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

1.4.2 Source and Query Feeds
In this section, we analyze and discuss feeds from their provenance point of view, as well
as the way they are created and used. We distinguish between source feeds and query
feeds. In the first case, we use them as input feeds consumed by the aggregator and ignore
the way they are created; we call them source feeds. In the other case, we talk about query
feeds when they represent the output feeds generated by an aggregator as the result of
aggregation queries.
Definition 1.4.4. Source feed
A source feed s represents the feed generated by a source, by constantly publishing new
items.
Definition 1.4.5. Query feed
Let q be the aggregation query applied by an aggregator on a set of source feeds S =
{s1, ...sk}. The result of q is called a query feed and is denoted by f = q(S).
As already mentioned in Section 1.3, in this dissertation we focus on the frequent case
of aggregation queries computed as a union of filters over source feeds sets and has the
general form: q(S) = �
j σj(Sj), where S = ∪jSj.
A filter applied on a set of source feeds σj(Sj) selects the items published by those source
feeds that satisfy a given item predicate. Item predicates are boolean expressions (using
conjunctions, disjunctions, negation) of atomic item predicates that express a condition
on an item attribute. Depending on the item attribute type, atomic predicates may be
applied on the publication date of an item, on the content of an item or on the set of
categories that describes an item. A complex model of aggregation queries specific to the
RoSeS system is presented in [TATV11].
We resume the example presented in Section 1.3 that shows how the query feed f =
IceVolEruFeed is created as a union of two filters applied on two different feed sources,
f = q(s1, s2) = σ1(s1) ∪ σ2(s2). The two input feed sources come from two different
websites, s1 =”The Guardian” and s2 =”The Big Picture”. The filter applied on the
first source σ1(s1) selects those items that contain the keywords ”iceland”, ”volcano” or
”eruption” in their content. The second filter applied on the second source σ2(s2) selects
only the items that contain the same three keywords within the set of keywords associated
to each item.
The items contained by the query feed f = q(S) published by the aggregator come from
the input source feeds in S and have passed the conditions imposed by the filters σj
(they are said to be relevant to query q). We consider that the items are sorted on their
publication date before being inserted in the query feed f. Other sorting criteria may also
be considered, such as the source feed sj from which the item comes from or some sort of
item importance score.
Any query q applied on a set of source feeds introduces a selectivity factor with values
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