Points of presence
The Fastly network comprises a large number of physical servers distributed all over the world, connected to the internet at high density internet exchange points.
Fastly servers are grouped into:
- "sites", which describes a co-located set of machines in a single physical facility, attached to the same internet transit; and
- "points of presence" (commonly "POPs"), which describes the clustering of multiple machines together to create a single pool of cache storage.
Commonly, all the machines that comprise one site also comprise one POP, with the POP and the site sharing the same name. The largest POPs in densely populated metropolitan areas may span multiple sites, with locations and connectivity chosen to serve the same human population.
In VCL services, the
server.datacenter variable reports the identity of the POP, while
server.hostname includes the ID of the site. The
server.identity variable includes the ID of both the site and the POP. Due to the effects of clustering, a single request to a VCL service may be processed by servers in more than one site, during the request's journey through the POP.
If a service has shielding enabled, a request that is not satisfiable from cache will transit two POPs. The following example shows one possible content retrieval path for a request from a user in Los Angeles to a Fastly-hosted domain with a service configuration set to shield in our DCA POP:
In Los Angeles, the LGB POP spans two sites (LAX and SNA), which act together as a single cache. In Virginia, the DCA POP is housed within a single site, also called DCA.
We recommend using the
server.datacenter value if you need to reference a particular POP in VCL, but be aware that POPs are added and removed from the Fastly network regularly.
Both POPs and sites use three-letter codes as identifiers, derived from the IATA airport code of a nearby airport. However, since POPs are rotated into and out of service regularly, codes are allocated based on availability and the location of the airport is not necessarily an accurate guide to the location of the Fastly site.
WARNING: Although we have historically used the term data center to refer to both a POP and a site, current use of the term "data center" may refer to POPs or sites depending on the context. Be sure you understand which concept is being referred to. The
server.datacenter variable, for example, refers to a POP.
Our network map shows current and planned locations of all Fastly POPs, information that is also available via the API. Announcements of POPs entering and leaving service, or of substantial changes to POP capacity, are made on our service status page.
The design of the Fastly network balances issues such as provider diversity, connectivity, traffic volume, and optimum cache size. For Fastly customers, POP variability is most notable in its effect on cache hit ratio (i.e., the ratio of inbound requests that are able to be satisfied from cache).
For example, 100 requests handled by 100 distinct servers in 100 distinct POPs will experience a much lower cache hit ratio than 100 requests handled by 100 distinct servers all participating in the same POP because, in the latter case, all the requests have access to the same shared pool of cache storage.
Other features provided by Fastly are unaffected by POP variability, such as our web application firewall, which is available on all Fastly servers.
The effects of site variability are negligible and likely undetectable.