The best combination of steps to improve application performance visibility during peak traffic events is ABD. Here's a detailed explanation:

Reasoning for choosing A: Configuring the Aurora MySQL DB cluster to publish slow query and error logs to Amazon CloudWatch Logs is crucial. During the promotional event, Aurora DB cluster performance metrics were insufficient for query performance analysis. Publishing slow query logs to CloudWatch allows for detailed analysis of slow-running queries, which can be a major cause of performance bottlenecks. Error logs provide insights into any database-related errors that might be occurring under heavy load. This directly addresses the issue of insufficient Aurora metrics for query performance analysis. This is the correct choice because it provides detailed database performance insights.

Reasoning for choosing B: Implementing the AWS X-Ray SDK to trace incoming HTTP requests on the EC2 instances and tracing SQL queries with the X-Ray SDK for Java is essential for understanding the flow of requests through the application and identifying performance bottlenecks in the web server layer. The problem description mentions that users reported errors and timeouts while adding items to their shopping carts. X-Ray allows tracing requests from the CloudFront distribution, through the EC2 instances, and to the Aurora database. This end-to-end tracing enables pinpointing exactly where the bottlenecks are occurring. This option is correct because it allows tracing requests and identifying bottlenecks across different components of the application.

Reasoning for choosing D: Installing and configuring an Amazon CloudWatch Logs agent on the EC2 instances to send the Apache logs to CloudWatch Logs is vital for capturing and analyzing web server logs. The operations team recovered logs created by the web servers after the promotional event, but some web servers were terminated before logs could be collected. By having a CloudWatch Logs agent on each EC2 instance, logs are streamed to CloudWatch Logs in real-time, even if the instance is terminated. This ensures that all logs are collected for analysis. These logs contain information about requests, errors, and other relevant data from the Apache web servers. This is the correct choice because it ensures that web server logs are collected and available for analysis, even if instances are terminated.

Reasoning for not choosing C: While streaming slow query and error logs to Amazon Kinesis could be an option, it's generally used for real-time processing and analytics, which is not the primary need described in the question. The main requirement is to improve application performance visibility to understand the causes of the errors and timeouts. CloudWatch Logs offers sufficient capabilities for analyzing database performance issues in this scenario and is a simpler and more cost-effective solution. Kinesis would add unnecessary complexity for the stated needs. Therefore, option C is not the best choice.

Reasoning for not choosing E: Enabling and configuring AWS CloudTrail to collect and analyze application activity from Amazon EC2 and Aurora is not the most effective solution for the specific problem described. CloudTrail primarily focuses on recording API calls made to AWS services for auditing and compliance purposes. While it can provide some insights into the overall activity, it doesn't provide detailed information about application performance, slow queries, or request tracing. The problem description emphasizes the need for detailed performance analysis of the web server and database layers, which CloudTrail doesn't address directly. Thus, option E is not the correct choice.

Reasoning for not choosing F: Enabling Aurora MySQL DB cluster performance benchmarking and publishing the stream to AWS X-Ray is not a standard practice and isn't directly supported by AWS services. While Aurora Performance Insights provides detailed database performance metrics, it does not directly integrate with X-Ray in the manner suggested. Performance Insights is the more appropriate tool for detailed Aurora performance analysis. Moreover, X-Ray is designed for tracing requests across application components, not for ingesting database performance benchmarking data. Therefore, option F is incorrect.

AWS X-Ray, https://aws.amazon.com/xray/
Amazon CloudWatch Logs, https://aws.amazon.com/cloudwatch/features/logs/
Aurora Performance Insights, https://aws.amazon.com/rds/aurora/performance-insights/