51
Traffic Server can participate in cache hierarchies, where requests not fulfilled in one cache can be routed to
other regional caches, taking advantage of the contents and proximity of nearby caches.
This chapter discusses the following topics.
•
Understanding cache hierarchies‚ on page 51
•
HTTP cache hierarchies‚ on page 51
•
ICP cache hierarchies‚ on page 54
Understanding cache hierarchies
A cache hierarchy consists of levels of caches that communicate with each another. Traffic Server supports
several types of cache hierarchies. All cache hierarchies recognize the concept of parent and child. A parent
cache is a cache higher up in the hierarchy, to which Traffic Server can forward requests. A child cache is a
cache for which Traffic Server is a parent.
Traffic Server can be a member of the following cache hierarchies:
•
An HTTP cache hierarchy (described in
HTTP cache hierarchies‚ on page 51
)
•
An ICP (Internet Cache Protocol) cache hierarchy (described in
ICP cache hierarchies‚ on page 54
)
HTTP cache hierarchies
In an HTTP cache hierarchy, if a Traffic Server node cannot find a requested object in its cache, it can search
a parent cache—which itself can search other caches—before resorting to retrieving the object from the origin
server.
You can configure a Traffic Server node to use one or more HTTP parent caches. You use more than one
HTTP parent cache so that if one parent is unavailable, another parent can service requests. This is called
parent failover and is described in Chapter , “Parent failover,” below.
Figure 7-1. illustrates a simple cache hierarchy, where a Traffic Server node is configured to use a parent
cache.
In this figure, a client sends a request to a Traffic Server node (which is a child in the cache hierarchy because
it is configured to forward missed requests to a parent cache). The request is a cache miss, so the Traffic Server
forwards the request to the parent cache. On the parent, the request is a cache hit, so the parent sends a copy
of the content to Traffic Server, where it is cached and then served to the client. (Future requests for this
content can now be served directly from the Traffic Server cache.)
7 Hierarchical Caching
NOTE
If you do not want all requests to go to the parent cache, you can configure Traffic
Server to route certain requests directly to the origin server (for example, requests that
contain specific URLs) by setting parent proxy rules in the
parent.config
configuration file (described in
parent.config‚ on page 179
).