s checkpoint is updated atomically based on Lambda’s synchronous response. Success = checkpoint moves forward; failure = checkpoint stays put, ensuring you don’t lose track of which records need processing.

As for the output, as mentioned earlier—since the synchronous invocation’s primary purpose is to send a success/failure signal to the stream, any business-related output from your Lambda is not consumed by the stream or any default AWS service. You have to build custom logic to route it if needed.

内容的提问来源于stack exchange，提问作者Laurence Gonsalves