XML Demystified

CHAPTER 8 Really Simple Syndication (RSS)
The skipHours child element requires at least one hour child element and can
have up to 24 hour child elements. Each hour child element must contain an integer
that represents the hour you want the aggregator to skip the update. The hour child
element uses the 24-hour clock where 0 is one o’clock in the morning and 23 is
midnight.
Here’s how to tell the aggregator to skip the updates after normal business
hours:
0
1
2
3
4
5
6
7
17
18
19
20
21
22
23
The ttl (time to live) element specifies the number of minutes that the RSS
document has before the copy of the RSS document in cache is refreshed. The
RSS document is placed in cache (memory) the first time it’s loaded. During the
session, the RSS document is displayed from cache when subsequent requests are
made for the RSS document.
Retrieving the RSS document from cache rather than from the server provides a
quick response; however, the contents of the copy of the RSS document in cache
can easily be outdated. In order to avoid this problem you use the ttl element to set
the number of minutes that the RSS document can remain in cache; after this
expires, the RSS document must be refreshed from the server.
Let’s say that your RSS documents almost always remains unchanged for two
hours. Therefore, you can set the ttl element to 120 minutes. This means that the
aggregator is asked to refresh its copy of the RSS document that’s stored in cache
every 120 minutes.
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