Content is stale if it has expired in cache. CDNs don't hold objects in cache forever: an object has a TTL (Time To Live) and once the TTL is zero, the object has expired and is stale. The CDN must contact the origin first before sending the object to clients.
Imagine the CDN gets a request for /images/logo.png and the image has expired in cache. The CDN tries to get fresh copy from origin but the origin is unavailable. What should the CDN do? Send a timeout error or serve the cached, stale object?
Serve Stale means the CDN serves an expired object from cache while the origin is unresponsive and/or returns an error. In most cases it's better for the user to get a good but expired/old/outdated response from the CDN than getting a timeout or other error response.
Some CDNs serve stale by default (e.g. StackPath) while others provide serve stale as a feature you can turn on (e.g. Tata Communications).
Fastly and CDN77 are two of the CDNs that support HTTP Cache-Control Extensions for Stale Content (RFC5861) which means you can control the CDN's stale content serving behaviour from headers the origin sends. For example, Cache-Control: max-age=600, stale-while-revalidate=30 instructs the CDN to cache the object for 10 minutes and, at the end of that 10 minutes, serve stale for up to 30 seconds while new content is being fetched.
CDNs and Serve Stale
Serve Stale does not mean the same on all CDNs.
See the table below for an overview of the behaviour and options per CDN.
Does the CDN support serving stale content? Is it free or a paid add-on?
Origin is unavailable
Serve stale if origin is unreachable or too slow?
Origin serves error
Can the CDN serve stale if origin serves error response?
Control via headers
Does the CDN support HTTP Cache-Control Extensions for Stale Content?
AvailableCan the CDN serve stale content? Free feature or paid add-on?
Origin is unavailableServe state if origin is unavailable
Origin serves errorServe stale if origin serves error response
Control via headersCDN supports HTTP Cache-Control Extensions for Stale Content
CDNs missing in this table? That is because we don't have the relevant info
More info per CDN
If "Ignore cache control" option is enabled, by default the CDN for Static Content serves stale content from cache by default in case the origin serves error, is down or unresponsive. VideoCDN does not have behaviour to serve stale content.
BaishanCloud supports the stale-while-revalidate and stale-if-error headers from origin, enabling the customer to control the serve stale behaviour.
CacheFly CDN can honour the serve stale headers from origin. Customer must ask support to activate this. Furthermore, Cachefly support engineers can alter the behavior either with a rewrite of the incoming headers or with logic to preserve stale content longer than the default. Cachefly can also provide (at a cost) permanent cache that can be used to keep stale content indefinitely, and build behavior around that cache to meet the customer's retention needs.
Edgio provides two relevant features: Stale content Delivery on Error and Stale While Revalidate. Both features are part of the Advanced Rules Engine package (extra costs apply).
Stale Content Delivery on Error: when active, the CDN caches will serve stale content when an error occurs during a connection to an origin server (note: error responses served by the origin like 404 and 502 will be passed along to the client!)
Stale While Revalidate: the customer can specify a length of time and select a time unit (e.g. seconds, minutes, hours, etc.) to allow stale content delivery.
Fastly can do it all. As a customer, you have full control over the stale content serving behaviour, either by way of the headers your origin sends or by using custom VCL code. Read the Fastly documentation Serving Stale Content to get detailed insight into how this all works at Fastly. Or, read the easier to consume blog post Stale-While-Revalidate, Stale-If-Error Available Today by Steve Souders on the Fastly blog.
Imperva supports the delivery of stale content in cases where the origin server is unresponsive. Imperva will respect stale content for a duration of 2 to 24 hours based on the time passed since the resource was last updated. Frequent updates will result in a shorter stale period. Non-frequent updates will result in a longer stale period.
Kingsoft Cloud supports the stale-while-revalidate and stale-if-error headers from origin, enabling the customer to control the serve stale behaviour. Customer can activate/configure this in the portal.
Tata can do serve stale but the customer needs the Tata support team to turn this on.
Tencent Cloud does not support the stale-while-revalidate and stale-if-error headers from origin but customers can configure the CDN to serve stale content in the event the origin server returns a 5xx error. This can be configured in both the console and the API.