AWS Certified Solutions Architect – Professional (SAP-C02) — Question 410

A software as a service (SaaS) company has developed a multi-tenant environment. The company uses Amazon DynamoDB tables that the tenants share for the storage layer. The company uses AWS Lambda functions for the application services.

The company wants to offer a tiered subscription model that is based on resource consumption by each tenant. Each tenant is identified by a unique tenant ID that is sent as part of each request to the Lambda functions. The company has created an AWS Cost and Usage Report (AWS CUR) in an AWS account. The company wants to allocate the DynamoDB costs to each tenant to match that tenant's resource consumption.

Which solution will provide a granular view of the DynamoDB cost for each tenant with the LEAST operational effort?

Answer options

• A. Associate a new tag that is named tenant ID with each table in DynamoDB. Activate the tag as a cost allocation tag in the AWS Billing and Cost Management console. Deploy new Lambda function code to log the tenant ID in Amazon CloudWatch Logs. Use the AWS CUR to separate DynamoDB consumption cost for each tenant ID.
• B. Configure the Lambda functions to log the tenant ID and the number of RCUs and WCUs consumed from DynamoDB for each transaction to Amazon CloudWatch Logs. Deploy another Lambda function to calculate the tenant costs by using the logged capacity units and the overall DynamoDB cost from the AWS Cost Explorer API. Create an Amazon EventBridge rule to invoke the calculation Lambda function on a schedule.
• C. Create a new partition key that associates DynamoDB items with individual tenants. Deploy a Lambda function to populate the new column as part of each transaction. Deploy another Lambda function to calculate the tenant costs by using Amazon Athena to calculate the number of tenant items from DynamoDB and the overall DynamoDB cost from the AWS CUR. Create an Amazon EventBridge rule to invoke the calculation Lambda function on a schedule.
• D. Deploy a Lambda function to log the tenant ID, the size of each response, and the duration of the transaction call as custom metrics to Amazon CloudWatch Logs. Use CloudWatch Logs Insights to query the custom metrics for each tenant. Use AWS Pricing Calculator to obtain the overall DynamoDB costs and to calculate the tenant costs.

Correct answer: B

Explanation

Option B is correct because since the DynamoDB tables are shared across multiple tenants, resource-level tagging (Option A) cannot distinguish individual tenant costs within a single table. Logging the exact Read Capacity Units (RCUs) and Write Capacity Units (WCUs) consumed per transaction to CloudWatch Logs provides the exact metrics needed to accurately allocate shared table costs. Other options either use inaccurate proxies like item counts (Option C) or response sizes (Option D), which do not map directly to DynamoDB's pricing model, and the AWS Pricing Calculator is not designed for programmatic billing reconciliation.