TTL and Invalidations

To address this challenge, CloudFront employs the concept of Time-to-Live (TTL).

Time-to-Live (TTL)

Time-to-Live (TTL) refers to the duration for which an object is considered valid and cached at edge locations. When an object’s TTL expires, CloudFront initiates an origin fetch to retrieve the latest version of the object from the origin server. If the object has been updated, CloudFront fetches the updated content from the origin server and refreshes its cache. If the object hasn’t been updated, CloudFront continues serving the cached content. This ensures that users receive up-to-date content, promoting a seamless browsing experience.

CloudFront sets a TTL of 24 hours for cached objects by default. However, users can customize TTL values to align with their specific caching requirements.

• Maximum TTL: The maximum TTL represents the upper limit for the TTL value that can be assigned to cached objects. It ensures that even if an object is not accessed frequently, it remains cached for a predetermined maximum duration, optimizing cache efficiency. Setting a higher maximum TTL can benefit content that rarely changes or has a long shelf life.

• Minimum TTL: The minimum TTL defines the lower limit for the TTL value assigned to cached objects. It ensures that objects remain cached for at least a specified duration, even if their TTL is set to expire sooner. The minimum TTL helps maintain consistency in caching behavior and prevents frequently accessed objects from being evicted prematurely.

In addition to this, CloudFront honors TTL directives specified by Cache-Control and Expires headers, allowing origin servers to exert control over caching behavior.

• Cache-Control header: The Cache-Control header includes directives instructing CloudFront on caching and serving content. Directives such as “max-age” specify the maximum amount of time (in seconds) that an object should be considered fresh and cached. CloudFront interprets this directive to set the TTL for cached objects accordingly.

• Expires header: The Expires header specifies an exact date and time when an object’s cached version should be considered stale and expire. CloudFront uses this information to determine the TTL for the cached object, ensuring it remains cached until the specified expiration date and time.

By adhering to TTL directives specified in Cache-Control and Expires headers, CloudFront enables origin servers to precisely control caching behavior. This granular control empowers organizations to optimize content delivery strategies and enhance user experiences by ensuring timely and efficient access to cached content.

Cache invalidation

While TTL mechanisms help prevent the serving of stale content, there are scenarios where immediate invalidation of cached objects is necessary. Cache invalidation allows users to expire cached objects immediately, regardless of their TTL, ensuring they receive the latest content without delay. In CloudFront, cache invalidations are performed at the distribution level and apply to all edge locations within that distribution.

It’s important to note that cache invalidations incur a cost and may take time to propagate across all edge locations. Therefore, they should be used judiciously and typically reserved for scenarios where immediate updates are critical, such as correcting errors or pushing time-sensitive content changes. CloudFront supports flexible invalidation patterns, allowing users to specify wildcard patterns to invalidate multiple objects based on path patterns.

Enhancing cache management with versioning

While cache invalidations are effective for immediate content updates, they can be resource-intensive and unsuitable for scenarios requiring frequent updates. In such cases, adopting a versioning approach can streamline content management and reduce reliance on cache invalidations.

Versioning involves appending version identifiers to filenames, such as “v1”, “v2”, and so forth. Each versioned file represents a distinct resource, enabling seamless updates without the need for continuous cache invalidations. This approach enhances logging and access management by clearly delineating between different versions of content.

By embracing versioning, organizations can optimize content delivery and maintenance processes. Applications can effortlessly point to the latest versioned file, eliminating the need for frequent cache invalidations and ensuring efficient content delivery without compromising performance.