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HTTP ETags - Entity Tags

What are ETags

HTTP ETags (Entity Tags) are a mechanism for validating cached content in the HTTP protocol. ETags are essentially a string of characters that is assigned to a specific resource, such as a web page or an image, by a server. The idea behind ETags is to provide a way for the client (such as a web browser) to verify whether a cached version of a resource is still up-to-date, without having to request the entire resource from the server.

Example of use

When a client requests a resource from a server, the server can send back an ETag header along with the response. This header contains the ETag value for the resource. The client can then cache the resource and store the ETag value for later use.

The next time the client requests the same resource, it can send an "If-None-Match" header along with the request, containing the ETag value from the previous response. The server can then compare the ETag value in the "If-None-Match" header with the current ETag value for the resource. If the values match, it means that the cached version of the resource is still up-to-date, and the server can respond with a "304 Not Modified" status code, indicating that the client can use its cached version of the resource.

If the ETag values do not match, it means that the resource has been updated on the server, and the server will send a full response with the updated resource and a new ETag value.

Here is an example of how HTTP ETags are implemented in the CodeNOW demo application - Using ETag in Demo Reservation Application.

Benefits of using ETags

ETags are an efficient way to reduce the amount of data transmitted over the network, as clients can often use their cached versions of resources instead of requesting the entire resource from the server each time. This can result in significant performance improvements, especially for large or frequently-accessed resources.

Strong vs. Weak ETags

There are two types of ETags: strong and weak. Strong ETags are considered to be a definitive representation of a resource, meaning that two resources with the same strong ETag value are guaranteed to be exactly the same. Weak ETags, on the other hand, are considered to be a representation of the resource at a specific point in time, and may change even if the resource itself has not changed.


In conclusion, HTTP ETags are an important mechanism for optimizing the performance and efficiency of HTTP-based applications. By enabling clients to cache resources and validate their freshness, ETags can help reduce network traffic and improve response times.