A new feature should be available only to a specific group of premium customers. A developer needs to be able to toggle the feature on and off rapidly (for example, in response to performance or feedback) and validate changes before deploying, without causing user disruption. What should the developer use?

AWS AppConfig is the correct choice because it is specifically designed for managing and deploying application configurations, including feature flags. It allows for rapid toggling of features, validation of configurations before deployment, and controlled rollouts, minimizing user disruption. This aligns perfectly with the requirement to enable a feature for a specific group of premium customers and rapidly toggle it. AWS Secrets Manager is for storing sensitive information like API keys or database credentials, not for general application configurations or feature flags. AWS Config is used for assessing, auditing, and evaluating the configurations of your AWS resources, not for application-level feature toggling. AWS Systems Manager Parameter Store can store configuration data, but it lacks the advanced validation, controlled deployment, and rapid toggling capabilities that AppConfig offers for feature flags.