The caches in a Traffic Control CDN are servers running the Apache Traffic Server software. See ATS documentation for more information. Caches in a Traffic Control CDN are deployed in cache groups.
A cache group is a logical group of caches that Traffic Router tries to use as a combined cache. Traffic Router treats all servers in a cache group as though they are in the same physical location, though they are in fact only in the same region (network). A cache group has one single set of geographical coordinates even if the caches that make up the cache group are in different physical locations. The caches in a cache group are not aware of the other caches in the group - there is no clustering software or communications between caches in a cache group.
There are two types of cache groups: EDGE and MID. Traffic Control is a two tier system, where the clients get directed to the EDGE cache group. On cache miss, the cache in the EDGE cache group obtains content from a MID cache group, rather than the origin, which is shared with multiple EDGEs. EDGE cache groups are configured to have one single parent cache group.
Often the EDGE to MID relationship is based on network distance, and does not necessarily match the geographic distance.
A cache group serves a particular part of the network as defined in the coverage zone file. See The Coverage Zone File and ASN Table.
Consider the example CDN below:
There are two MID tier cache groups, each assigned with three EDGEs. The lax, den and chi EDGE locations are configured with the West MID as their parent, and the nyc, phl, and hou EDGEs, are configured with the East MID as their parent. On a cache miss, the EDGEs use their assigned parent.
All caches (and other servers) are assigned a Profile in Traffic Ops.
A Profile is a set of configuration settings and parameters, applied to a server. For a typical cache there are hundreds of configuration settings to apply. The Traffic Ops parameter view contains the defined settings, and bundled into groups using Profiles. Traffic Ops allows for duplication, comparison, import and export of Profiles.