The Unified Modeling Process in Web-Deployed, Object-Oriented Database Systems 245 in Java

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Multiple servers. An OO database system may manage a distributed database, that is, a set of ODL schemas and the corresponding OO databases, distributed on a set of disks inside a network. From the security point of view, several CORBA systems support both SSL and twofactor (but not three-factor) authentication in their implementation, although these features are still immature. Although these features are currently proprietary, recently they have been standardized for Version 3.0 of the CORBA specification. A word of caution is necessary however: Full usage of CORBA in implementing the colloquium between Web components and OO data sources is still in evolution, and unexpected problems may arise. Again, experience [DOOR Esprit project 1997] has shown that establishing a connection of a user interface to OO databases via some CORBA implementations may be awkward, and even not possible. Database vendors are solving these problems, but they still affect deployment in many ways. When accessing an OO database "from outside," CORBA's Interface Definition Language (IDL) allows us to define a CORBA wrapper, that is, an external syntactical interface to the database. Using IDL, the designer defines the syntactical interface selecting the methods needed for the particular purpose of the view. This view can then be used by a client outside the database (i.e., a client that runs in a process but is not a native client). The designer of the syntactical interface has then to implement the wrapper on top of the OO database, inside another process that is a native client. It should be noted that the definition of a syntactical interface does not guarantee any quality of service. CORBA architecture acts as a software bus between the application layer and the database. This deployment technique involves flattening the dynamic relationships among objects defined during the OMP process, translating them into interactions on the CORBA software bus. CORBA-Based Deployment Example We shall now use a very simple example to outline a standard CORBA-based deployment technique aimed at providing a standard link between design-level classes and the implementation-level OO database interface. Of course, this example intends only to show how, in principle, CORBA can make deployment independent from technology and project-related choices (providing a monolithic or Web-based client, using a Java servlet or an applet as a query interface, etc.), bringing it back under the methodology's umbrella. The basic idea can be outlined as follows. A CORBA Server is deployed as a generic OQL server. In our example, the server is a Factory object, used to create Servants that deal with clients' requests, according to the well-known Factory design pattern [Mowbray 1997]. Web-based Clients (e.g., Web-deployed Java servlets or applications) ask the Factory object for an O-Connection to be created and opened. The O-Connection syntactical interface should be designed to provide roughly the same functionality as the JDBC Statement for relational databases. Often, however, a subset of those functionalities may turn out to be sufficient. Each O-connection is the Web-based equivalent of a database session, during which clients may freely execute OQL commands. Before providing a description of this deployment technique, we need a few notions about CORBA-compliant invocations of distributed services. CORBA objects must
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The Unified Modeling Process in Web-Deployed, Object-Oriented Database Systems 245
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provide a syntactical interface written in an Interface Definition Language (IDL); objects themselves are to be implemented via inheritance or delegation as Servants in other words, network objects whose methods can be invoked remotely by other objects that possess a valid reference to them. On the other hand, a CORBA server object will instantiate the Object Request Broker (ORB), the standard software bus connecting clients and servants; then it will forward clients' requests to servants, hiding their physical location. Normally, the server starts by instantiating a servant and connecting it to the ORB, so that clients can obtain a remote reference to it. Such creation is realized by an Object Adapter (the simplest implementation of which is the Basic Object Adapter or BOA of the version 2.0 of the standard). However, servants can also be created by other servants connected to the ORB (again, according to the Factory design pattern). In our example, we shall utilize this latter method for deploying servants allowing remote access to OO databases. To invoke a servant, clients instantiate a local object (proxy or stub) representing a remote service to it. When the client invokes a method of the local proxy, the proxy marshals the input parameters and communicates them to the server via the ORB. The ORB forwards them to the BOA, which decodes the parameter according to the local type system and passes them to the skeleton, which is the entry point to the servant's implementation. If any return parameters are specified for the call, the procedure is repeated. By allowing both communicating objects to provide local proxies CORBA can be used to support a symmetric programming model. Implementation Guidelines We are now ready to provide some implementation guidelines. First of all, we observe that implementations of servants Factory, O-Connection (together with the ObjectSet object that will hold query results) can be done either via inheritance or delegation; for the sake of simplicity, here we shall briefly discuss the inheritance-based one. Using this technique, the interface implementation can be obtained simply by defining Java classes inheriting from the skeletons, as depicted in Figure 11.18. Then, the server object (Server.java) must be implemented, which instantiates the ORB, BOA, and Factory. Its sample code is:
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//ORB init org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(args, props); //BOA init org.omg.CORBA.BOA boa = orb.BOA_init (args, props); //Factory init Factory_impl f = new Factory_impl(orb); //connect Factory to the ORB orb.connect (f,"FactoryServant");
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In the preceding example, for the sake of simplicity we used the well-known Named Servant technique to associate a string name with a servant, while connecting it to the ORB via the connect method. Note that, when instantiating the Factory servant, the ORB is passed to the constructor. This way, the CreateConnection method instantiates an O-Connection object, connecting it to the ORB, according to the Factory design pattern.
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