Glossary of terms

These are common Fastly, HTTP, and networking terms you may encounter within our service guides.


Access Control List. A list of permissions that can be attached to an object allowing customers to quickly check a client's IP against a list of known net blocks and then make decisions based on the result.


Fastly's customer summit.


The user interface through which customers access Fastly's CDN services.


See origin server.


The specific HTTP header that controls who can cache a response, under which conditions, and for how long. Fastly respects Cache-Control headers returned from origin servers as one approach to cache management. See also surrogate-control, max-age, and s-maxage.


Fastly's community discussion forum.

HTTP headers used to perform certain functions like authenticating login in secure website areas, information tracking, remembering user preferences, and customizing how information is presented.


An open-source command line tool for transferring data with URL syntax from or to a server using one of many supported protocols. Fastly users can issue cURL commands to verify requests are caching in the Fastly network.


Domain Name System. A system for naming computers and network services that translates a domain's numbered IP address into an easy-to-remember alphabetic name.

Edge Dictionary

A type of container Fastly users can create to store data as key-value pairs and turn frequently repeated statements into a single function that acts as constant.

Egress traffic

Bandwidth used when traffic travels from Fastly points of presence (POPs) to the end user.


Edge Side Includes. An XML-based markup language that allows content assembly by HTTP surrogates. Allows Fastly users to cache pages that contain both cacheable and uncacheable content (such as user-specific information).


A way of compressing information to make it faster to transmit. Fastly allows users to dynamically gzip content based on file extension or content type.

An HTTP field that precedes the main content of information being sent in a request or response and describes the length of the content, type of content, or other characteristics of the information.

Host (header)

Information used in addition to the IP address and port number to uniquely identify a domain. See also Normalizing the host header.

Ingress traffic

Bandwidth used when end users make requests that send traffic to Fastly points of presence (POPs).

Instant Purge

A feature of Fastly's purging functionality that allows users to actively invalidate content in Fastly caches within milliseconds. See also Soft Purge.


An HTTP Cache-Control directive that specifies how long (in seconds) an object will remain in the cache before Fastly removes the object from storage. See also surrogate-control, cache-control, and s-maxage.


A tool that combines traceroute and ping programs in a single network diagnostic tool. Frequently used in debugging network connections.

Origin server

The location or address from which Fastly's network requests the content it will serve.

Origin Shield (Shielding)

A specific Fastly point of presence (POP) designated by users as the primary source of content through which all content requests from other POPs will be directed in lieu of contacting a customer's origins directly.


On-the-fly packaging. A feature of Fastly's Video on Demand media and streaming services that allows customers to dynamically package video for delivery in multiple HTTP streaming formats. Also known as "just in time" video content packaging.


Point of Presence. Datacenter within which Fastly's globally distributed cache servers reside.


An HTTP Cache-Control directive that allows users to select which objects are not cached. Fastly will not cache responses with a Cache-Control value of private.


The process of picking out one or more objects from the Fastly cache and discarding it along with its variants. See also Instant Purge and Soft Purge.


A function that directs requests for information from their originally intended locations to a more desirable destination.

The header sent by a server in response to an HTTP request and then used to create a cookie on a user's origin. Fastly supports a method for extracting a named value out of Set-Cookie headers no matter how many there are. By default, Fastly will not cache responses that contain a Set-Cookie header.


An HTTP cache control directive similar to max-age, but applied only to shared caches. See also surrogate-control and cache-control.

Soft Purge

A type of purging that allows users to easily mark content as outdated (expired) instead of immediately deleting it from Fastly's caches. See also Instant Purge.


Fastly's network status monitoring site. Allows customers to quickly check whether anomalies they see may be due to a known problem currently being worked on by Fastly or if their issues more likely stem from problems within their own infrastructure.


The main email address of Fastly's Customer Support team through which customers can ask questions and receive assistance.


An HTTP response header that allows origin servers to use control directives to dictate how intermediate caches, including Fastly, should handle response entities. Surrogate-Control will not affect browsers. See also cache-control, max-age, and s-maxage.

Surrogate Key

A unique identifier that allows customers to group content together for faster processing. Fastly uses surrogate keys as part of its purging strategy.

Synthetic response

Custom responses generated within the CDN that users can set if a specific URL is requested or a specific condition, such as a status code, is met. These responses require no origin server interaction.


A cryptographic protocol Fastly follows that ensures privacy between communicating applications and their users on the internet.


Uniform Resource Locator. An address used to find a site or application's objects on the internet.


Caching software that helps content-heavy dynamic websites as well as heavily consumed APIs load faster. Fastly's core caching infrastructure is based on a heavily modified version of Varnish.


Varnish Configuration Language. A scripting language used to configure and add logic to Varnish caches. Fastly users can create custom VCL files with specialized configurations.


A network debugging tool designed to provide key info to help a Fastly user troubleshoot issues with Fastly's Customer Support team.

Back to Top