Fastly's Legacy Full-site Delivery services
Last updated 2019-08-09
Fastly offers full-site delivery that allows you to speed up websites and mobile apps by pushing content closer to users, providing improved and secure experiences across the world.
The Fastly CDN Service responds to HTTP GET requests initiated from end users' using your 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 your 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 you 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 you to specify how to forward and transform specific header types.
Fastly has no set hard limit on how long objects will remain cached. Instead, Fastly supports the expiration of content via Time to Live (TTL) settings that you configure. TTL settings work as timers on your cached content. When content has resided in the cache for the entire TTL interval, that content is given the status of "expired." Before Fastly delivers requested content that is expired, the cache checks to see if the content is still valid by checking with your application server first.
If the application server says the content remains unchanged, the cache sets the content's status to "valid" and resets its TTL value. If the object has been changed, it is declared "invalid" because the content has expired. The application server delivers updated content. Fastly's CDN Service caches the updated content with the status of "valid," and its TTL timer begins to run.
The fetch and refresh process may take a second or more, and during that time, a Fastly cache may receive dozens or hundreds of end-user requests for that content. Fastly's request collapsing feature groups these requests and fulfills them at once when the application server response is received.
Fastly offers you the option of setting a global, default TTL for cached content control. When set, Fastly's CDN service caches objects in a consistent manner even when applications are inconsistent in doing so.
When configuring Fastly's CDN Service, you can designate a specific point of presence (POP) to serve as a shield for your origin servers. This server is referred to as a "shield" because it protects your application servers from continuous requests for content. By default, no origin shield is enabled for you. You must specifically enable shielding to use it.
If Fastly's caches do not have the content being requested, they fetch it from the shield server instead of your origin servers. Fastly caches fetch content from your origin server only when the shield server does not have the content being requested.
You can designate multiple servers as your origin servers. When two or more application servers are provisioned as origin servers, Fastly's CDN Service will distribute requests to fetch content across those application servers. This type of load balancing is enabled by default. You must explicitly disable it if you don't want to use it.
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 your origin server. Request collapsing is enabled by default.
Fastly supports an Instant Purge feature that allows you to actively invalidate content. Rather than requiring your network operations and application staff to guess how frequently each bit of content may change, Fastly allows you to generate an HTTP Purge method that is sent to the CDN Service whenever an application changes or deletes data in its database. The Fastly CDN Service invalidates the associated content throughout the service's cache network, causing a new version of that content to be retrieved from the application server the next time it is requested.
Fastly allows URL-based and key-based purging, as well as purging of all content at once via specific, configurable purging commands. Fastly currently supports Ruby, Python, PHP, and Perl libraries for instant purging.
When purging by URL or surrogate key, Fastly's CDN Service can process thousands of changes per second. The invalidation process takes less than 300 milliseconds, making it possible to deliver dynamic content that changes rapidly and unpredictably. Using Instant Purge, you can eliminate cache-to-origin HTTP traffic that all other CDN services generate to determine if expired objects are still valid.
You have the option to configure Fastly's CDN Service to perform health checks on your application servers and measure their responsiveness. You can use health check responsiveness measurements to fine-tune the distribution of fetch requests. Health checks are not enabled by default. You must specifically enable them.
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 shields application servers by instructing Fastly's CDN Service to continue to serve stale or expired (but likely still valid) content to end users for a set amount of time. This allows you 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. You must specifically configure you service to serve stale content to use grace mode.
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