What is ETag? Benefits & Characteristics

ETag

In web development, managing requests between servers and browsers is crucial to performance and user experience. One of the tools used in this process is the ETag.

Around 25% of responses across the top websites utilize the ETag header, based on a 2023-02-01 HTTP Archive study that crawled over 1 billion resources.

It's an important piece of technology that helps ensure the data sent to users is fresh and efficient.

What is ETag?

An ETag (short for Entity Tag) is an identifier assigned by a web server to a specific version of a resource. It's part of the HTTP headers and acts like a fingerprint for files.

When you load a web page, your browser requests the server for the page's content. In response, the server sends the requested data along with an ETag header.

Think of an ETag as a version tracker. It allows the browser to check if the content has changed since the last visit. This is extremely helpful for avoiding unnecessary data transfers and optimizing web performance.

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How ETags Are Generated

When a server creates an ETag for a resource, it generates a unique identifier based on the file's contents, the timestamp, or a combination of both. The generation of ETags can vary depending on the server’s configuration. Typically, there are two main approaches:

Weak ETags: These ETags check for major changes to the content. They are useful when small changes (like metadata updates) aren’t significant for reloading the whole file.
Strong ETags: These ETags represent the entire file, meaning even a small change will result in a new ETag.

The ETag header looks something like this in HTTP responses:

ETag: "686897696a7c876b7e"

This value will be unique for the resource and will change only if the resource changes.

Common Etag Misconfigurations

While generation is smooth, there are times when Etags are not properly set up to achieve their desired purpose. Here’s how you can fix common scenarios:

Misconfiguration	Issue Caused	Recommended Solution
ETag based on timestamp only	Cache invalidated unnecessarily	Use content-based ETag instead
Using strong ETags for cache static content	Leads to frequent revalidations	Use weak ETags for static assets
Not disabling ETags on dynamic pages	Security and tracking risks	Disable ETags for sensitive pages like login forms
Lack of ETag configuration in CDNs	Stale content served across edge servers	Configure CDN to honor origin ETag updates

Types of ETags

As mentioned earlier, there are two main types of ETags:

1. Weak ETags

These are less sensitive to minor changes. They’re ideal when small tweaks don't significantly affect the overall content. Weak ETags are denoted with a W/ prefix in the HTTP header. For example:

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ETag: W/"686897696a7c876b7e"

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Weak ETag Type	Description	Use Case	Pros	Cons
Timestamp-Based Weak ETag	ETag generated using the resource's last modification timestamp	Static resources with minor updates	Easy to implement, less computational overhead	Can cause unnecessary cache invalidation for trivial changes
Metadata-Based Weak ETag	ETag generated based on metadata (e.g., file size, permissions)	For resources where metadata changes but content remains stable	Lightweight and efficient	Doesn’t detect content changes accurately
Combined Hash Weak ETag	Weak ETag created using a combination of content hash and timestamp	Resources where only significant updates matter	Efficient for low-frequency updates	May not detect smaller content changes
Application-Defined Weak ETag	Custom weak ETag based on application logic, such as content blocks	Dynamic websites with partial updates (e.g., ads or banners)	Customizable and flexible	Requires careful configuration

2. Strong ETags

These are highly sensitive and require that even the smallest change in content will trigger a new ETag. Strong ETags are used when data accuracy is crucial, such as with frequently updated pages.

Understanding the distinction between these types is important, as it can impact both the performance and the integrity of the data your users receive.

Strong ETag Type	Description	Use Case	Pros	Cons
Content-Based Hash Strong ETag	ETag generated from a hash of the entire resource content	Perfect for static resources where accuracy is critical	Ensures accurate cache validation	Computationally expensive for large files
Byte-Level ETag	Strong ETag generated using the precise byte content of the resource	Large files with frequent minor updates (e.g., media files)	Extremely accurate for detecting any content change	Increased CPU usage and slower to generate
Version-Controlled ETag	Strong ETag created using a version number or unique ID from version control	Versioned assets (e.g., APIs, software updates)	High precision, easy to track changes	Requires integration with versioning systems
Composite Strong ETag	ETag created using a combination of file size, content hash, and metadata	Resources that are frequently updated but need strict validation	Offers both accuracy and flexibility	Complex to configure and maintain

How ETags Work in CDNs

CDNs use ETags to reduce latency and improve load times by caching resources on their edge servers. When a resource is cached, its associated ETag is stored with it.

Each time a user requests that resource, the CDN checks its cache:

Check for Cached Content: The CDN first verifies if the resource is stored in its cache.
Validate with ETag: If cached, the CDN compares the stored ETag with the origin server’s ETag to ensure the content is up-to-date.

CDN Responses Based on ETag Validation

The CDN responds in one of two ways:

ETag Matches (Fresh Content): If the ETag matches, the CDN serves the cached content without bothering the origin server. This saves bandwidth and reduces server load.
ETag Mismatch (Stale Content): If the ETag has changed, the CDN fetches the updated content from the origin server, caches it with the new ETag, and serves it to the user.

ETags and Cache Invalidation in CDNs

When content changes, the ETag on the origin server updates, marking the old cached version as stale. This triggers cache invalidation, and the CDN retrieves the updated resource.

ETags automate cache invalidation, ensuring users receive fresh content without manual intervention.

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CDN Optimization with ETags

Granular Cache Control: ETags ensure that only changed content is updated in the cache, maintaining efficient resource management.
Edge Server Efficiency: ETags help ensure consistency across all CDN edge servers, preventing outdated content from being served.
Bandwidth Savings: ETags reduce unnecessary data transfers by validating content freshness, minimizing origin server requests and bandwidth use.

ETag Handling in Distributed CDNs

In distributed CDNs with multiple data centers, each edge server relies on ETags to ensure they serve the latest content.

This ensures users receive the freshest version, regardless of location, while maintaining redundancy in the system to prevent disruptions.

How Browsers Handle ETags

When a browser interacts with a server to fetch content, ETags play a significant role in determining whether the content needs to be re-downloaded or if the cached version can be used.

1. Storing the ETag

When a browser makes an HTTP request to a server for a resource (such as an image, JavaScript file, or CSS file), the server responds with the requested content.

If the server is configured to use ETags, the response will include an ETag header. The browser saves both the content and the ETag for future requests.

For example, the response might include a line like:

ETag: "abc123"

The browser now knows that this ETag corresponds to the current version of the resource. The ETag gets stored in the browser’s cache alongside the actual content.

2. Checking the ETag for Future Requests

When the user revisits the website or reloads the page, the browser wants to know if the resource it has cached is still up-to-date.

Instead of downloading the entire file again, the browser sends a request to the server with the saved ETag using the If-None-Match HTTP header. This tells the server, "Here’s the version I currently have. Let me know if it’s still valid."

A typical request might look like this:

GET /style.css HTTP/1.1

If-None-Match: "abc123"

3. Server Response: 304 Not Modified

When the server receives the request with the If-None-Match header, it checks the ETag value against the current version of the resource on the server.

If the content hasn’t changed, the server sends back a 304 Not Modified HTTP response code, which means, "The file hasn’t changed; you can use the cached version." This way, the browser avoids re-downloading the file, saving bandwidth and speeding up the page load.

If the content has changed, the server will send the updated file along with a new ETag. The browser will update its cache with the new content and ETag.

Gavin from InfoQ has conducted an online experiment, showcasing how returning a 304 Not Modified with an empty body can be a great way of reducing bandwidth and computation.

4. ETags and Browser Caching Limits

It’s important to note that browsers have limits on how long they store ETags and other cached resources. These limits vary by browser, but generally, ETags are stored until the cache is cleared, either manually by the user or automatically by the browser when the cache reaches its storage limit.

Browsers also handle security concerns, like preventing sensitive or dynamic content from being cached with ETags. Developers need to be mindful when configuring ETags to ensure that user privacy and security aren’t compromised by caching mechanisms.

Benefits of Using ETags in CDNs

Using ETags in CDNs comes with several benefits:

Improved Load Times: When ETags are used effectively, users get cached versions of files if they haven't changed, leading to faster loading times. The CDN doesn't have to pull the resource from the origin server every time, only when necessary.
Reduced Bandwidth Usage: By caching resources and validating them with ETags, unnecessary data transfers are avoided. This minimizes the amount of bandwidth used, particularly when delivering large assets like images, videos, or scripts.
Efficient Caching: ETags provide a reliable mechanism to check whether cached files are up to date. If the file hasn't changed, the CDN delivers the cached version. If it has, the CDN fetches the latest version from the origin server.
Cost Efficiency: Reducing bandwidth and server load with ETag headers can save money, especially for websites that experience heavy traffic. Less server load means fewer resources used, which can translate to lower costs for infrastructure.

ETags and Security

While ETag headers are generally a good thing for performance, they can have security implications. For example, ETags in login systems can sometimes allow user sessions to be tracked in ways that weren’t intended. Attackers can potentially use ETags to track users across different sites or during a browsing session.

To mitigate these risks, developers should ensure that ETag usage doesn’t expose sensitive data or allow tracking. Disabling ETags for certain types of content, such as dynamic or sensitive pages (like login systems), is a common practice to ensure security while still benefiting from ETags on less critical resources.

Security-Focused ETags (In-Between Weak and Strong)

ETag Type	Description	Use Case	Security Features	Pros	Cons
Session-Based ETag	ETag generated based on a user session or token	Dynamic pages, personalized content (e.g., user dashboards, account pages)	Prevents cache sharing across sessions or users	Limits exposure to cross-session data	Requires more resources for session tracking
IP-Hashed ETag	ETag created using a combination of user’s IP address and content hash	Resources served to individual users, particularly in secure environments	Limits cache reuse between different IP addresses	Provides better isolation between users	Breaks caching for users with dynamic IPs
Role-Based ETag	ETag customized based on the user’s role or permission level	Multi-user applications with different access controls (e.g., admin vs. user views)	Ensures only authorized users get the right cached content	Enhances security for content segmentation	More complex to generate and manage
Encrypted ETag	ETag generated using encrypted metadata or encrypted hash of content	High-security environments where content is sensitive (e.g., financial or medical data)	Adds a layer of protection to prevent malicious manipulation of ETags	Provides strong data integrity and confidentiality	Increased computational overhead
Tokenized ETag	ETag created using authentication tokens (e.g., JWTs) combined with content hash	API responses, user-authenticated content delivery	Ties the cache validation to user authentication	Prevents unauthorized access to cached data	Dependency on token validity and token management

Characteristics of Security-Based ETags:

Session Awareness: Many security-based ETags take user sessions into account, ensuring that cached content isn't shared across different sessions or users. This prevents potential data leaks.
IP or Role Sensitivity: Some ETags may include user-specific data, such as IP addresses or user roles, to ensure that the cached resource is only valid for that particular user or role, enhancing security in environments where sensitive data is involved.
Encryption: Encrypted ETags ensure that even if an ETag is intercepted, it cannot be tampered with or used to infer sensitive information, which is essential for secure applications.

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Published on:

October 14, 2024