Roxana - Gabriela HORINCAR Refresh Strategies and Online ... - LIP6

the update date of the entry and the entry categories. Most of these elements are optional.
An example of an RSS document is shown in Figure 1.1.
Figure 1.1: A sample RSS feed
The postings are arranged in the reverse chronological order where new postings are appended
in the front and old postings are pushed downwards and removed. For the majority
of current implementations, an RSS document contains the most recent 10 or 15 postings.
New postings are added to the feed at any time without notifying their subscribers; thus,
the subscribers have to poll the RSS feeds regularly and check for updates.
1.2 Feed Aggregation System
We define a content aggregator as a system that gathers web content coming from different
online information sources for subsequent reuse, such as content distribution to users to suit
their needs. Feed aggregators help users access and stay up to date with the new content
disseminated through RSS feeds. They reduce the time and effort needed to regularly
check websites for updates, creating a unique and personalized information space. There
are various examples of content aggregation systems, but in this dissertation we focus on
the case of RSS feeds syndication systems.
We place our work in the context of the RoSeS project (Really Open Simple and Efficient
Syndication) [rosa]. RoSeS is a feed aggregation system of RSS [rssa] feed sharing in which
personalized feeds defined by content-based aggregation queries are delivered to users.
As shown in Figure 1.2, a RoSeS aggregator mediates the interaction between a set of users
and the set of data sources to which it is subscribed to. Users define a set of content-based
aggregation queries (introduced in Section 1.3) over a subset of the sources. Data sources
constantly generate new pieces of information, called items, entries or postings. One data
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