Appendix A Caching Solutions and Performance 115
Step 4 If the data in the cache is stale, the appliance connects to the origin server and
asks if the document is still fresh. If the document is still fresh, the appliance
sends the cached copy to the user immediately.
Step 5 If the object is not in the cache (a cache miss) or the server indicates that the
cached copy is no longer valid, the appliance gets the document from the Web
server, simultaneously streaming it to the user and the cache (Figure 2).
Subsequent requests for the object will be served faster.
Figure 2 A cache miss
Caching is more complex than the preceding overview suggests. In particular, the
overview does not answer these questions:
✔ How does the Intel NetStructure Cache Appliance ensure freshness given the
different protocols it supports?
✔ How does the appliance revalidate stale HTTP objects?
✔ How does the appliance test an HTTP object for freshness?
✔ How does the appliance decide to serve an HTTP object?
✔ How do you configure the appliance’s HTTP freshness options?
✔ How does the appliance serve correct HTTP alternates?
✔ How does the appliance treat requests for objects that cannot or should not be
cached?
The following sections discuss these questions.
Ensuring cached object freshness
The Intel NetStructure Cache Appliance handles object freshness differently
depending on protocol.
FTP FTP documents stay in the cache for a time period specified by the system
administrator. See Freshness‚ on page 36.
NNTP News articles are refreshed each time the appliance polls parent news servers for
changes in group lists, article overview lists, and article updates. See Maintaining
the cache: updates and feeds‚ on page 141.
request
a cache miss
The Intel Cache simultaneously caches
and serves the document to the client
Intel NetStructure Cache Appliance
1
2
3
client
cache
local
server
origin
miss
