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3.2 RFID Complex Event Specifications
Major RFID event processing frameworks use expressive languages to
specify complex RFID events. As an RFID complex event specification
language, SASE [42, 68] language is SQL-like, and supports sequencing,
negation operation (!), parameterized predicates and sliding windows.
The SASE language can be used to filter, correlate and transform prim-
itive RFID events into complex events to answer semantic queries. The
overall structure of SASE language is:
EVENT < event pattern >
[WHERE < qualification > ]
[WITHIN < window > ]
The EVENT clause describes a sequence pattern, and its components
are occurrence or non-occurrence of component events in a temporal
order. The WHERE clause specifies constraints on those events. The
WITHIN clause specifies the sliding window for the whole sequence of
events. For example, the complex event corresponding to shoplifting in
a retail store can be specified as Q 1 :
Q 1 : EVENT SEQ(SHELF x, !(COUNTER y), EXIT z)
WHERE x.Oid=y.Oid=z.Oid
WITHIN a hour
In Q 1 , SEQ denotes sequence pattern. SHELF , COUNTER and EXIT
are different event types. The sign ' ! ' denotes non-occurrence of an event
(also called as a negation event).
To enhance its expressibility and adaptability, the SASE language has
been extended to support Kleene closure [69]. NEEL [71] is a complex
event specification language for the definition of embedded sequences (or
nested sequences) of RFID events, and is essentially an extension of the
SASE language.
Based on Snoop [72], an expressive event specification language for
active databases, Wang and et al [73, 74] formalize the semantics and
specification language of RFID events, and propose powerful rules for
RFID data filtering, transformation, aggregation, and real-time moni-
toring. For clarity and convenience, here we refer to this specification
language as RCEDA (i.e., RFID Complex Event Detection Algorithm)
language.
The RCEDA language defines three basic non-temporal constructors
including OR (
), and five temporal constructors
including SEQ(;) , TSEQ(:) , SEQ + (; + ) , TSEQ+(: + ) and WITHIN .Most
temporal constructors come with two types of temporal constraints: the
), AND (
)and NOT (
¬
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