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

define tens or hundreds of such aggregation queries on hundreds or thousands of source
feeds.
For example, a user wants to create a feed with news about volcano eruptions in Iceland
fetched from ”The Guardian” and images published by ”The Big Picture” on the same
topic. This can easily be translated into the following aggregation query which applies a
simple disjunctive filtering condition on the union of the corresponding feeds:
CREATE FEED IceVolEruFeed AS
RETURN "http://feeds.guardian.co.uk/theguardian/rss" AS $guardian |
"http://www.boston.com/bigpicture/index.xml" AS $picture
WHERE $guardian[ITEM CONTAINS "iceland" OR
ITEM CONTAINS "volcano" OR
ITEM CONTAINS "eruption"] AND
$picture[CATEGORY = "iceland" OR
CATEGORY = "volcano" OR
CATEGORY = "eruption"];
This query filters all items of both sources which contain at least one of the three terms
”iceland”, ”volcano” and ”eruption”. The overall aggregation infrastructure is illustrated
in Figure 1.3 which shows two aggregators nodes aggregating feeds from two sources.
Figure 1.3: Aggregation queries
1.4 RSS Feed Aggregation Model
In this section we present feeds from several points of view. First, we define two different
semantics for RSS feeds. A feed can be seen both as a continuous stream of all items
published until a certain moment and, at the same time, as a publication window that
holds only a finite subset of items most recently published. And last, we analyze feeds by
their provenance. We distinguish between source feeds and query feeds and explain the
way they are created and used. The concepts introduced and discussed in this section
serve not only for a better understanding of feeds and aggregation in general, but they are
also useful for the further concept definitions we present later.
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