Understanding Cache-Control and ETag for efficient web caching
Tags: http, webdev, webperf, caching
Published: 2023-08-10 07:49:17

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
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.
```

## 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](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/i4dl969kcoehmqjtetmk.png)<figcaption>Combining both ETag and Cache-Control allows for the most efficient caching possible.</figcaption>

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.