Component Search Service and Deployment of Distributed ...

that supports the trading service in a distributed environment.It can be viewed as an object through which other objectscan advertise their capabilities and match their needsagainst advertised capabilities [10]. The trading allows customersto discover and select services at run time and bindto them dynamically. These services are early exported bysuppliers. In the following, we present some trading systemsand discuss some of their related aspects.3.1. Trading in ANSAThe trading in the ANSA system is defined as follows:the activity of choosing services, such that they match someservice requirement. The choice is based on the comparisonof the specification of a service required (provided bya prospective consumer) and the service specifications suppliedby service providers or their agents [2]. An ANSAtrader is defined as an object performing the trading activitiesbased on the entire or on a subset of the service offers.The trader has not a special status, it is considered like anyother object [2]. A trading service is not the only servicethat can store knowledge about and references to other services;on the contrary, any object and service can do this.Service offers selection is based on conformance of servicetype and a constraint expression which must characterizethe required service.3.2. Information exchange in UDDIUniversal Description, Discovery and Integration(UDDI) defines a framework for publishing and discoveringinformation about web services [13]. Consideredservices consist of specific business functionalities exposedby companies in order to be used by other companies.UDDI adopts an approach based on the so-calleddistributed registry which includes businesses and theirservice descriptions. Built on SOAP (Simple Object AccessProtocol) [14] and XML, UDDI intends to ensure interoperabilitythrough a cross-language and cross-platformapproach. Service description in UDDI must provide threekinds of information: white, yellow and green (technical)pages-equivalent information, like the OMG specifications[7]. UDDI business registry provides a framework forexchanging service information on Internet. The UDDIregistry is a distributed service with multiple root nodesthat replicate data within each other. Service descriptionconsists of four types of information: business information,service information, binding information, and informationabout specifications for services. These types are structuredin a hierarchy and correspond to the following entities:¢businessEntity: serves as the top-level informationmanager concerning a business unit. It includes supportfor yellow pages taxonomies (particular industry,product category, geographic situation).¢businessService: is a sub-structure of the businessEntitystructure. Its role is to keep information relatedto either a business process (purchasing services, shippingservices, etc.) or category of services.¢bindingTemplate: within each businessService liveone or more technical web service descriptions in abindingTemplate structure (connection address of theservice, etc.).To discover a web service using UDDI, the first step isto locate the businessEntity information of the service. Theprogrammer can drill down for more detail about a businessServiceor request a full businessEntity structure. Theprogrammer then selects a particular bindingTemplate anduses it for contacting the service.As we can see it, the trading in those systems is based onthe notion of service represented by an interface. Searchingfor service offers requires the use of a matching approachwhich allows the trader to select the well-suited service offercorresponding to a requested one.3.3. Service offers selectionAn importer uses a trader to search for services matchingsome criteria. A service type, which represents the informationneeded to describe a service, is associated with eachtrader service [10]. It comprises an interface type and somenamed property types. Services types can be structured intoa hierarchy reflecting interface type inheritance and determiningthe substitution rules between service types.Service type conformance allows clients wishing to usea service which they can’t find to substitute it for anotherservice which conforms to the requested one.Conformance in ANSA is based on the no surprises rulein which an interface is substituted for another supportingall its operations. Moreover, each operation of the secondinterface exists in the first one with the same name andits type conforms. Operation conformance means that theyhave the same lists of parameters and the same terminationconditions [2].4. Component TradingComponent-based application development requires acomponent description language like IDL3 [9] or CDL [5].This kind of languages describes the context of components(required/provided interfaces, emitted/received events, etc.)and semantic relationships between component specificationsand implementations (e.g. component £ is equivalent

implies that components must be described in an abstractfashion in order to allow developers to find and integratethe appropriate components. They argue that these abstractionsmust be represented in ontologies. Furthermore, theydiscuss interface adapting if the specifications do not matchexactly. Interface adapting consists of adapting the outputsof one method to inputs of another method (adapting oneIDL specification to another).service must be able to distinguish these requests and searchfor active component instances rather than for implementationpackages. Figure 3 shows the main components of thetool:Component Search ServiceIn conclusion, for a component to be reusable easily, itmust be quickly found and understood by developers. Semantictrading which emphasizes on the functionality ofcomponents contributes significantly to achieve these objectives.5. Component search serviceRequestReplyKnowledge BaseTranslatorSearch ToolsNaming ServiceURL LocatorIn this section, we describe a search service we are currentlydeveloping within the framework of a project calledCÉSURE. CÉSURE is a project where the objective is toconceive a platform for applications of service providedto mobile users [1]. CÉSURE applications are distributedapplications focusing on the mobility of users through thesmart card support and developed using a component-basedmodel. CÉSURE is intended to provide tools for development,deployment and maintenance of applications. A numberof system utilities are developed in this scope. This includescomponent search, monitoring, fault tolerance (componentreplication, failure detection, application reconfiguration,etc.) and deployment tools.In CÉSURE, the deployment of an application consistsof searching component packages, active instancesand hosts on which component implementations must beinstantiated. For example, a user which has subscribedto a weather service, may wish to use his service from amobile terminal. When it connects, the search tool mustsearch for a component corresponding to a man/machineinterface which must be instantiated on the user’s terminal.Moreover, an active component corresponding to a weatherserver must be found by the search tool. Therefore, thesearch tool must be able to search for component packages,instances as well as hosts on which components will be instantiated.5.1. Component search service architectureIn this subsection, we present the architecture of thecomponent search service which we propose. The aim ofthe service is to respond to component search requests whatevertheir form. In other words, the tool serves as namingand trading services at the same time. The semantic tradingis used. Furthermore, since distributed application deploymentmay use already activated component instances, theChoice Policy(preferences, OCL + ...)Trading ServiceFigure 3. Component search service architecture.¢Translator: this is the main component of the service.It is intended to receive requests and reply to theclient (a deployment tool in general). The translatoruses the knowledge base to transform original requestsinto well-suited, easily understandable and completerequests using the ontology stored in the knowledgebase. Note that, syntactic and semantic informationsare combined in the search process.¢Search Tools: they represent the search space. Theyconsist of some classical search tools in which servicematching is based on interface type compatibility.This includes CORBA trading and naming services, aswell as a tool for locating component packages throughURLs. So, component packages, instances or interfacescan be searched by the same tool. Moreover, thesearch criteria may be a description of the entity (componentpackage, service), a symbolic name or an absolutelocation of the entity. Following the case, thetrading, the naming or the URL locator is used.¢Knowledge base: the Knowledge-base constitutes abase in which all informations (ontologies) are kept. Inaddition, it includes the matching rules between termsfrom different domains. The matching rules constitutethus an integration of ontologies encoding the concepts

¢component behav: expresses the behavior of the component.Behavioral information consists, in particular,of invariants and pre/post conditions. For each componentport (interface), a number of invariants maybe specified and for each operation of the interface,a number of pre/post conditions can be defined. Thebehavioral information specification may be expressedin an Eiffel-like language.¢reuse conds: this structure is kept for eventually expressingthe reuse conditions of a component. Theseconditions may be inspired from the reuse contractsstudied by De Hondt [4].¢component funcs: describes component functionalitiesin a textual or other form. Component importersmay use this possibility to search for some componentby describing its functionalities. When a componentis obtained, its context dependencies including interfacedescription, syntax and behavioral conditions areknown. So, the component can be safely used.5.2.3 Component search scenarioWhen a request is submitted to the search server, it goesthrough the translator, the heart of the model. The translatortransforms the request, rich semantically, to a syntacticrequest, using the matching rules of ontologies stored in theknowledge base. The resulting request is then likely to beunderstood in other domains and adapted to the traditionalsearch forms.Matching rules stored in the knowledge base and resultingfrom ontology integration are used in a dictionary usagemanner. This allows translating terms originating from adomain in another domain. So, requests initiated from adomain can be understood and satisfied in another differentdomain, without user intervention.All the search criteria of components in the search requestare optional. The search may concern a part of a componentdescription. So, clients may provide only a descriptionof the component’s functionalities to access the full semanticand syntactic characteristics of the component. Thisopportunity may be very useful in the development phase ofapplications.6. Component search service and deploymentThe deployment phase is the last step in the developmentprocess of applications. The deployment of a distributedapplication consists of installing and instantiating its componentson the hosting servers, configuring them and assemblingthem following the application assembly specification.In many cases, some components used in the assemblyare already active, their instantiation is thereforeunnecessary. The component search service is responsiblefor identifying and locating these components. The searchservice is typically used by a deployment tool in order tosearch for appropriate components and sometimes to optimizetheir deployment. Deployment tools, provided by anORB or other vendors, vary from simple tools which justguarantee the deployment constraints, to more elaboratedtools which ensure more advanced properties, like componentlocation services and load balancing.6.1. Deployment process in CCMIn CCM, the deployment is intended to hook-up a socalledlogical component topology to a physical computingenvironment [9]. In CCM, the deployment is triggered bylogical component assembly specified in an assembly file,and concerns components and component homes.AssemblyFactoryDeploymentApplicationComponentInstallationServerActivatorComponentServerContainerFigure 4. CCM deployment architecture.AssemblyComponentHomeComponentIn the following, we summarize the basic steps whichcontribute in the deployment process in CCM [9]:1. Determine the target host configuration, on whichcomponents will be installed and instantiated. This isdone most likely in collaboration with the user. Componentsare deployed in some cases following a processor host collocation constraints;2. Install non-already installed component implementationson appropriate platforms;3. Instantiate components and component homes on particularhosts. This can take into account the processand host collocation constraints;

4. Connect components as specified in the application’sassembly file.Figure 4 illustrates the architecture of the deploymenttool specified by CCM and the relationships between theobjects participating in the application deployment process.The deployment is carried out by a deployment applicationand some helper objects (component repositories, assemblyand component factories, containers, etc.).6.2. Deployment process in CÉSUREIn this subsection, we will show how the infrastructureservices are used at the deployment phase of the distributedapplication in CÉSURE.Although the deployment process is well specified andstandardized in CCM, this specification presents a numberof drawbacks including:¢poorness of the mechanism which does not addresssome issues like the localization of component implementationsand the management of host configuration;¢vagueness of the deployment process which doesnot fix a number of details such as the localizationof helper objects (Assembly, ComponentInstallation,etc.). This is caused, in our opinion, by the fact thatthe process is not implemented and tested.The additional services (component search and monitoringservices) can be considered as Common Object Services(COS) and can be retrieved by the ORB, like in standardCORBA programming. Therefore, these services maybe centralized, distributed or semi-distributed. The deploymenttool may be unaware of their localization. However,an important issue is that the infrastructure must optimizethe use of these services, by finding the nearest service agentin case of a distributed architecture for example.Figure 5 depicts out the enhanced deployment processwhich we propose for CÉSURE applications. The mainsteps of CCM deployment process are adopted and adapted.Furthermore, some additional steps are included followingthe contribution of the additional provided services. Thedeployment process proceeds as follows:1. The DeploymentApplication is started. It begins bylocating the component search and monitoring servers.Then, it addresses the search service in order to geta host configuration on which components will be instantiated.The location of components is either specifiedby the application in the assembly descriptor file,or searched by the component search server. The locationof hosts is determined:by the application in the assembly file,¢or by the search service in conjunction with the¢monitoring server following some constraints.Component search and monitoring servers cooperatein order to keep an up-to-date state information onhosts. The host configuration management is ensuredby the monitoring server which supervises host activities(load state, etc.) and informs systematically thesearch server of the state evolution of the configuration.So, the search process for hosts may involve preciseload information in the selection criteria.2. The second phase in the deployment process is the installationof the retrieved and non-installed componentimplementations on the appropriate platforms. This isdone by calling the install operation on the ComponentInstallationobject. The parameters of this operationare the implementation identifier (id) and a stringdenoting the component file location.3. After the component installation phase is complete, thedeployment application creates an Assembly object.This is done by calling an AssemblyFactory objecton the host where the assembly object must be created.The parameters of this operation are especiallythe assembly descriptor file location (steps 3 and 3.1on the figure). The role of the assembly objects is tocoordinate the creation and destruction of componentassemblies.4. The next step consists of creating the assembly by connectingcomponent instances. Before connecting thecomponents, component and home instances must befirst created. Component instances creation is orchestratedby the Assembly object on the basis of collocationconstraints and on host configuration obtained atthe first phase.5. In order to create a component, the Assembly objectmust create a component server, create a containerwithin the server, install a home object within theserver and then use the home to create the component[9]. This procedure is carried by a ServerActivatorobject which resides on the host where the componentinstance is to be created 2 . Thus, the component creationis achieved in the following steps:(a) The Assembly object calls the createcomponent server operation on theServerActivator of the host on which the componentis to be created. An empty server is2 There is one ServerActivator instance on each host. Its reference mustbe known by the Assembly object.

Deployment Host2aComponentInstallationdServerActivatorDeploymentApplicationServicesMonitoringServer 1 3bcefe’6, 7ComponentServerContainer ContainerHome Home Home HomeComponent Component Component ComponentComponent Component Component ComponentHost 11.1ComponentSearchServer...ComponentInstallationServerActivatorAssemblyFactory3.1AssemblyComponentServerContainer ContainerHome Home Home HomeHost nComponentComponentComponentComponentComponentComponentComponentComponentFigure 5. Deployment steps in CÉSURE.then created and the ComponentServer object’sreference is returned.(b) When the Assembly object obtains the referenceof the created ComponentServer, it uses it tocreate a Container within the server. This isachieved by calling the create container operationon the ComponentServer object. The containeridentifier is passed to the operation whichcreates the container and returns a reference to itsinterface.(c) The next step consists of installing the ComponentHomewithin the container. The Assemblyobject calls then the install home operation onthe Container interface, giving it the identifier ofthe component. The operation returns a referenceto the created component home interface.(d) In the ComponentHome creation operation, thecontainer calls the get implementation operationof the ComponentInstallation object. Thisoperation gets a component implementationidentifier and returns the absolute location of thisimplementation. The container then loads theimplementation and instantiates the ComponentHomeobject.(e) The next step consists of creating the Component.This is accomplished by the ComponentHomeobject, when the Assembly object callsthe create component operation provided by itsinterface. The return value of the operation is areference to the created component (of type CC-MObject).(f) The component is then configured, if required.6. When all components are created, the Assembly objectconnects them based on the information specifiedin the connect block of the assembly descriptor. Thecomponent connection is accomplished by calling thereceptacle connect operation of the CCMObject reference.7. After that, the Assembly object calls configurationcomplete on each object in the assembly to signalthat all its initial connections have been fixed.7. ConclusionThe development of large scale distributed applicationsrequires some interoperability level, in order to allow heterogeneousapplications to cooperate. Search tools likenaming and trading services provide a framework facilitatingthis interoperability, through the notion of service offersexported by some suppliers and made at the disposal of

service importers or clients. Search approaches are generallybased on the conformance of the type and sometimesthe properties of services. However, in componentbasedmodels, this type of search becomes inadequate. Indeed,component-based distributed applications are constructedby assembling some software components. Matchingis therefore based on component substitutability whichexpress the conditions under which a component may besubstituted for another. These conditions include syntacticproperties characterizing component dependencies (interfaces,events, etc.) as well as semantic properties, includinginformation on component functionalities and reuseconditions. This is even more significant in large scale environments,in which terms from different domains may besubject to ambiguities and miseinterpretation.Search service is an important tool facilitating the distributedapplications development, deployment and interoperability.Therefore, developing and experimenting searchservices based on semantic trading is very important, in orderto measure the efficiency, feasibility and performance ofthe approach. Indeed, semantic trading constitutes an essentialframework for developing and supervising distributedapplications especially in large scale environmentsThe assembly, created in CCM deployment steps, specifiesan initial configuration and does not address the evolutionof the component connections. The component connectionsmay change following the application evolution andunderlying environment state changes. These changes includeespecially failures, load unbalance, etc. The applicationmust be able to adapt to these situations and to continueto evolve. Therefore, tools allowing to keep these propertiesmust be considered in the deployment and supervision processof applications.Within the framework of the component search service,other issues and problems must be addressed. This concernsmany aspects and in particular the versioning problem. Indeed,when multiple versions of the same component mayco-exist, the search server must be able to choose the appropriateversions among all the available ones. A solution tothis problem may be the consideration of the version property,which must be specified by the client, in the search criteria.Another solution may be a policy used by the searchservice which allows the automation of this task, such as thechoice of the most recent version systematically.References[3] J. H. Gennari, A. R. Stein, and M. A. Musen. Reusefor knowledge-based systems and Corba components. In10th Knowledge Acquisition for Knowledge-Based SystemsWorkshop (KAW’96), Banff Canada, November 1996.[4] K. D. Hondt, C. Lucas, and P. Steyaert. Reuse contractsas component interface descriptions. In WCOOP’97 - 2ndInternational Workshop on Component-Oriented Programming,pages 43–49, Turku Centre for Computer Science,1997.[5] J. Kiniry. CDL: A component description language. In submittedto the Advanced Topics Workshop (ATW) of the 5 thUSENIX Conference on Object-Oriented Technologies andSystems (COOTS’99), March 1999.[6] R. Marvie and P. Merle. CORBA: From objects to components.In Tutorial for the 14th European Conferenceon Object Oriented Programming (ECOOP’2000), Nice &Cannes, France, June, 2000.[7] The Common Object Services Specifications, Volumes 1 &2: Architecture and specification - revision 2.0. John Wileyand Sons Inc., 1994.[8] The Common Object Request Broker: Architecture andspecification - revision 2.0. OMG TC Document 97-02-25,July 1996.[9] Corba Component. OMG Documents orbos/99-07-01orbos/99-07-02 orbos/99-07-03 ptc/99-10-04 Joint RevisedSubmission, 1999.[10] Trading object service specification. Associated OMG Documents- Trader Service V1.0, May 2000.[11] C. Szyperski. Component Software: Beyond Object-Oriented Programming. Addison-Wesley, 1998.[12] S. Terzis and P. Nixon. Component trading: The basis fora component-oriented development framework. In Workshopon Component-Oriented Programming (WCOP’99),European Conference for Object-Oriented Programming(ECOOP’99), Lisbonne, Portugal, June 1999.[13] Universal Description, Discovery and Integration.UDDI technical white paper. UDDI white papers,http://www.uddi.org, September 2000.[14] W3C. The simple object access protocol (SOAP) 1.1. W3CNote 08 May 2000 http://www.w3.org/TR/SOAP.[15] N. Wang, D. C. Schmidt, and C. O’Ryan. Component-BasedSoftware Engineering: Putting the Pieces Together, chapterAn Overview of the CORBA Component Model. Addison-Wesley, 2001 (still).[16] J. Warmer and A. Kleppe. OCL: The constraint languageof the UML. Journal of Object-Oriented Programming, 13,May 1999.[17] G. Wiederhold. Interoperation, mediation and ontologies.In International Symposium on Fifth Generation ComputerSystems (FGCS94), Workshop HCKB, volume W3, pages33–48, Tokyo, Japan, December 1994.[1] Cesure. Infrastructure système. D5, Novembre 2000.[2] J. P. Deschrevel. The ANSA model for trading and federation.Technical Report ANSA phase III APM.1005.01,Architecture Projects Management (APM) Ltd., Cambridge,UK, July 1993.