---
title: HTTP Cache
eleventyNavigation:
  key: HTTP Cache
  parent: Customizing
  order: 5
---

The HTTP cache temporarily stores web documents (e.g. HTML pages, CCS documents, images) to reduce the latency induced by the server when it needs to serve up a page and/or reduce its workload.

- [Activate HTTP cache on WordPress](/en/docs/web-hosting/sites/customizing/activate-http-cache-on-wordpress)

## Concept

When a user attempts to access a page, the corresponding web server will generate a page and send it over the network. Then the cache intercepts the response to store it in its local memory before serving it up to the user.

![Caching a resource when it is requested](images/http-cache_part-1.png)

When a request for the same page is sent by the same or another user, the cache will deliver it as it now has a copy of the requested resource. The web server will no longer be queried.

![Returning a resource cached previously](images/http-cache_part-2.png)

The standard's specifications are set out in [RFC 7234](https://tools.ietf.org/html/rfc7234).

## Use the HTTP cache

### 1. Check that your application manages a cache

For the cache to query the upstream to determine if the requested resource has been modified, the application **must** provide the `Etag` and/or `Last-Modified` header.

A response **CANNOT** be cached if:
- the `Vary` header is `*`,
- the `Content-Type` header is not present,
- the resource `Content-Type` is not one of the following values:  `text/html`, `text/xml`, `text/plain`, `application/xml`, `application/html+xml`, `application/rss+xml`, `application/rdf+xml`, `application/atom+xml`, `text/css`, `text/javascript` ;
- the `Cache-Control` header takes one of the following values: `private`, `no-store`, `no-cache`, `no-transform` ;
- the `Set-Cookie` header is present,
- the `Authorization` header exists, but `Cache-Control` takes none of the following values: `public`, `must-revalidate`, `proxy-revalidate`, `s-maxage` ;
- The *HTTP status code* is not one of the following: 200, 203, 204, 206, 300, 301, 404, 405, 410, 414, 501.

### 2. Activate the HTTP cache

This is done in **Web > Sites > Edit the [site] - ⚙️ > Cache**.

![](images/admin-panel_add-site-cache.png)

### Using `PURGE`

`PURGE` can be executed in three different ways at alwaysdata:

1. by using the full URL of the resource (e.g. `https://test.alwaysdata.net/foo/bar`). This will remove the related cache entry and its variations (generated by the `Vary` header);
2. by adding the `X-Cache-Purge-Match: wildcard` header and adding a wildcard to your URL (e.g. `https://test.alwaysdata.net/*`). This will remove all entries matching the URL template;
3. adding the `X-Cache-Purge-Match: startswith` header and adding a partial path to your URL (e.g. `https://test.alwaysdata.net/foo`). This will remove all entries matching the URL template (and thus `https://test.alwaysdata.net/foo/bar`).

> [!NOTE]
> While the HTTP cache is appropriate in most cases, you can also [run Varnish](/en/docs/web-hosting/sites/http-servers/user-program) on your alwaysdata account.


> Icons: The Noun Project