Explanation

When documents are uploaded to an S3 bucket and trigger Lambda functions directly, there can be issues with:

1. Concurrency limits - Lambda has default concurrency limits that can be exceeded during traffic spikes
2. Event loss - If Lambda throttling occurs, S3 events can be lost
3. No retry mechanism - Direct S3-to-Lambda invocation doesn't have built-in retry capabilities

Why Option D is correct:

• Amazon SQS provides a buffer between S3 events and Lambda processing
• Messages are persisted in the queue until successfully processed
• Built-in retries - Lambda can retry failed messages
• Decouples producers and consumers - S3 events are sent to the queue, Lambda processes from the queue
• Handles traffic spikes - The queue absorbs bursts of uploads

Why other options are incorrect:

• Option A: Increasing timeout doesn't address the root cause of missed document processing
• Option B: Replication and staging add complexity without solving the event processing reliability issue
• Option C: Adding another Lambda function doesn't solve the fundamental problem of event loss during throttling

Best Practice: Using SQS as a buffer between event sources and Lambda functions is a common pattern for ensuring reliable message processing, especially when dealing with unpredictable traffic patterns like those caused by marketing campaigns.