About Fastly's Delivery Acceleration features
Cached content sometimes must be refreshed when that content becomes "stale" or expires. When multiple end-users request content that is in the process of being refreshed, request collapsing groups those requests to be satisfied together, as soon as the content is received. This accelerates content delivery by keeping Fastly's CDN Service from repeating duplicate requests to a customer's origin server. Request collapsing is enabled by default.
When an application server becomes unavailable for any reason, end users will normally receive error messages indicating the content they've requested cannot be retrieved. When enabled, grace mode instructs Fastly's CDN Service to accelerate content delivery to end users by continuing to serve stale or expired (but likely still valid) content for a set amount of time. This allows customers to return otherwise unavailable application servers to normal operations and still serve content rather than error messages to end users. By default, grace mode is not configured. Customers must specifically configure their service to serve stale content.
HTTP request fulfillment
The Fastly CDN Service responds to HTTP GET requests initiated from end users' using a customer's website, or from a program making calls to an Internet-hosted API.
Fastly's CDN Service supports forwarding HTTP headers to end users when they are received from a customer's origin server. Alternatively, headers can be added, removed, or modified using our edge scripting language either before or after caching a response from the origin. This includes the Cache-Control and Expires headers as well as the Surrogate-Control header. HTTP header support allows customers to send one set of instructions to the Fastly cache servers and another set of instructions to downstream caches, such as proxies or browsers. In particular, the Surrogate-Control header allows customers to specify how to forward and transform specific header types.