Issue link: https://read.uberflip.com/i/1146286
The Problem with ETags | 91 Entity Tags ETags provide another way to determine whether the component in the browser's cache matches the one on the origin server ("entity" is another word for what I've been calling a "component": images, scripts, stylesheets, etc.). ETags were intro- duced in HTTP/1.1. An ETag is a string that uniquely identifies a specific version of a component. The only format constraint is that the string must be quoted. The ori- gin server specifies the component's ETag using the ETag response header. GET /i/yahoo.gif HTTP/1.1 Host: us.yimg.com HTTP/1.1 200 OK Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT ETag: "10c24bc-4ab-457e1c1f" Content-Length: 1195 ETags were added to provide a more flexible mechanism for validating entities than the last-modified date. If, for example, an entity changes based on the User-Agent or Accept-Language headers, the state of the entity can be reflected in the ETag. Later, if the browser has to validate a component, it uses the If-None-Match header to pass the ETag back to the origin server. If the ETags match, a 304 status code is returned, reducing the response by 1195 bytes. GET /i/yahoo.gif HTTP/1.1 Host: us.yimg.com If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT If-None-Match: "10c24bc-4ab-457e1c1f" HTTP/1.1 304 Not Modified The Problem with ETags The problem with ETags is that they are typically constructed using attributes that make them unique to a specific server hosting a site. ETags won't match when a browser gets the original component from one server and later makes a conditional GET request that goes to a different server—a situation that is all too common on web sites that use a cluster of servers to handle requests. By default, both Apache and IIS embed data in the ETag that dramatically reduces the odds of the validity test succeeding on web sites with multiple servers.