HTTP Cache

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

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.

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.

The standard’s specifications are set out in RFC 7234.

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, application/xml, application/html+xml, application/rss+xml, application/rdf+xml, application/atom+xml, 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.

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).