Understanding Caching in AWS CloudFront

AWS CloudFront optimizes content delivery through its network of edge locations, each maintaining its own cache. Here's a succinct overview of how CloudFront caching works and the strategies involved in maximizing cache effectiveness:

CloudFront Caching Mechanism

• Edge Locations and Cache Storage: Each CloudFront edge location holds a cache. The number of caches corresponds to the number of edge locations.
• Cache Key: Objects in the cache are identified by a Cache Key, usually consisting of the host name and the resource portion of the URL.
• Cache Operation: When a request is made, the edge location checks if the object is cached and still valid based on its Time to Live (TTL). If not, the request is forwarded to the origin server, and the returned response is cached.

Cache Key Complexity and Cache Policies

• Default Cache Key Components: By default, a Cache Key includes the host name and resource path from the URL.
• Enhanced Cache Key: For more complex scenarios where content varies by user, device, language, etc., additional information like HTTP headers, cookies, or query strings can be incorporated into the Cache Key.
• Cache Policy Configuration: Users can define what to include in the Cache Key through a CloudFront cache policy, specifying the handling of HTTP headers, cookies, and query strings.

Optimization and Invalidation

• Maximizing Cache Hit Ratio: To reduce requests to the origin server and enhance performance, it's crucial to cache as much content as possible at the edge locations.
• Invalidation: If necessary, items can be forcibly removed from the cache before their TTL expires through an invalidation process.

Origin Request Policy vs. Cache Key

• Origin Request Policy: Allows inclusion of additional headers, cookies, or query strings in requests sent to the origin, but these are not used in the Cache Key.
• Distinction from Cache Key: While the Cache Key determines what is stored in the cache, the origin request policy defines what is sent to the origin, irrespective of caching decisions.

This overview should clarify the strategic use of caching in CloudFront, emphasizing the importance of cache key configuration and the roles of cache and origin request policies in efficient content delivery.