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copyright lastupdated keywords subcollection
years
2018, 2024
2024-10-09
cis

{{site.data.keyword.attribute-definition-list}}

Caching concepts

{: #caching-concepts}

This document contains some concepts and definitions that are related to caching and how it affects your {{site.data.keyword.cis_full}} deployment. {: shortdesc}

What is caching?

{: #what-is-caching}

Caching is the process of storing files on the edge servers, which is done to improve the response time to serve those files to customers. By storing the files closer to the customers, you can decrease the time that it takes for the data to stream across the network, which commonly is called the latency.

Cached files have a specified expiration time, called Time-to-live (TTL), after which they are purged from the cache. It is also possible to purge files from the cache manually. After files are removed from the cache, {{site.data.keyword.cis_short_notm}} goes back to your origin server to reload your files and update the cache with the latest versions.

A deeper explanation of the cache settings and your caching options is located in the Caching and Page Rules section.

Cached content

{: #what-content-is-cached}

By default, {{site.data.keyword.cis_short_notm}} caches static files, which include many types of image and text files (non-HTML files). The cache includes only files from your websites and not third-party resources from social networking sites, for instance. Also, {{site.data.keyword.cis_short_notm}} currently does not cache by MIME type.

Caching HTML

{: #how-do-i-cache-html}

{{site.data.keyword.cis_short_notm}} does not cache HTML files by default because they are not considered to be static. However, if static HTML can be clearly distinguished from dynamic HTML it is possible to cache HTML files by using the Page Rules feature.

Default caching behavior

{: #default-cache-behavior}

{{site.data.keyword.cis_short_notm}} caches static content based on where your visitors come from, which {{site.data.keyword.cis_short_notm}} data center your visitors reach, and how often visitors request a resource at the specific data center.

{{site.data.keyword.cis_short_notm}} caches only resources within the {{site.data.keyword.cis_short_notm}} data center that serves the request and doesn’t cache the following resources:

  • Off-site or third-party resources (for example, Facebook and Flickr)
  • Content that is hosted on unproxied DNS records

By default, {{site.data.keyword.cis_short_notm}} respects the origin web server’s cache headers in the following manner unless overridden by an edge cache TTL page rule:

  • If the cache-control header is set to private, no-store, no-cache, or max-age=0, or if a cookie is in the response, then {{site.data.keyword.cis_short_notm}} does not cache the resource.
  • Otherwise, if cache-control is set to public and the max-age is greater than 0, or if the expires header is a date in the future, {{site.data.keyword.cis_short_notm}} caches the resource.
  • If both max-age and an expires header are set, max-age is used.

By default, {{site.data.keyword.cis_short_notm}} caches certain HTTP response codes with the following edge cache TTL when no cache-control directive or expires response header are present:

Response code TTL
200, 206, 301 120 m
302, 303 20 m
404, 410 3 m
403 1 m
{: caption="Default cache response codes" caption-side="bottom"}

{{site.data.keyword.cis_short_notm}} provides several cache customization options:

The maximum file size that {{site.data.keyword.cis_short_notm}} caches is 512 MB for Trial and Standard Next customers and 5 GB for Enterprise customers. Enterprise customers can open a Support case to request caching of larger files. {: tip}

File extensions cached by default

{: #default-file-extensions}

{{site.data.keyword.cis_short_notm}} uses file extensions to cache content. The following file extensions are cached automatically:

  • 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

{{site.data.keyword.cis_short_notm}} does not cache by MIME type, and doesn't cache HTML by default. {{site.data.keyword.cis_short_notm}} does cache a website's robots.txt. You can cache more content by creating page rules.

Understanding {{site.data.keyword.cis_short_notm}} cache responses

{: #understanding-cis-cache-responses}

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

Response code Definition
HIT The resource was found in the {{site.data.keyword.cis_short_notm}} cache.
MISS The resource was not found in the {{site.data.keyword.cis_short_notm}} cache and was served from the origin web server.
EXPIRED The resource was found the cache but has since expired and was served from the origin web server.
STALE The resource was served from cache but is expired. {{site.data.keyword.cis_short_notm}} couldn’t contact the origin to retrieve the updated resource.
BYPASS The origin server instructed {{site.data.keyword.cis_short_notm}} to bypass 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. {{site.data.keyword.cis_short_notm}} 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 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 was not cached by default and your current {{site.data.keyword.cis_short_notm}} caching configuration doesn't instruct {{site.data.keyword.cis_short_notm}} to cache the resource. Instead, the resource was requested from the origin web server. Use page rules to implement custom caching options.
{: caption="Cache response codes and definitions" caption-side="bottom"}

Using query string sorting

{: #query-string-sorting}

Enterprise Only {{site.data.keyword.cis_short_notm}} treats URLs that have query strings in different orders as separate files in the cache. This means that if one user requests:

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

And another user requests:

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

{{site.data.keyword.cis_short_notm}} goes back to the origin, even though we have the file in our cache.

Query String Sort sorts the query strings before they hit our cache, resulting in a higher cache hit rate. Enable Query String Sort by using the toggle in the Caching page.

Serving stale content

{: #serve-stale-content-caching}

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