A model for Context-aware Databases

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What is Context? Ο καθένας ορίζει το context διαφορετικά...

“location, identities of nearby people and objects, and changes to those objects”

“environment or situation” etc…

Δύο πιο γενικοί ορισμοί: ContextContext is any information that can be used

to characterize the situation of an entity. A system is context-awarecontext-aware if it uses context

to provide relevant information and/or services to the user, where relevancy depends on the user’s task.

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Motivation Πολλά διαφορετικά μοντέλα για την περιγραφή του

context. Όμως τελικά η διαχείριση της πληροφορίας με

βάση το context γίνεται στο επίπεδο των εφαρμογών. Redundancy: Η λογική και οι βασικές

λειτουργίες υλοποιούνται από κάθε εφαρμογή ξεχωριστά.

Οι υλοποιήσεις εξαρτώνται στενά από τη φύση της κάθε εφαρμογής και τις επιλογές κατά τον σχεδιασμό της.

Maintenance overhead.

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Motivation Ο στόχος είναι:

Ομοιόμορφος χειρισμός της πληροφορίας και του context που την χαρακτηρίζει.

Διαχείριση του context ως «first class citizen» από τα ΣΔΒΔ.

Πλεονεκτήματα: Καλύτερος σχεδιασμός. High level query language. Efficient access/optimization. Μηχανισμός για consistency checking ως

προς το context.

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Context Definition World

environment under which data obtain a substance. Dimension

the set of parameters used to specify the worlds. example set of dimensions D = {device, payment}

Context specifier [device=PC] [device=PDA, payment in {credit card, cash}]

Universal context: [] Empty context: [-]

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Example Web site about digital cameras. Information entity dcameradcamera with

attributes: Brand, Model, MPix, Photo, Price

Dimensions: devicedevice ranging over {PC, PDA, CELL} paymentpayment ranging over {Credit Card,

Cash} Photo: defined only for worlds with

browsing device in {PC,PDA} Price: defined in every world but its

values may change

World Device

Payment

w1 PC Credit Card

w2 PDA Credit Card

w3 CELL Credit Card

w4 PC Cash

w5 PDA Cash

w6 CELL Cash

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Context Relation dcamera

Brand Model MPix Photo Price

<e1>

<e2>

<e3>

<e4>

...w1w2w3w4

Attribute

Entity

World

w5w6

Facet

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Context-Project πModel[ ], MPix[ ], Price[ ] dcamera

<e1>

<e2>

<e3>

<e4>

...w1w2w3w4

Brand Model MPix Photo Price

w5w6

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Context-Project πModel[Device=PC,Payment=CreditCard], MPix[Device=PC,Payment=CreditCard],

Price[Device=PDA,Payment in {CreditCard,Cash}] dcamera

<e1>

<e2>

<e3>

<e4>

...w1w2w3w4

Brand Model MPix Photo Price

w5w6

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World Project κ[w1,w4] dcamera

Brand Model MPix Photo Price

<e1>

<e2>

<e3>

<e4>

...w1w2w3w4w5w6

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Entity Context-Select σentity (BRAND[ ]=‘Kodak’ AND MPix[]>3)

dcamera

Brand Model MPix Photo Price

<e1>

<e2>

<e3>

<e4>

...w1w2w3w4w5w6

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Entity Context-Select σentity (BRAND[ ]=‘Kodak’ AND MPix[]>3)

dcamera σentity (Price[Device=PC,Payment=Cash] <

Price[Device=PC,Payment=CreditCard]) dcamera σentity (BRAND[ ] = Kodak AND Price < 250)

dcamera NO context specifier evaluate the condition to true if there exists at least one world where the condition holds.

Πως αποτιμάται το condition στους κόσμους όπου δεν ορίζεται ένα attribute??

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Facet Context-Select σ facet (Price < 500) dcamera

<e1>

<e2>

<e3>

<e4>

...w1w2w3w4

Brand Model MPix Photo Price

w5w6

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Context Relation accessories

Name Model Desc Price

<e1>

<e2>

<e3>

<e4>

...w1w2w3w4w5w6

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Context Cartesian Project/Join

Brand ... Price Name Model ... Price

w1w2w3w4

<e1>

<e1>

<e1>

<e2>

...

w5w6

<e’1>

<e’2>

<e’3>

<e’1>

...

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Illustrating Example

Ctx-Rel1 context cartesian product(dcamera, accessories)

Ctx-Rel2 σentity (dc.Model[ ] = ac.Model[ ] AND BRAND[ ]=‘Kodak’ AND

dc.Price[Device=PC,Payment=Cash]<400)Ctx-Rel1

Result πdc.Model[Device=PC,Payment=Cash],

dc.Price[Device=PC,Payment=Cash],

ac.Name[Device=PC,Payment=CreditCard],

Ac.Price[Device=PC,Payment=CreditCard]Ctx-Rel2

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CR vs Relational model Relational implementation:

A single demoralized relation. Number of different worlds relations.

The link between the facets of the same entity is lost.

The link between the same attribute under different worlds is lost.

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Future Work Extend the relational calculus and algebra to

incorporate context. Context-aware query language for the CR

model. Possible uses of CRModel

Time/histories of data. Personalization/profiles. Web/mobile services.

Design efficient access methods to take context into account.

Query optimization.

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