What is the primary function of S3 CORS?

The primary function of S3 CORS (Cross-Origin Resource Sharing) is to control access from web applications across different origins. This is essential for web applications that need to interact with resources stored in different domains. When a web application is served from one domain and attempts to request resources (like objects stored in an S3 bucket) from another domain, the browser enforces the same-origin policy to prevent unauthorized access.

S3 CORS allows you to specify which origins are permitted to access your S3 resources, which HTTP methods (such as GET, POST, PUT) are allowed, and what headers can be included in the requests. This configuration ensures that only authorized web applications can interact with S3 objects, thereby maintaining the security and integrity of the data.

The other options focus on different aspects of S3’s functionality. Managing lifecycle policies pertains to transitioning and expiring objects in S3, reducing data transfer costs relates to optimizing the costs associated with transferring data in and out of S3, and managing data retention policies involves strategies for retaining or deleting data based on specific criteria. While these are important aspects of S3, they do not pertain to the specific function of CORS, which is exclusively concerned with cross-origin interactions.