Glossary of terms

The following words and phrases are either common industry concepts that are important to Fastly and our technology or proprietary terms we've coined to specifically describe our own products and platforms. Here you can find definitions of these terms to help you navigate the world of Fastly.

  1. A
  2. B
  3. C
  4. D
  5. E
  6. F
  7. G
  8. H
  9. I
  10. L
  11. M
  12. N
  13. O
  14. P
  15. R
  16. S
  17. T
  18. U
  19. V
  20. W


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


  • A public application programming interface (API) provided by Fastly to permit control of and configuration of Fastly services. An alternative to using the web interface or command line interface (CLI).Learn more


  • A term used to describe the current state of a service version. Fastly service configurations contain both versioned and versionless elements. The elements of a service configuration that are versioned can only be edited in a service version that is in draft. When ready to deploy, the service version is activated, replacing the currently active version.



  • A non-Fastly internet host from which Fastly will fetch content to serve in response to end-user requests. Also known as an Origin.


  • CapacityThe ability for a network route to handle a certain amount of traffic (e.g., "the bandwidth of the link is 100Gbps").
  • Data transferThe amount of data actually transferred over a network link, e.g. "billable bandwidth". However, "egress traffic" is preferred to avoid confusion.

Cache hit ratio

  • The percentage of end-user requests that are able to be satisfied by Fastly by reading from cache. There are many ways to calculate this and it can be significantly impacted by the effects of shielding and segmented caching. See also origin offload.

Cache key

  • A mathematical derivative of predetermined attributes of a request, used to match a request to an appropriate response in the cache, and also to allocate requests to an appropriate fetch node during the clustering process. In VCL services the cache key can be adjusted in vcl_hash.

Cache object

Cache server

  • A single Fastly computer server that participates in our Edge Cloud. Cache servers are physically organized into sites and logically organized into POPs.

Cache settings object


  • A software system responsible for making an HTTP request that is handled by a server (often Fastly). A client is not a person, but a person can use a client to make a request. Clients are downstream of Fastly.


  • The process of taking the currently active version of a service and creating a new draft of it. Changes made to draft service versions are not applied until the version is activated.

Cloud Optimizer

  • A Fastly product designating Fastly as the origin for other CDNs with the aim of consolidating requests and reducing traffic to origin.Learn more


  • A process by which incoming requests are initially handled by a random cache server (the "delivery node"), and then handed off to a second server (the "fetch node") determined based on the cache key for the request. This allows all cache servers in a POP to act as a single contiguous pool of storage.Learn more

Command line interface (CLI)

  • An installable command-line tool, which can be used to manage Fastly services as an alternative to the web interface or API.Learn more


  • One of the two platforms offered by Fastly to run edge code. Compute services run Webassembly binaries that are compiled outside of Fastly using tools that we provide in the Fastly CLI. The other platform is VCL.

Concierge TLS

  • A professional services and support product providing advanced configuration support for TLS.Learn more


  • A configuration mechanism that can be added to a service, and which generates VCL code based on selections made in the web interface or using the API. Conditions are used to limit the circumstances in which other logic added via configuration objects (such as headers and cache settings) are executed.


  • An entity that owns a group of services, and has multiple users. Charges incurred by all services owned by a customer will be billed to that customer.

Data center

  • Outdated usageHistorically this term was used interchangeably with site and POP, but today site and POP are distinct concepts.


  • The caching and reverse proxy capabilities of the Fastly edge cloud, the features most commonly associated with a CDN. See also Full-Site Delivery.

Delivery node

  • The machine that first handles a request when Fastly cache servers perform clustering. A delivery node is responsible for running vcl_recv, vcl_hash, vcl_deliver and vcl_log in the VCL flow, and is the machine to which the end user is connected. The other server involved in clustering is the fetch node.


  • A configuration primitive in VCL used to create pools of servers for load balancing.Learn more


  • An internet hostname, such as or that is associated with a service. Requests received by Fastly cache servers will be processed by the service that owns the domain specified in the Host header included with the request.


  • A way to describe things that are closer to the end user than the current network node. For example, a Cache-Control header can be set in a Fastly configuration for the benefit of 'downstream caches', which would include the end-user's browser cache.

Dynamic server pools

Dynamic site acceleration (DSA)

  • The increase in performance of a website associated with terminating the end-user request at the edge, and carrying the remainder of the connection to the origin on already-established connections. This performance boost can be realized without caching any content at the edge.


  • A collection of servers deployed across the world that act together to identify as the same set of hosts, so that end-users attempting to connect to one of the relevant hostnames will establish a connection with a server that is close to them both physically and in terms of network routing. The Fastly edge cloud is such a network.

Edge Dictionary

  • A Fastly product that provides persistent state in the form of string key-value pairs. Edge Dictionaries can be read from a VCL service, and may be updated via the web interface or Fastly API. In VCL services, Edge Dictionaries are exposed as VCL tables.Learn more

Edge Side Includes (ESI)

  • An extension to HTML. ESI tags are intended to be parsed and removed or replaced at the edge, before a resource is delivered to the end-user. While a version of the ESI spec is published as a W3C note, the version supported by Fastly is only a subset of this and supported only in our VCL platform.Learn more

Edge cache

  • Collectively, the cache storage deployed across all Fastly cache servers.

Edge code

  • Programming code written by Fastly customers, that is ultimately executed by Fastly cache servers. On the Compute platform, code can be written in a variety of languages and is compiled before being uploaded to Fastly. On the VCL platform, edge code is written in VCL, uploaded as source, and compiled by Fastly.

Edge code errors

  • Unhandled errors, panics or exceptions in Compute programs usually produce descriptive output on stderr, and if the program terminates before generating a response, will cause a blank HTTP 500 (internal server error) to be served to the client.Learn more

Egress traffic

  • Traffic that begins inside of a network and proceeds through various routes to a destination somewhere outside of the network. Technically, any traffic flowing from one system to another is both ingress and egress depending on your perspective. Fastly bills for Fastly egress, which includes requests to origin and responses to clients. Responses from origin to Fastly is origin egress.



  • A time to live (TTL) is stored for every object as it enters cache. When an object has been in cache for longer than its TTL, the object is considered expired. Expired objects may remain in cache until evicted and in qualifying circumstances may be served as a stale response.

Fastly TLS

  • A Fastly product that provides TLS provisioning with Fastly-managed or customer-managed certificates. Suitable for 1-1000 domains.

Fastly debug

  • HeaderThe Fastly-Debug HTTP header is a request header used to trigger a Fastly service to include debug information in the response
  • WebsiteA website, often used by the Fastly Support team, to gather information about your connectivity to Fastly.


  • A person who is employed by Fastly.

Fetch node

  • The machine that fetches the resource from the upstream server and stores it when Fastly cache servers perform clustering. The fetch node is responsible for running vcl_miss, vcl_pass, vcl_hit and vcl_fetch in the VCL flow. The other server involved in clustering is the delivery node.


  • A tool made available by Fastly for testing and debugging Fastly VCL. Can also refer to individual tests created using the tool (in general, without an article, "Fiddle" means the tool, while with an article "a fiddle" or "the fiddle" refers to an individual test).Learn more

Full Site Delivery

  • All the capabilities of the Fastly edge cloud that are not sold as separate products. Full site delivery (FSD) includes the features most commonly associated with a CDN, such as reverse proxy and caching, and also Fastly-specific features like edge code, shielding, and log streaming. It does not include products such as image optimization or the Web Application Firewall.Learn more


  • A number, automatically incorporated into the cache key of cache objects in the Fastly edge cache, in order to facilitate purge-all. It can be read from the req.vcl.generation variable.

Guru mediation

  • Outdated usageAn HTML page delivered via a synthetic HTTP response generated by Fastly when an unhandled error occurs in a VCL service. The name was inherited from the Varnish project but we changed it from meditation to mediation. Superseded by the 54113 error.

Header object

  • A configuration mechanism that can be added to a service and that generates VCL code based on selections made in the web interface or using the API. Cache settings objects are used to control the TTL of resources.

Health check

  • A regular HTTP request made by Fastly to an origin server to check that the origin is in a fit state to accept traffic.Learn more


  • When a request to the Fastly edge cache is satisfied by returning a resource from the cache and does not require making a fetch to an origin. Some features, including Segmented Caching and shielding, make it possible for a single request to result in a combination of hits and misses.

Image optimization (IO)

  • A Fastly product that allows images passing through the Fastly edge cloud to be transformed as required by the client.Learn more

Ingress traffic

  • Any traffic flowing from one system to another is both ingress and egress depending on your perspective. Fastly bills for Fastly egress, which includes requests to origin, and responses to clients. Responses from origin to Fastly is origin egress.


  • A single execution of a Compute program, triggered by an inbound HTTP request that is routed to the Fastly service. An instance is short-lived and cannot interact with other instances of the same program which may be running concurrently to handle other requests.

Instant Purge

  • A Fastly product that describes the ability to send purges using API calls or the web interface that removes a single item or a group of items identified by the same surrogate key, from the Fastly Edge cache. Does not apply to purge-all.

Internet Exchange Points (IXP)

  • A physical facility and infrastructure that allows two or more network operators such as Internet Service Providers (ISPs) and Content Delivery Networks (CDNs) to exchange data destined for each other's networks. Fastly intentionally chooses locations with busy internet exchange points when deciding where to build Points of presence.


  • A process that makes a cache object ineligible for use to satisfy future requests. Invalidation can be caused by the expiration of an object that has no stale lifetime, by a stale object reaching the end of its stale lifetime, by explicit purge, or by eviction). Invalidated content is effectively deleted from Fastly caches, but in practice may remain on disk until overwritten by new cache entries.

Load balancing

  • The practice of selecting a backend server from a pool of available, appropriate and often functionally identical servers based on a strategy designed to maximize performance and availability. On our VCL platform, load balancing is implemented using directors.

Managed CDN

  • Fastly edge cloud platform software running on dedicated infrastructure within a customer's private network.

Media shield

  • Outdated usageA legacy Fastly product intended for media customers that deploys a Fastly service upstream of other Content Delivery Networks with the aim of consolidating requests and reducing traffic to origin. Succeeded by cloud optimizer, Media Shield for Live, and Media Shield for VODLearn more


  • A cache lookup that results in no match, meaning that the desired resource is not in the cache on that cache server. With clustering enabled, a miss normally indicates that the desired resource is not in cache anywhere within that Fastly POP.


  • Within our Compute platform, a namespace is an identifier for a complete set of execution processes across the Fastly server fleet. Fastly may allocate services or entire customer accounts into appropriate namespaces based on load, isolation, routing or other similar requirements.



On-the-Fly Packaging (OTFP)

  • A Fastly product that enables video content to be packaged automatically such that it supports a variety of client-requested formats.Learn more


  • A non-Fastly internet host from which Fastly will fetch content to serve in response to end-user requests. Also known as a Backend. Origin servers are upstream of Fastly.

Origin Shield

Origin connect

  • A dedicated private physical link connecting a Fastly POP to a customer's origin.Learn more

Origin egress

  • Traffic flowing from an origin server to Fastly. Origin egress traffic is not billable by Fastly directly, but in some situations, due to partnerships between Fastly and popular hosting providers, origin egress to Fastly may be subject to discounted or free pricing from the origin hosting provider.

Origin offload

  • The number of requests to a Fastly service that do not trigger a fetch to origin, as a proportion of all requests to the service. An 80% origin offload indicates that using Fastly has reduced the load on the origin server to only a fifth of what it would otherwise be.

Platform TLS

  • A Fastly product that hosts customer-uploaded TLS certificates at scale for customers offering mass hosting or supporting multi-brand portfolios. Suitable for thousands of individual certificates.

Point of Presence (POP)


  • In VCL snippets, header objects, and conditions, priority determines the order in which objects for the same subroutine are rendered into the final service VCL. Lower numbered priorities are rendered and executed first (they have a higher priority). Where two objects in the same scope have the same priority, the order is not guaranteed.

Private Network Interconnect (PNI)

  • A direct connection between two networks which allows data to flow between them without being routed on the internet. Fastly establishes PNIs at many of our POPs, with connections to major cloud infrastructure providers, such as Amazon Web Services. These improve the reliability, performance and cost efficiency of origin connections.


Purge all

  • A purge operation that makes all cacheable content in a service unreachable by incrementing its generation.

Purge by URL

  • A purge operation that targets a single object based on the cache key. If the object has variants, all are purged.

Purge by key

Range requests

  • An HTTP request that includes the Range header, which instructs a server to extract and deliver only a portion of a resource, rather than the whole thing. Fastly supports range requests from clients, and if segmented caching is enabled, we also make range requests to origin servers.


  • An HTTP request. Requests include those made by the end user to Fastly, between Fastly POPs, and from Fastly POPs to origin.

Request settings object

  • A configuration mechanism that can be added to a service, and which generates VCL code based on selections made in the web interface or using the API. Request settings objects are used to control a variety of request behaviors, such as forcing a cache miss.Learn more


  • Something on the internet to which a user can send HTTP requests, often to download it - for example, HTML pages, images, stylesheets, videos, or API endpoints. Fastly stores cacheable resources in our cache servers. A resource stored in the cache is a cache object.

Response object

  • A configuration mechanism that can be added to a service that generates VCL code based on selections made in the web interface or using the API. Response objects are used to create synthetic content and to trigger that content based on request conditions.Learn more

Segmented Caching

  • The splitting apart of large resources into smaller, equally sized chunks to enable them to be cached separately. This is intended to reduce origin load in the case of range requests, and allow the edge cache to operate more efficiently.Learn more


  • A logical configuration that captures a selection of inbound traffic (using domains or service pinning), connects it to origin servers, applies rules, and runs edge code. Service versions are used to create immutable configurations and apply multiple changes to a service in a single deployment. The mechanism used to run edge code and other logic on requests processed by the service depends on the service type, which can be Compute or VCL.

Service authorization

  • A rule that sets a permission level for a specific user in relation to a service, as part of role-based access control (RBAC).Learn more

Service pinning

  • A means of directing inbound requests to a specific Fastly service by using a dedicated set of IP addresses, instead of a hostname. This allows a Fastly service to handle traffic for sites that have a very large number of domains, such as sites that themselves provide hosting services to others.

Service version

  • A revision of a service's configuration. In general, a service configuration is updated by cloning the currently active version, making changes, and activating them. Only draft versions can be edited. Once activated, a version becomes immutable, even if it is no longer the active version. Previously-active versions have a status of 'locked'.

Shared TLS Certificate

  • Outdated usageAn entry for a customer's domain added to a shared TLS certificate using the SAN field.

Shared TLS Wildcard Certificate

  • Outdated usageAn entry for a customer's wildcard domain (e.g. * added to a shared TLS certificate using the SAN field.

Shield POP


  • A means of reducing origin traffic by forcing all origin requests to go through a nominated Fastly point of presence. Requests that are not initially handled by that nominated POP will therefore be processed by two POPs, the service configuration will execute twice, and the request will have a second chance to be satisfied within the Fastly Edge cloud (thus avoiding an origin request).Learn more


  • Fastly infrastructureA co-location of cache servers in a single physical facility.
  • WebsiteA website that uses Fastly as part of its architectural design.

Solid state drive (SSD)

  • A hard disk drive with no moving parts. Fastly uses only solid state drives in our edge cache.


  • Content that is expired but remains useable in limited circumstances because it is within a period of time defined by 'stale-if-error' or 'stale-while-revalidate' headers.Learn more

Streaming Delivery

  • The use of Fastly to deliver streaming media formats such as HLS, LLHLS, and MPEG-DASH as part of the core Full Site Delivery platform. Streaming formats include any content that is consumed by the client as it is being received, rather than after it has been downloaded.Learn more

Subscriber Provided Prefix

  • A Fastly product that allows you to have your IP spaces announced, routed, and served by Fastly infrastructure for use with production services. When you purchase this product, you provide your own IP address space to Fastly rather than use Fastly IP addresses.Learn more

Surrogate Key

  • A label that can be attached to a cache object to allow a group of objects assigned the same tag to be purged in one operation (see purge by key).


  • An HTTP response header that provides the same semantics as Cache-Control but is intended to be parsed and observed by edge caches such as Fastly. It is stripped from the response before the response is forwarded to the downstream client.


  • VCLThe synthetic statement in VCL that instructs Fastly to create an HTTP response from a provided string.
  • ConceptA synthetic response is one that is constructed at the edge, not by an origin server.

Time To First Byte (TTFB)

  • VCLThe time.to_first_byte variable in VCL measures the time between Fastly receiving a request and beginning to serve a response. For hits, this can be as little as a few microseconds.
  • CommonIn general, time to first byte refers to the time between when a client has finished sending an HTTP request and when it starts to receive the response. Unlike the time.to_first_byte variable available in VCL, this includes network latency and is usually tens to hundreds of milliseconds.

Time To Live (TTL)

  • The amount of time a resource will remain in cache before it becomes stale or expires. Usually expressed in seconds unless a unit is specified.

Trace ID

  • A universally unique identifier (UUID) generated by Fastly for each incoming request on our Compute platform.Learn more


  • A way to describe things that are closer to the origin than the current network node. For example, for a service that uses shielding, the shield POP is upstream of the edge POP, and the origin is upstream of the shield POP.

VCL services

  • One of two types of Fastly service, the other being a Compute service.


  • An open source reverse caching proxy, from which Fastly's VCL platform has evolved. Fastly VCL has been extensively extended and is no longer compatible with the open source project.

Varnish Configuration Language (VCL)

  • An evolution of the domain-specific scripting language used to configure and add logic to Varnish caches as originally designed by the Varnish Cache project. Fastly VCL has been extensively extended and does not include features added to more recent versions of the open source project.


  • Some elements of service configuration are not part of the immutable definition of a service version. When these are updated, the service is immediately affected, without the need to clone and activate a new version of the service. For example, edge dictionaries are attached to a service as part of a version, but the entries in the dictionary are versionless.

Web Application Firewall (WAF)

  • Signal SciencesAn application security monitoring system that proactively monitors for malicious and anomalous web traffic directed at your web servers
  • Fastly WAFA legacy Fastly product that offers a feature set similar to a subset of the Signal Sciences WAF. Available via the Fastly legacy WAF API and the WAF 2020 API

Web interface

  • A means of configuring Fastly services via a web browser. The Fastly web interface, available at, is a client of the same Fastly API that customers use directly.

WebAssembly (Wasm)

  • A binary format for executable programs, designed to be portable between computer architectures, that can also be produced by compilers for multiple programming languages. Fastly's Compute platform runs WebAssembly programs, which are compiled from other languages such as Rust and JavaScript.

WebAssembly System Interface (WASI)

  • WebAssembly System Interface, or WASI, is a family of APIs for WebAssembly being designed and standardized through the WASI Subgroup of the W3C WebAssembly Community Group. WASI is an effort to provide general-purpose APIs for supporting non-Web use cases.