An AWS Lambda event source mapping is the essential component that connects a stream-based event source, such as an Amazon DynamoDB stream, to a Lambda function. This mapping is a resource managed by the Lambda service that reads records from the stream and invokes the target function synchronously with a batch of items. Although DynamoDB Streams are enabled on the table and the function's execution role has permissions, the Lambda service will not poll the stream and invoke the function until this explicit mapping is created. This is the missing step required to establish the trigger.

A. The StreamViewType parameter determines the content of the stream records (e.g., new image, old image). It does not affect the invocation mechanism itself.

C. Amazon DynamoDB Streams cannot be directly mapped to an Amazon SNS topic. This is not a supported integration pattern for triggering the function.

D. The function timeout setting is relevant only after a function has been invoked. The problem described is a failure to invoke, not a failure during execution.

1. AWS Lambda Developer Guide: In the section "Using AWS Lambda with Amazon DynamoDB

" it states

"To associate a function with a stream from Amazon DynamoDB

you first create a Lambda function... Then

you create an event source mapping to tell Lambda to send records from your stream to the Lambda function."

Source: AWS Lambda Developer Guide

"Using AWS Lambda with Amazon DynamoDB

" section on "Event source mapping."

2. Amazon DynamoDB Developer Guide: The documentation on triggers explains the architecture: "AWS Lambda polls the shard for records and invokes your Lambda function synchronously when records are detected in the shard. The event source mapping passes the stream records to the Lambda function as an event payload."

Source: Amazon DynamoDB Developer Guide

"Processing new items with DynamoDB Streams and Lambda

" section on "DynamoDB Streams and AWS Lambda Triggers."

3. AWS Lambda API Reference: The CreateEventSourceMapping API action is used to create the trigger. Its description confirms its purpose: "Creates a mapping between an event source and an AWS Lambda function. Lambda reads items from the event source and invokes the function."

Source: AWS Lambda API Reference

CreateEventSourceMapping action documentation.