ETag
An ETag (entity tag) is an HTTP header that serves as a unique identifier for a specific version of a resource. It is a crucial mechanism for efficient cache validation. When a browser needs to check if a cached file is still fresh, it can send the file's ETag to the server. If the version on the server is identical, the server can respond with a lightweight 304 Not Modified status, telling the browser to use its cached copy without needing to re-download the entire file.
Related web performance terms
- EXIF (Exchangeable Image File Format)
EXIF is metadata embedded within image files (like JPEGs) that contains information such as the camera settings, date, time, and sometimes GPS location where the photo was taken.
- Early Hints (HTTP 103)
Early Hints (HTTP 103 response status) is a mechanism that allows the server to send resource hints to the browser before the final HTML response is ready.
- Edge Cache (Fastly)
The Edge Cache is Fastly's distributed edge caching infrastructure that serves cached content from Fastly's Points of Presence (PoPs) around the world.
- Element Timing
Element Timing is a browser API that allows you to measure when specific elements on a page are rendered. This is particularly useful for creating custom performance metrics when Google's standard…