What isETag

An ETag is an HTTP header used for web cache validation and conditional requests from browsers for resources. It is a persistent identifier assigned to a user’s browser that works similarly to the “last-modified” HTTP response header. ETags use content hashing like the MD5 hash generator to allow the requestor to verify that the last response matches the previous one. ETags are assigned to each resource on a page at any URL and can be quickly compared to determine if two versions of a resource are the same or different. ETags are used with the “cache-control HTTP response header” to determine the cache lifetime for a resource.

FAQs on Entity Tag (ETag) for Web Cache Validation and Conditional Requests

What is an Entity Tag (ETag)?

An Entity Tag (ETag) is an HTTP header used for web cache validation and conditional requests from browsers for resources. ETags use persistent identifiers (PIE) assigned to the user’s browser. The ETag is an opaque identifier that the web server assigns to each resource on the page at any URL.

What is the Difference Between “last-modified” and ETag Response Headers?

ETags work similarly to the “last-modified” HTTP response header. The main difference between the two is that ETag uses content hashing like the MD5 hash generator. This allows the requestor to verify that the last response matches the previous one. In this way, ETags are similar to fingerprints, and they can be quickly compared to determine whether two versions of a resource are the same or different.

How is ETag Used with Cache-Control Response Header?

ETag is used with the “cache-control HTTP response header”. The cache-control response header is used to determine the cache lifetime for a resource. Cache-control is used with the “max-age” and “public” or “private” values. If the value of the cache-control-response header is not clear to the requestor (client), the client can check the ETag value for comparing the resources so that the refreshed version of the web page can be displayed in the browser with refreshed resources.

Can ETags Be Used to Restore Data of Deleted Cookies?

Although a user can remove HTTP cookies, ETags store the same information along with a hidden backup to restore the data of deleted cookies. ETags, unlike cookies, cannot be deleted by the user. Therefore, ETags can be used to restore data even in situations where cookies are deleted.

The gist

Entity Tag (ETag) is a useful HTTP header that enables web cache validation and allows clients to request resources conditionally from the server. ETags use content hashing to identify resources uniquely and can be used to compare two versions of a resource. ETag and cache-control response headers are used to determine the cache lifetime and freshness for a resource. ETags are especially useful for refreshing resources in web pages to ensure that the updated version is displayed in the client’s browser.

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