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Step 5 If the object is not in the cache (a cache miss) or the server indicates that the cached copy is no longer valid, the appliance gets the document from the Web server, simultaneously streaming it to the user and the cache (Figure 2).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.
3The Intel Cache simultaneously caches and serves the document to the client
Intel NetStructure Cache Appliance | |
miss | localcache |
1 request |
|
client | 2 a cache miss |
originserver |
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.
Appendix A Caching Solutions and Performance | 115 |
