Caching

Caching is the process of storing files on edge servers to improve the response time when serving those files to customers. By storing files closer to users, caching reduces the time it takes for data to travel across the network, commonly known as latency.

By default, CIS caches static files, which include many types of image and text files (non-HTML). The cache includes files only from your websites and doesn't cover third-party resources, such as those from social networking sites. Currently, CIS does not cache based on MIME type.

CIS doesn't cache HTML files by default because they are generally considered dynamic. However, if static HTML can be clearly distinguished from dynamic HTML, it is possible to cache HTML files by using page rules.

Cached files have a specified expiration time, called Time-to-Live (TTL), after which they are purged from the cache. You can also manually purge files from the cache at any time. After files are purged, CIS goes back to your origin server to reload the files and update the cache with the most recent versions.

For detailed information on cache settings and options, see Using page rules with caching.

Default caching behavior

CIS caches static content based on your visitors' locations, the CIS data center they connect to, and how frequently the resource is requested at that data center.

Caching occurs only within the CIS data center that serves the request. The following resources aren't cached by CIS:

Off-site or third-party resources
Content hosted on DNS records that aren't proxied

By default, CIS respects the origin web server’s cache headers unless an edge cache TTL page rule overrides them:

CIS doesn't cache resources if the cache-control header is set to private, no-store, no-cache, or max-age=0, or if a cookie is present in the response.
CIS caches resources if cache-control is public with a max-age greater than 0, or if the expires header specifies a future date.
When both max-age and expires headers are set, max-age takes precedence.

When both the cache-control directive and the expires headers aren't present, CIS caches certain HTTP response codes with the following default edge cache TTL:

Default cache response codes
Response code	TTL
200, 206, 301	120 m
302, 303	20 m
404, 410	3 m
403	1 m

CIS provides several options to customize caching:

Set caching behavior for individual URLs by using CIS page rules.
Use CIS edge functions or the API to customize caching.
Adjust caching levels, TTL, and more through the CIS CLI.

The maximum file size that CIS caches is 512 MB for Trial and Standard Next customers, and 5 GB for Enterprise customers. Enterprise customers can request caching of larger files by opening a support case.

File extensions cached by default

CIS uses file extensions to cache content. The following file extensions are cached automatically:

File extensions cached by default
File extensions
`bmp` `class` `css` `csv` `doc` `docx` `ejs` `eot` `eps`	`gif` `ico` `jar` `jpg` `js` `mid` `midi` `otf` `pdf`	`pict` `pls` `png` `ppt` `pptx` `ps` `svg` `svgz` `swf`	`tif` `tiff` `ttf` `webp` `woff` `woff2` `xls` `xlsx`

CIS doesn't cache content based on MIME type, and by default, it doesn't cache HTML pages. However, it does automatically cache your website's robots.txt file. To cache extra content, such as HTML pages, you can set up custom page rules to control what gets cached.

Serving stale content

Serve stale content keeps a limited version of the site online if the server goes down. Even if the content expires, CIS continues serving cached content to users when origin servers are offline.

Understanding CIS cache responses

The output of the CF-Cache-Status header shows whether a resource is cached.

Cache response codes and definitions
Response code	Definition
`HIT`	The resource was found in the CIS cache.
`MISS`	The resource wasn't found in the CIS cache and was served from the origin web server.
`EXPIRED`	The resource was found in the cache but has since expired and was served from the origin web server.
`STALE`	The resource was served from cache, but is expired. CIS wasn't able to contact the origin to retrieve the updated resource.
`BYPASS`	The origin server instructed CIS to bypass the cache by using a `cache-control` header set to `no-cache`, `private`, or `max-age=0`. `BYPASS` is returned when you enable origin `cache-control`. CIS also sets `BYPASS` when your origin web server sends cookies in the response header.
`REVALIDATED`	The resource is served from cache, but is stale. The resource was revalidated by either an `If-Modified-Since` header or an `If-None-Match` header.
`UPDATING`	The resource was served from the cache, but is expired. The resource is being updated by the origin web server. `UPDATING` is typically seen only for popular cached resources.
`DYNAMIC`	The resource wasn't cached by default and your current CIS caching configuration doesn't instruct CIS to cache the resource. Instead, the resource was requested from the origin web server. Use page rules to implement custom caching options.

Using query string sorting

Enterprise Only: CIS treats URLs with query strings in different orders as separate files in the cache. For example, if one user requests:

/video/123456?title=0&byline=0&portrait=0&color=987654

and another requests:

/video/123456?byline=0&color=987654&portrait=0&title=0

CIS fetches the file from the origin again, even if it’s already cached.

The Query String Sort feature sorts query strings "before" caching, increasing cache hit rates. You can enable Query String Sort by using the toggle on the Caching page.