[IEEE 2011 IEEE International Conference on Pervasive Computing and Communications Workshops (PerCom Workshops) - Seattle, WA, USA (2011.03.21-2011.03.25)] 2011 IEEE International Conference on Pervasive Computing and Communications Workshops (PERCOM Workshops) - Decoupling context-aware services

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  • Decoupling Context-aware ServicesSren Debois

    IT University of Copenhagendebois@itu.dk

    Arne John GlenstrupIT University of Copenhagen

    panic@itu.dk

    Francesco ZanittiIT University of Copenhagen

    frza@itu.dk

    AbstractWe present a novel software architecture for context-aware applications based on a distributed, non-monolithic, simpleand extensible relational model for representing context; aservice-oriented architecture for computing these relations in adecoupled, flexible fashion; and with data driven, event basedcommunication providing the kind of fine grained dynamicservice composition required in mobile and volatile environments.A prototype implementation is running a Bluetoothsensor-basedactive map of users at our home university.

    I. INTRODUCTION

    In this paper, we introduce a novel software architecturefor context-aware services. Within the last 18 years, theconstruction of software architectures for context aware ap-plications has been a hot topic [1], [2], [3]; context-awareservices joined the fray within the last five [4], [5]. Thearchitecture we propose (cf. Figure 2) has the same aims asmany of these works: it seeks to promote code reusabilityby encouraging compositionality of system design. It distin-guishes itself from these other approaches principally by beingdata-driven: Services are composed not by specifying whichservices invoke which other services, but rather by specifyingwhich services need what context information. For example,three different services might provide the same kind of locationdata: one based on Bluetooth (BT), one on WiFi, and one onGPS. Services consuming this location data need not concernthemselves with the origin of that location data, only the dataitself.

    This data-driven composition mechanism is of course notin itself novel; other architectures have separated context datafrom its computation. The novelty here is that we make avery fine-grained division of the computation of context intosuch decoupled services. As a consequence we get (a) veryfine-grained re-usability of code: very small computations canbe meaningfully re-used; and (b) a very neat mechanism forsensor-subsystem switch-over, as alluded to above.

    Our architecture is intended for the scenario where a largearea, say, a shopping mall, an airport, a train station, ora supermarket is equipped with physical sensors, such astemperature or location sensor, tracking the whereabouts ofcustomers. The tracking could be based on the clients mobiledevices, RFID equipped shopping carts, or other positioningtechnologies provided by the merchants.

    This work funded in part by the Danish Research Agency (grant no.: 2106-080046) and the IT University of Copenhagen (the Jingling Genies projects).Authors are listed alphabetically by last name.

    A key point is that our architecture is flexible and extensiblein the sense that several different merchants can deploy eachtheir system, perhaps relying on different technologies, yetstill allowing the context sensitive applications to operateseamlessly.

    While this scenario is not new [6], it is recently of renewedinterest: it is finally happening outside the laboratory. Theadvent of the ubiquitous hand-held mobile phone with BT,GPS, WiFi or RFID provides a cheap, non-intrusive means ofzone-based tracking of individuals in public spaces [7], [8],[9], [10], just to mention some location tracking technologies.At the same time, institutions such as airports, train stations,supermarkets etc. are beginning to deploy the necessary in-frastructure for context aware systems: tracking equipmentinstalled to find customer movement patterns [11], [12], [13]conveniently doubles as a source of location information forcontext aware services.

    Such environmentsairports, supermarkets, etc.are asso-ciated with particular activities, and knowledge about theseactivities is a source of context information different fromphysical, sensor-obtained data. This knowledge is stored indatabases, relating, e.g., passengers with flights, flights withdeparture times and gates, etc. Examples of activities providingcontext information are people going to airports to take planesand to supermarkets to buy groceries.

    In most cases the expected activity of users is to a degreepredictable from existing databases. In airports, the activitiesare almost fully formalised: a database somewhere knowspassengers names, passports and flight numbers, as well asplane departure times, gate numbers, etc. In supermarkets, adatabase somewhere knows exactly what goods are availablein the supermarket, where they are, and how fast those goodsusually disappear in the course of a normal business day. If asupermarket in question has a frequent-shopper program, somedatabase somewhere might also have a very precise idea aboutwhat particular individual shoppers tend to buy.

    Thus, the present architecture targets a scenario wherecontext awareness is based both on physical sensors and somewell-defined activities we may get from existing databasesascenario in fact being realized commercially right now.

    The architecture proposed in the present paper is not onlyproposed, it is in fact implemented and available for down-load [14]. A prototype active map tracking people at ouruniversity is also available online [15].

    8th IEEE Workshop on Context Modeling and Reasoning

    978-1-61284-937-9/11/$26.00 2011 IEEE 450

  • II. CONTEXT-AWARE SERVICES

    We contend uncontroversially that the central challenge ofcontext-aware computation consists of computing the actualcontext from whatever data is available. Software architecturesfor context-awareness distinguish themselves primarily bywhat way they assist that computation.

    Its important to note that for even the simplest of examples,such computation is non-trivial. To be concrete, suppose acontext-aware application needs to know whether some pas-senger in an airport has passed through security. This is asimple boolean piece of information, obtainable by simplyinspecting the current location of the passenger in question.Even so, in even the most naive of implementations, thiscomputation will typically require (at least) two steps. First,the information that the device with MAC-id 0x2198f172 iscurrently in range of sensor 0x31 must be converted to theinformation that passenger Jones is in the Lufthansa First ClassLounge. Second, we must check whether this Lounge is in factsituated before or after security.

    The opportunity for code re-use in the development ofcontext-aware applications stems from the observation thatwithin a given application domainsay, a specific airportsome of these computations will be almost universally useful.In the above example, the ability to map particular sensor in-puts to particular passengers high-level location would likelyhave such usefulness. In fact, also the information whether ornot a passenger has passed security is conceivably interestingto variety of applications, even if it is not universally useful.

    As usual for Software Oriented Architecture (SOA), weconsider a context-aware service to be something whichimplements a reusable, distinct computation of context. Theapplication in the above example then uses two services: onefor the low-level sensor-to-passenger mapping, and one for thehigh-level location-to-security-state mapping.

    However, put this way, there is no discernible difference be-tween a context aware service and a context aware procedure.To maximise the potential reuse of context-aware services, weshall further decouple them in the following section.

    III. CONTEXT-AWARE SERVICE INTERFACES

    Let us call the mapping of low-level sensor-data to high-level location L, and the mapping of high-level locationto security state H . Suppose that the low-level sensors itinterfaces with is the Ekahau WiFi-positioning system [7]. Inplaces where WiFi is not available, the airport can augment thelocation tracking with a BT-based positioning system. How dowe modify the existing context-aware application to use alsothe new BT-based tracking system?

    By use of the service H , the remainder of the applicationis already shielded from this change. But what should happento H and L themselves? In a procedural world, H and Lwould be tightly coupled. For H to also accommodate thenew location system we must implement a service L takingBT-based positions to high-level locations, and we would haveto modify H so that it always queried both L and L.

    But there is a better way! We know that the service H isonly interested in high-level locations. It is not interested inother services. There is no need for H to know where its high-level location information comes from; whether it be L, L orsome third entity is absolutely irrelevant to H .

    Thus we propose that services be further decoupled sothat the composition is not formed by one service directlyaccessing another service, but rather by one service requiringparticular data, no matter who provides it. In composition, therelevant identifier for a data-stream from a service is no longerits name, but rather the particular kind of data it produces andconsumes.

    However, this definition leaves us with a practical problem:how does a service specify the particular kind of data itproduces or consumes?

    IV. RELATIONAL CONTEXT MODELS & RELATIONTRANSFORMERS

    Describing what context-data is produced is of course a mat-ter of selecting a context meta-model. Fortunately, the problemof describing classes of context models, of constructing acontext meta-model, is by now fairly well studied, and hasproduced suggestions using various techniques, ranging fromtuple-space [2] and object-orientation [4] to ontology-based[16], and even combinations (e.g. OO with ontology [17]).

    The great strength of ontologies is that they come withdescription logics as declarative languages for specifying howcontext computation is done. However, the ontological meta-model invites a centralized approach, with one centralizeddatabase and reasoning engine. This centralization is inher-ently at odds with the breaking down of the computation intosmall, re-usable pieces that we are looking for.

    However, we salvage the key abstraction from ontology-based context-models: at its core, an ontology defines howobjects are related. So perhaps to describe context, it issufficient to agree on the meaning of some set of relations?It seems so; any data relevant to context-aware applicationswould seem to immediately fit the relational model. For theabove example, an ekahau relation relates WiFi MACs andsensor IDs; in particular, one data point could be

    ekahau [wifimac=0x2198f172, wifisensor=0x31].A different relation would indicate the high-level locations:

    loc [passenger=Jones, area=Lufthansa FirstClass Lounge]

    Similarly, a relation relating BT MAC-addresses to other BTMAC-addresses might represent the information about whichmobile phones are currently being detected by which BTsensor. A relation relating passenger names to flight numbersmight represent the information about which passengers arebooked on which flights. A relation relating flights, gatenumbers and departure times might represent informationabout which flights are scheduled to depart from which gatesand at which times, etc.

    Incidentally, notice that the meta-model of relation is ac-tually that of relational databases. This observation should, at

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  • least on an intuitive level, justify our expectation that whatevercontext we wish to describe, it should be expressible in thisrelational model.

    So, the interface of a service is the type of relations itconsumes and produces. Both of the services L, L producethe loc relation; the service H consumes that relation.

    Do notice the pleasing abstractions here: the context isrepresented as some set of relations; computation on context isthen essentially a function on relations, what we might thinkof as a relation transformer. The entire computation of high-level context from low-level context is performed by lettingsome set of such relation transformers interact.

    V. DATA DRIVEN EVENT BASED COMMUNICATION

    At this point, the only remaining key idea of our architectureis the mechanism needed to connect services wishing tocommunicate on some relation, e.g., to connect the loc relationoutput of L, L to the loc relation input of H . The archi-tecture supports a basic push-based event publish-subscribemechanism, on top of which other interaction patterns can beconstructed if needed.

    The routing of events is completely data-driven; a servicesubscribes to observe changes in a relation, and any eventsemitted changing the state of that relation are routed to theservice. For instance, to subscribe to all changes in the ekahaurelation, a service issues a subscription (pseudocode):

    subscribe ekahau [wifimac=?, wifisensor=?]To monitor only a specific area, the service fixes a value inthe subscription:

    subscribe ekahau [wifimac=?, wifisensor=0x31]A subscription will not by itself make events appear; that

    requires services publishing that event. It is, however, accept-able for a service to subscribe to events not currently beingpublished by anyonethey might appear later.

    VI. ARCHITECTURE SUMMARY

    To summarize, in our architecture, the computation of whatis the current context is broken down into units of computationcalled services. Services consume small parts of the context-information and produce in turn new small parts. The partsthemselves are represented as relations, and services nevercommunicate directly: rather, they indicate their interest inparticular relations. Notice that a service needs to know onlythe relations it is interested in; this makes fairly easy to add,remove and change services.There is no central place hostingthe definition of context. This definition is itself dynamic, andchanges with the available services. Actual communicationbetween services rely on an event-distribution mechanism, ontop of which one can build query-mechanisms. The interfacebetween the services and the event-distribution sub-system islisted in the example listed in Table I (pseudocode, refer to[14] for actual interface).

    We emphasize that in this architecture there is no centralizedrepresentation of the current context. The architecture onlyforces representation of a part of the context the instant that

    part must be transmitted as an update to a relation fromone service to another. This is the only situation in whichthe architecture requires explicit representation of the contextbeing manipulated. Individual services are of course free topreserve as much or as little of this information as they mightlike; one might for instance implement a location history-service by listening for and locally storing location-events,answering queries based on these stored events.

    The architecture helps disseminating changes in relationsamong the different services, as long as they agree on theinterface i.e. the definition of the relation they are interestedin or that they provide. This dissemination process happen atrun-time, so it is possible to change it dynamically,...

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