What is an ETag and how does it work?

An ETag (Entity Tag) is an HTTP response header that helps web browsers determine whether a webpage or file has changed since the last time it was loaded. Instead of downloading the same content again, the browser checks the ETag value and only downloads the file if it has been updated.

How Does It Work?

Whenever a browser visits a website, the server sends an ETag along with the webpage or file.

The next time you visit the same page, your browser sends that ETag back to the server.

• If the ETag matches, the server replies that the content hasn't changed, so the browser uses its cached version.

• If the ETag is different, the server sends the updated content.

This process saves both time and bandwidth.

Why Is It Used?

The main purpose of an ETag is to improve website performance.

Instead of downloading the same images, CSS files, or webpages repeatedly, the browser only downloads files that have actually changed. This makes websites load faster and reduces unnecessary server requests.

Imagine you visit a news website every morning.

The website logo hasn't changed, so your browser doesn't need to download it again. It simply checks the ETag, confirms the file is unchanged, and loads it from the cache. However, the latest news articles have changed, so those are downloaded again.

Types

There are two common types of ETags:

• Strong ETags: Detect every change in a file.

• Weak ETags: Allow minor differences while treating the content as essentially the same.

Advantages & Limitations

Advantages

• Faster page loading.

• Reduced bandwidth usage.

• Better user experience.

• Improved server efficiency.

Limitations

• Can create caching issues if not configured properly.

• May behave differently across multiple servers if ETags aren't synchronized.

ETag vs Last-Modified

Both help browsers check whether content has changed.

• ETag compares a unique identifier.

• Last-Modified compares the file's modification date.

ETags are generally more accurate because they detect content changes even when timestamps don't change.

Best Practices

• Enable ETags for static resources whenever appropriate.

• Combine ETags with proper cache-control headers.

• Test caching behavior after website updates.

ETag at a Glance

An ETag is a simple but powerful caching mechanism that helps browsers avoid downloading unchanged files. By reducing unnecessary requests, it improves website speed, saves bandwidth, and creates a smoother browsing experience for users.

Must Read: What is HTTP-2?

The Entity Tag or ETag is the HTTP Header which is used for the validation of web cache and for the conditional requests from browsers for resources. They can be also referred to as identification elements that have been tagged to the user’s browser. The user is allowed to remove HTTP cookies and they store the information which is same along with backup if in case there is a need to reconstitute the data of deleted cookies.

Working of the ETag is explained below:
I. Firstly, Server receives a normal HTTP request for a particular source, for example, ABC.
II. Server side will now prepare the response. The server-side logic wants the browser to cache ABC locally. All the browsers cache the resources always by default.
III. The server will include the header ETag with its value.
IV. The response will now be sent from the server to the header, content of ABC in the body with status code 200.
V. Then, the same browser will make another request for the same resource ABC.