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Andreas Bergström
Andreas Bergström

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Understanding Cache-Control and ETag for efficient web caching

Web performance is essential in delivering a seamless user experience. One way to boost this performance is by implementing efficient caching strategies. Among numerous techniques available, two HTTP headers that play a central role in this endeavor are Cache-Control and ETag. These headers can help your website load faster by minimizing unnecessary network requests while ensuring that your users always see the most up-to-date content.

Cache-Control: Mastering the Basics

The Cache-Control header, as its name suggests, dictates how HTTP responses should be cached by browsers and other intermediate caches. It uses various directives to control the caching behavior:

  • max-age: Specifies how long (in seconds) a resource is considered fresh. After this time, the cache deems the resource stale and tries to re-fetch it.
  • no-cache: The cache must validate the stored copy with the server before serving it to the client, even if it's not yet stale.
  • no-store: The response should not be stored in any cache.
  • public and private: Define whether the response can be cached in a public cache (like a CDN) or only in a private cache (like a user's browser).
  • must-revalidate: Once the resource is stale, the cache must not use its stored copy without successful validation with the server.

For instance, Cache-Control: max-age=3600 instructs the client to consider the cached resource fresh for one hour. After that, the cache should consider the resource stale and try to re-fetch it.

ETag: The Freshness Validator

While Cache-Control provides general caching directives, ETag (Entity Tag) works as a validator, ensuring the freshness of a cached resource. An ETag is a unique identifier for a specific version of a resource. The server includes this tag in its response, and the client stores this ETag value along with the cached resource.

When the client needs the same resource again, it sends the stored ETag value in an If-None-Match header. The server then compares the current ETag value of the resource with the client's value. If they match, the client's cached version is still fresh, and the server returns a 304 Not Modified status, allowing the client to use its cached copy. If they don't match, the server sends the new resource with a new ETag.

Using Cache-Control and ETag together

Cache-Control and ETag often work together to effectively manage caching. Here's an example of how:

HTTP/1.1 200 OK
Cache-Control: max-age=3600
ETag: "12345"
Content-Type: text/html

In this scenario, `Cache-Control` instructs the client to cache the response and consider it fresh for one hour. Meanwhile, the `ETag` header is provided for validation in case the resource needs to be re-fetched after that hour.
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Benefits of Using Both Cache-Control and ETag

  1. Optimal Efficiency and Freshness:
    By combining the expiration-based caching mechanism of Cache-Control with the validation-based approach of ETag, developers can ensure that a client doesn't even attempt to re-fetch a resource until necessary, but when it does, it's validated for freshness.

  2. Reduced Server Load: With Cache-Control set to a specific max-age, requests for a resource don't hit the server until that age is reached. This means fewer requests overall, reducing the server's load. However, once that age is reached or if must-revalidate is set, having ETag ensures that a full resource doesn't have to be resent if nothing has changed.

  3. Bandwidth Efficiency: By setting an ETag, if the resource hasn't changed when validation is sought, the server can send back a 304 Not Modified without sending the actual resource data again, thus saving bandwidth.

  4. Granular Control Over Caching: Using Cache-Control, you can define caching rules for different types of resources. For instance, images and stylesheets that change infrequently can have a longer max-age, while dynamic content might have a shorter one or even a no-cache directive. This granularity ensures optimal user experience and server performance.

Why not just use ETag and skip cache-control?

http headers showing etag and cache-control
Combining both ETag and Cache-Control allows for the most efficient caching possible.

Certainly, you can use only ETag for cache validation, but there are caveats:

  1. Increased Server Requests: Without Cache-Control, clients might check with the server more frequently for resource validation. Even if the server is only returning 304 Not Modified statuses, it still means increased requests to handle and more round trips, causing slight delays.

  2. Missed Optimization Opportunities: Cache-Control directives like immutable can be invaluable for resources you're certain won't change, like versioned files. Without using these directives, you could be missing out on optimal caching behaviors.

  3. Lack of Control Over Where Content is Cached: Cache-Control allows you to specify whether a resource can be cached publicly (e.g., by CDNs) or only privately (e.g., by a user's browser). Without it, you lose this specificity.

  4. Potential Over-Reliance on Validation: If you only rely on ETag, you're effectively forcing a check with the server every time a resource is requested, which might not always be necessary. It's like having a conversation where you continuously ask, "Has this changed?" instead of being told, "This won't change for the next hour."

In summary, while ETag alone offers a robust mechanism to ensure the freshness of cached content, combining it with Cache-Control provides a more comprehensive and efficient caching strategy. It's the synergy between the "how long to cache" of Cache-Control and the "has this changed" of ETag that delivers the best results in web performance.

Love the cache

Cache-Control and ETag are instrumental in efficient web caching. Understanding and implementing these headers can drastically improve your website's load time and performance. While Cache-Control sets the caching guidelines, ETag validates the cached resources' freshness, ensuring that your users always get the latest and fastest content. By properly leveraging these two, developers can deliver a smoother and more efficient user experience.

Top comments (1)

didof profile image
Francesco Di Donato

Very well explained and straight to the point. Bravo!