About Cost Anomaly Detection (CAD)

AWS Cost Anomaly Detection (CAD) uses machine learning to identify unusual spending patterns in your AWS account. The expected spend is calculated based on historical usage patterns and trends. In your specific case, there could be a few reasons why the anomaly wasn't detected:

1. Time delay: CAD runs approximately three times a day, but there can be a delay of up to 24 hours in detecting anomalies after usage occurs. This is because CAD uses data from Cost Explorer, which itself has a delay of up to 24 hours.

2. Historical data: For new services or accounts, CAD needs sufficient historical data to establish a baseline. It typically requires about 10 days of historical service usage data before it can start detecting anomalies effectively.

3. Threshold sensitivity: The 10% threshold you've set might not be sensitive enough to capture this specific spending pattern. CAD evaluates spend patterns over time, and if the increase happened gradually or if there were similar spending patterns in the past, it might not trigger an alert.

4. Anomaly definition: CAD looks for unexpected spikes or changes in spending patterns. If the increase from $150 to $300 happened gradually or if there have been similar fluctuations in the past, CAD might not consider it anomalous.

5. Monitor configuration: Ensure that your monitor is set up correctly to cover all relevant AWS services and accounts.

To improve anomaly detection in your case, you could try adjusting the threshold to be more sensitive, or create more specific monitors for services or accounts where you want closer monitoring. You might also want to review your CAD settings and ensure they align with your specific needs and spending patterns.

Remember that CAD is a tool to assist in identifying unusual patterns, but it may not catch every fluctuation, especially if the change doesn't significantly deviate from historical patterns or if it happens gradually over time.

Sources
Detecting unusual spend with AWS Cost Anomaly Detection - AWS Cost Management
Getting started with AWS Cost Anomaly Detection - AWS Cost Management
AWS Cost Anomaly Detection - Amazon Web Services