What Is WebLogic Server?
The following sections provide an overview of the WebLogic Server e-commerce platform:
The WebLogic Server Solution
Today's business environment demands Web and e-commerce applications that accelerate your entry into new markets, help you find new ways to reach and retain customers, and allow you to introduce new products and services quickly. To build and deploy these new solutions, you need a proven, reliable e-commerce platform that can connect and empower all types of users while integrating your corporate data, mainframe applications, and other enterprise applications in a powerful, flexible, end-to-end e-commerce solution. Your solution must provide the performance, scalability, and high availability needed to handle your most critical enterprise-scale computing.
As the industry-leading e-commerce transaction platform, WebLogic Server allows you to quickly develop and deploy reliable, secure, scalable and manageable applications. It manages system-level details so you can concentrate on business logic and presentation.
WebLogic Server contains Java 2 Platform, Enterprise Edition (J2EE) technologies. J2EE is the standard platform for developing multitier enterprise applications based on the Java programming language. The technologies that make up J2EE were developed collaboratively by Sun Microsystems and other software vendors, including BEA Systems.
J2EE applications are based on standardized, modular components. WebLogic Server provides a complete set of services for those components and handles many details of application behavior automatically, without requiring programming.
WebLogic Server 6.1 with J2EE 1.2 and J2EE 1.3 Functionality
BEA WebLogic Server 6.1 is the first e-commerce transaction platform to implement advanced J2EE 1.3 features. To comply with the rules governing J2EE, BEA Systems provides two separate downloads: one with J2EE 1.3 features enabled, and one that is limited to J2EE 1.2 features only. Both downloads offer the same container and differ only in the APIs that are available.
WebLogic Server 6.1 with J2EE 1.2 Plus Additional J2EE 1.3 Features
With this download, WebLogic Server defaults to running with J2EE 1.3 features enabled. These features include EJB 2.0, JSP 1.2, Servlet 2.3, and J2EE Connector Architecture 1.0. When you run WebLogic Server 6.1 with J2EE 1.3 features enabled, J2EE 1.2 applications are still fully supported. The J2EE 1.3 feature implementations use non-final versions of the appropriate API specifications. Therefore, application code developed for BEA WebLogic Server 6.1 that uses the new features of J2EE 1.3 may be incompatible with the J2EE 1.3 platform supported in future releases of BEA WebLogic Server.
WebLogic Server 6.1 with J2EE 1.2 Certification
With this download, WebLogic Server defaults to running with J2EE 1.3 features disabled and is fully compliant with the J2EE 1.2 specification and regulations.
Product CD Installers for J2EE 1.2 and 1.3
In addition to being available at http://commerce.bea.com/downloads/products.jsp, both distributions are provided on the WebLogic Server 6.1 product CD. (On Windows machines, the installer for WebLogic Server with J2EE 1.3 features enabled starts automatically when you insert the CD.)
WebLogic Server consolidates Extensible Markup Language (XML) technologies applicable to WebLogic Server and XML applications based on WebLogic Server. A simplified version of the Standard Generalized Markup Language (SGML) markup language, XML describes the content and structure of data in a document and is an industry standard for delivering content on the Internet. Typically, XML is used as the data exchange format between J2EE applications and client applications, or between components of a J2EE application. The WebLogic Server XML subsystem supports the use of standard parsers, the WebLogic FastParser, XSLT transformers, and DTDs and XML schemas to process and convert XML files.
Application Deployment Across Distributed, Heterogeneous Environments
WebLogic Server provides essential features for developing and deploying mission-critical e-commerce applications across distributed, heterogeneous computing environments. These features include the following:
WebLogic Server 6.1 uses Web Services Description Language (WSDL) 1.1, an XML-based specification, to describe Web services. WebLogic Web Services use Simple Object Access Protocol (SOAP) 1.1 as the message format and HTTP as a connection protocol.
What Is WebLogic Express?
BEA WebLogic ExpressTM is a scalable platform that serves dynamic content and data to Web and wireless applications. WebLogic Express incorporates the presentation and database access services from WebLogic Server, enabling developers to create interactive and transactional e-business applications quickly and to provide presentation services for existing applications.
WebLogic Express offers many services and APIs available with WebLogic Server, including WebLogic JDBC features, JavaServer Pages (JSP), servlets, Remote Method Invocation (RMI), and Web server functionality.
WebLogic Express differs from WebLogic Server in that WebLogic Express does not provide Enterprise JavaBeans (EJB), Java Message Services (JMS), or the two-phase commit protocol for transactions.
WebLogic Server Application Architecture
WebLogic Server is an application server: a platform for developing and deploying multitier distributed enterprise applications. WebLogic Server centralizes application services such as Web server functionality, business components, and access to backend enterprise systems. It uses technologies such as caching and connection pooling to improve resource use and application performance. WebLogic Server also provides enterprise-level security and powerful administration facilities.
WebLogic Server operates in the middle tier of a multitier (or n-tier) architecture. A multitier architecture determines where the software components that make up a computing system are executed in relation to each other and to the hardware, network, and users. Choosing the best location for each software component lets you develop applications faster; eases deployment and administration; and provides greater control over performance, utilization, security, scalability, and reliability.
WebLogic Server implements J2EE, the Java Enterprise standard. Java is a network-savvy, object-oriented programming language, and J2EE includes component technologies for developing distributed objects. This functionality adds a second dimension to the WebLogic Server application architecture—a layering of application logic, with each layer selectively deployed among WebLogic Server J2EE technologies.
The next two sections describe these two views of WebLogic Server architecture: software tiers and application logic layers.
Software Component Tiers
The software components of a multitier architecture consist of three tiers:
Client applications access WebLogic Server directly, or through another Web server or proxy server. WebLogic Server generally connects with backend services on behalf of clients. However, clients may directly access backend services using a multitier JDBC driver.
Figure 1-1 illustrates the three tiers of the WebLogic Server architecture.
Figure 1-1 Three-Tier Architecture
WebLogic Server clients use standard interfaces to access WebLogic Server services. WebLogic Server has complete Web server functionality, so a Web browser can request pages from WebLogic Server using the Web's standard HTTP protocol. WebLogic Server servlets and JavaServer Pages (JSPs) produce the dynamic, personalized Web pages required for advanced e-commerce Web applications.
Client programs written in Java may include highly interactive graphical user interfaces built with Java Swing classes. They can also access WebLogic Server services using standard J2EE APIs.
All these services are also available to Web browser clients by deploying servlets and JSP pages in WebLogic Server.
CORBA-enabled client programs written in Visual Basic, C++, Java, and other programming languages can execute WebLogic Server Enterprise JavaBeans and RMI (Remote Method Invocation) classes using WebLogic RMI-IIOP. Client applications written in any language with support for the HTTP protocol can access any WebLogic Server service through a servlet.
The middle tier includes WebLogic Server and other Web servers, firewalls, and proxy servers that mediate traffic between clients and WebLogic Server. The Nokia WAP server, part of the BEA mobile commerce solution, is an example of another middle tier server that provides connectivity between wireless devices and WebLogic Server.
Applications based on a multitier architecture require reliability, scalability, and high performance in the middle tier. The application server you select for the middle tier is, therefore, critical to the success of your system.
The WebLogic Server cluster option allows you to distribute client requests and back-end services among multiple cooperating WebLogic Servers. Programs in the client tier access the cluster as if it were a single WebLogic Server. As the workload increases, you can add WebLogic Servers to the cluster to share the work. The cluster uses a selectable load-balancing algorithm to choose a WebLogic Server in the cluster that is capable of handling the request.
When a request fails, another WebLogic Server that provides the requested service can take over. Failover is transparent whenever possible, which minimizes the amount of code that must be written to recover from failures. For example, servlet session state can be replicated on a secondary WebLogic Server so that if the WebLogic Server that is handling a request fails, the client's session can resume uninterrupted on the secondary server. WebLogic EJB, JMS, JDBC, and RMI services are all implemented with clustering capabilities.
The backend tier contains services that are accessible to clients only through WebLogic Server. Applications in the backend tier tend to be the most valuable and mission-critical enterprise resources. WebLogic Server protects them by restricting direct access by end users. With technologies such as connection pools and caching, WebLogic Server uses back-end resources efficiently and improves application response.
Backend services include databases, enterprise resource planning (ERP) systems, mainframe applications, legacy enterprise applications, and transaction monitors. Existing enterprise applications can be integrated into the backend tier using the Java Connector Architecture (JCA) specification from Sun Microsystems. WebLogic Server makes it easy to add a Web interface to an integrated backend application.
A database management system is the most common backend service, required by nearly all WebLogic Server applications. WebLogic EJB and WebLogic JMS typically store persistent data in a database in the backend tier.
A JDBC connection pool, defined in WebLogic Server, opens a predefined number of database connections. Once opened, database connections are shared by all WebLogic Server applications that need database access. The expensive overhead associated with establishing a connection is incurred only once for each connection in the pool, instead of once per client request. WebLogic Server monitors database connections, refreshing them as needed and ensuring reliable database services for applications.
WebLogic Enterprise Connectivity, which provides access to BEA WebLogic EnterpriseTM systems, and Jolt® for WebLogic Server, which provides access to BEA Tuxedo® systems, also use connection pools to enhance system performance.
Application Logic Layers
WebLogic Server implements J2EE component technologies and services. J2EE component technologies include servlets, JSP Pages, and Enterprise JavaBeans. J2EE services include access to standard network protocols, database systems, and messaging systems. To build a WebLogic Server application, you must create and assemble components, using the service APIs when necessary.
Components are executed in the WebLogic Server Web container or EJB container. Containers provide the life cycle support and services defined by the J2EE specifications so that the components you build do not have to handle underlying details.
Web components provide the presentation logic for browser-based J2EE applications. EJB components encapsulate business objects and processes. Web applications and EJBs are built on J2EE application services, such as JDBC, JMS (Java Messaging Service), and JTA (Java Transaction API).
Figure 1-2 illustrates WebLogic Server component containers and application services.
Figure 1-2 Application Logic Layers
The following sections discuss the presentation layer, business logic, and application services.
Presentation Logic Layer
The presentation layer includes an application's user interface and display logic. Most J2EE applications use a Web browser on the client machine because it is much easier than deploying client programs to every user's computer. In this case, the presentation logic is the WebLogic Server Web container. Client programs written in any programming language, however, must contain either logic to render HTML or their own presentation logic. A client that accesses a Web service must assemble a SOAP message that describes the Web service it wants to invoke, and include the necessary data in the body of the SOAP message.
Web Browser Clients
Web-based applications built with standard Web technologies are easy to access, maintain, and port. Web browser clients are standard for e-commerce applications.
In Web-based applications, the user interface is represented by HTML documents, JavaServer Pages (JSP), and servlets. The Web browser contains the logic to render the Web page on the user's computer from the HTML description.
JavaServer Pages (JSP) and servlets are closely related. Both produce dynamic Web content by executing Java code on WebLogic Server each time they are invoked. The difference between them is that JSP is written with an extended version of HTML, and servlets are written with the Java programming language.
JSP is convenient for Web designers who know HTML and are accustomed to working with an HTML editor or designer. Servlets, written entirely in Java, are more suited to Java programmers than to Web designers. Writing a servlet requires some knowledge of the HTTP protocol and Java programming. A servlet receives the HTTP request in a request object and typically writes HTML or XML in its response object.
JSP pages are converted to servlets before they are executed on WebLogic Server, so ultimately JSP pages and servlets are different representations of the same thing. JSP pages are deployed on WebLogic Server the same way an HTML page is deployed. The .jsp file is copied into a directory served by WebLogic Server. When a client requests a .jsp file, WebLogic Server checks whether the page has been compiled or has changed since it was last compiled. If needed, it calls the WebLogic JSP compiler, which generates Java servlet code from the .jsp file, and then it compiles the Java code to a Java class file.
A client program that is not a Web browser must supply its own code for rendering the user interface. Non-browser clients usually contain their own presentation and rendering logic, depending on WebLogic Server only for business logic and access to back-end services. This makes them more difficult to develop and deploy and less suited for Internet-based e-commerce applications than browser-based clients.
Client programs written in Java can use any WebLogic Server service over Java RMI (Remote Method Invocation). RMI allows a client program to operate on a WebLogic Server object the same way it would operate on a local object in the client. Because RMI hides the details of making calls over a network, J2EE client code and server-side code are very similar.
Java programs can use the Java Swing classes to create powerful and portable user interfaces. Although by using Java you can avoid portability issues, you cannot use WebLogic Server services over RMI unless the WebLogic Server classes are installed on the client. This means that Java RMI clients are not suited to e-commerce applications. They can be used effectively, however, in enterprise applications in which an internal network makes installation and maintenance viable.
Client programs written in languages other than Java and Java client programs that do not use WebLogic Server objects over RMI can access WebLogic Server using HTTP or RMI-IIOP.
HTTP is the standard protocol for the Web. It allows a client to make different types of requests to a server and to pass parameters to the server. A servlet on WebLogic Server can examine client requests, retrieve parameters from the request, and prepare a response for the client, using any WebLogic Server service. For example, a servlet might respond to a client program with an XML business document. Thus an application can use servlets as gateways to other WebLogic Server services.
WebLogic RMI-IIOP allows CORBA-enabled programs to execute WebLogic Server enterprise beans and RMI classes as CORBA objects. The WebLogic Server RMI and EJB compilers can generate IDL (Interface Definition Language) for RMI classes and enterprise beans. IDL generated this way is compiled to create skeletons for an ORB (Object Request Broker) and stubs for the client program. WebLogic Server parses incoming IIOP requests and dispatches them to the RMI run-time system.
Web Service Clients
Client applications that invoke WebLogic Web Services can be written using any technology: Java, Microsoft SOAP Toolkit, and so on. The client application assembles a SOAP message that describes the Web service it wants to invoke and includes all the necessary data in the body of the SOAP message. The client then sends the SOAP message over HTTP/HTTPS to WebLogic Server, which executes the Web service and sends a SOAP message back to the client over HTTP/HTTPS.
WebLogic Server provides a Web Services run-time component, which is a set of servlets and associated infrastructure needed to create a Web service. One element of the run-time is a set of servlets that handle SOAP requests from a client. You do not need to write these servlets; they are automatically included in the WebLogic Server distribution.
For Java-based Web Services clients, WebLogic Server also provides an optional Java client JAR file. The JAR file includes everything a client application needs to invoke a WebLogic Web Service, such as the WebLogic Web Services Client API and WebLogic FastParser. Unlike other Java WebLogic Server clients, you do not need to include the weblogic.jar file with Web Services clients, thus making for very thin client applications.
Business Logic Layer
Enterprise JavaBeans are the business logic components for J2EE applications. The WebLogic Server EJB container hosts enterprise beans, providing life cycle management and services such as caching, persistence, and transaction management.
There are three types of enterprise beans: entity beans, session beans, and message-driven beans. The following sections describe each type in detail.
An entity bean represents an object that contains data, such as a customer, an account, or an inventory item. Entity beans contain data values and methods that can be invoked on those values. The values are saved in a database (using JDBC) or some other data store. Entity beans can participate in transactions involving other enterprise beans and transactional services.
Entity beans are often mapped to objects in databases. An entity bean can represent a row in a table, a single column in a row, or an entire table or query result. Associated with each entity bean is a unique primary key used to find, retrieve, and save the bean.
An entity bean can employ one of the following:
When container-managed persistence is used, the WebLogic EJB compiler can generate JDBC support classes to map an entity bean to a row in a database. Other container-managed persistence mechanisms are available. For example, TopLink for WebLogic Foundation Library, from WebGain, provides persistence for an object relational database.
Entity beans can be shared by many clients and applications. An instance of an entity bean can be created at the request of any client, but it does not disappear when that client disconnects. It continues to live as long as any client is actively using it. When the bean is no longer in use, the EJB container may passivate it: that is, it may remove the live instance from the server.
A session bean is a transient EJB instance that serves a single client. Session beans tend to implement procedural logic; they embody actions more than data.
The EJB container creates a session bean at a client's request. It then maintains the bean as long as the client maintains its connection to the bean. Sessions beans are not persistent, although they can save data to a persistent store if needed.
A session bean can be stateless or stateful. Stateless session beans maintain no client-specific state between calls and can be used by any client. They can be used to provide access to services that do not depend on the context of a session, such as sending a document to a printer or retrieving read-only data into an application.
A stateful session bean maintains state on behalf of a specific client. Stateful session beans can be used to manage a process, such as assembling an order or routing a document through a workflow process. Because they can accumulate and maintain state through multiple interactions with a client, session beans are often the controlling objects in an application. Because they are not persistent, session beans must complete their work in a single session and use JDBC, JMS, or entity beans to record the work permanently.
Message-driven beans, introduced in the EJB 2.0 specification, are enterprise beans that handle asynchronous messages received from JMS Message Queues. JMS routes messages to a message-driven bean, which selects an instance from a pool to process the message.
Message-driven beans are managed in the WebLogic Server EJB container. Because they are not called directly by user-driven applications, they cannot be accessed from an application using an EJB home. A user-driven application can, however, instantiate a message-driven bean indirectly by sending a message to the bean's JMS Queue.
Application Services Layer
WebLogic Server supplies the fundamental services that allow components to concentrate on business logic without concern for low-level implementation details. It handles networking, authentication, authorization, persistence, and remote object access for EJBs and servlets. Standard Java APIs provide portable access to other services that an application can use, such as database and messaging services.
Network Communications Technologies
Client applications connect with WebLogic Server using standard networking protocols over TCP/IP. WebLogic Server listens for connection requests at a network address that can be specified as part of a Uniform Resource Identifier (URI).
A URI is a standardized string that specifies a resource on a network, including the Internet. It contains a protocol specifier called a scheme, the network address of the server, the name of the desired resource, and optional parameters. The URL you enter in a Web browser, for example, http://www.bea.com/index.html, is the most familiar URI format.
Web-based clients communicate with WebLogic Server using the HTTP protocol. Java clients connect using Java RMI (Remote Method Invocation), which allows a Java client to execute objects in WebLogic Server. CORBA-enabled clients access WebLogic Server RMI objects using RMI-IIOP, which allows them to execute WebLogic Server objects using standard CORBA protocols.
In the following table, the scheme in a URI determines the protocol for network exchanges between a client and WebLogic Server.
Table 1-1 Network Protocols
HyperText Transfer Protocol. Used by Web browsers and HTTP-capable programs.
Hypertext Transfer Protocol over Secure Sockets Layer (SSL). Used by Web browsers and HTTPS-capable client programs.
WebLogic T3 protocol for Java-to-Java connections, which multiplexes JNDI, RMI, EJB, JDBC, and other WebLogic services over a network connection.
WebLogic T3 protocol over Secure Sockets Layer (SSL).
Remote Method Invocation (RMI), the standard Java facility for distributed applications.
Internet Inter-ORB protocol, used by CORBA-enabled Java clients to execute WebLogic RMI objects over IIOP. Other CORBA clients connect to WebLogic Server with a CORBA naming context instead of a URI for WebLogic Server.
Internet Inter-ORB protocol over Secure Sockets Layer (SSL).
WebLogic Web Services use Simple Object Access Protocol (SOAP) 1.1 as the message format and HTTP as a connection protocol.
The following sections provide more information about these protocols.
HTTP, the standard protocol of the World Wide Web, is a request-response protocol. A client issues a request that includes a URI. The URI begins with http:// and the WebLogic Server address, and the name of a resource on WebLogic Server, such as an HTML page, servlet, or JSP page. If the resource name is omitted, WebLogic Server returns the default Web page, usually index.html. The header of an HTTP request includes a command, usually GET or POST. The request can include data parameters and message content.
WebLogic Server always responds to an HTTP request by executing a servlet, which returns results to the client. An HTTP servlet is a Java class that can access the contents of an HTTP request received over the network and return an HTTP-compliant result to the client.
WebLogic Server directs a request for an HTML page to the built-in File servlet. The File servlet looks for the HTML file in the document directory of the WebLogic Server file system. A request for a custom-coded servlet executes the corresponding Java class on WebLogic Server. A request for a JSP page causes WebLogic Server to compile the JSP page into a servlet, if it has not already been compiled, and then to execute the servlet, which returns results to the client.
T3 is an optimized protocol used to transport data between WebLogic Server and other Java programs, including clients and other WebLogic Servers. WebLogic Server keeps track of every Java Virtual Machine (JVM) with which it connects, and creates a single T3 connection to carry all traffic for a JVM.
For example, if a Java client accesses an enterprise bean and a JDBC connection pool on WebLogic Server, a single network connection is established between the WebLogic Server JVM and the client JVM. The EJB and JDBC services can be written as if they had sole use of a dedicated network connection because the T3 protocol invisibly multiplexes packets on the single connection.
T3 is an efficient protocol for Java-to-Java applications because it avoids unnecessary network connection events and uses fewer OS resources. The protocol also has internal enhancements that minimize packet sizes
Remote Method Invocation (RMI) is the standard Java facility for distributed applications. RMI allows one Java program, called the server, to publish Java objects that another Java program, called a client, can execute. In most applications, WebLogic Server is the RMI server and a Java client application is the client. But the roles can be reversed; RMI allows any Java program to play the role of server.
RMI architecture is similar to the CORBA architecture. To create a remote object, a programmer writes an interface for a Java class that defines the methods that may be executed by a remote client. The WebLogic Server RMI compiler, rmic, processes the interface, producing RMI stub and skeleton classes. The remote class, stubs, and skeletons are installed in WebLogic Server.
A Java client looks up a remote object in WebLogic Server using the Java Naming and Directory Interface (JNDI), which is described later in this section. JNDI establishes a connection to WebLogic Server, looks up the remote class, and returns the stubs to the client.
The client executes a stub method as if it were executing the method directly on the remote class. The stub method prepares the call and transmits it over the network to the skeleton class in WebLogic Server.
On WebLogic Server, the skeleton class unpacks the request and executes the method on the server-side object. Then it packages the results and returns them to the stub on the client side.
WebLogic EJB and several other services available to Java clients are built on RMI. Most applications should use EJB instead of using RMI directly, because EJB provides a better abstraction for business objects. In addition, the WebLogic Server EJB container provides enhancements such as caching, persistence, and life cycle management that are not automatically available to remote classes.
Remote Method Invocation over Internet Inter-ORB Protocol (RMI-IIOP) is a protocol that allows CORBA client programs to execute WebLogic RMI objects, including enterprise beans. RMI-IIOP is based on two specifications from the Object Management Group (http://www.omg.com):
The Java-to-IDL specification defines how an Interface Definition Language (IDL) is derived from a Java interface. The WebLogic Server compilers for RMI and EJB give you the option of producing IDL when compiling RMI and EJB objects. This IDL can be compiled with an IDL APIs compiler to produce the stubs required by a CORBA client.
The objects-by-value specification defines how complex data types are mapped between Java and CORBA. To use objects-by-value, a CORBA client must use an Object Request Broker (ORB) with CORBA 2.3 support. Without a CORBA 2.3 ORB, CORBA clients can use only Java primitive data types.
Data exchanged with the HTTP and T3 protocols can be encrypted with the Secure Sockets Layer (SSL) protocol. Using SSL assures the client that it has connected with an authenticated server and that data transmitted over the network is private.
SSL uses public key encryption. Public key encryption requires you to purchase a Server ID, which is a certificate for your WebLogic Server from a Certificate Authority such as VeriSign. When a client connects to the WebLogic Server SSL port, the server and client execute a protocol that includes authenticating the server's Server ID and negotiating encryption algorithms and parameters for the session. WebLogic Server can also be configured to require the client to present a certificate, an arrangement that is called mutual authentication.
SOAP (Simple Object Access Protocol) is a lightweight, XML-based protocol used to exchange information in a decentralized, distributed environment. The protocol consists of an envelop that describes the SOAP message, encoding rules, and conventions for representing remote procedure calls and responses.
All information is embedded in a Multipurpose Internet Mail Extensions (MIME)-encoded package that can be transmitted over HTTP or other Web protocols. MIME is a specification for formatting non-ASCII messages so that they can be sent over the Internet.
Data and Access Services
WebLogic Server implements standard J2EE technologies to provide data and access services to applications and components. These services include the following APIs:
The following sections discuss these services in detail.
The Java Naming and Directory Interface (JNDI) is a standard Java API that enables applications to look up an object by name. WebLogic Server or a user application binds the Java objects it serves to a name in a naming tree. An application can look up objects, such as RMI objects, Enterprise JavaBeans, JMS Queues and Topics, and JDBC DataSources, by getting a JNDI context from WebLogic Server and then calling the JNDI lookup method with the name of the object. The lookup returns a reference to the WebLogic Server object.
WebLogic JNDI supports WebLogic Server cluster load balancing and failover. Each WebLogic Server in a cluster publishes the objects it serves in a replicated cluster-wide naming tree. An application can get an initial JNDI context from any WebLogic Server in the cluster, perform a lookup, and receive an object reference from any WebLogic Server in the cluster that serves the object. A configurable load-balancing algorithm is used to spread the workload among the servers in the cluster.
Java Database Connectivity (JDBC) provides access to backend database resources. Java applications access JDBC using a JDBC driver, which is a database vendor-specific interface for a database server. Although any Java application can load a vendor's JDBC driver, connect to the database, and perform database operations, WebLogic Server provides a significant performance advantage by offering JDBC connection pools.
A JDBC connection pool is a named group of JDBC connections managed through WebLogic Server. At startup time WebLogic Server opens JDBC connections and adds them to the pool. When an application requires a JDBC connection, it gets a connection from the pool, uses it, and then returns it to the pool for use by for other applications. Establishing a database connection is often a time-consuming, resource-intensive operation, so a connection pool, which limits the number of connection operations, improves performance.
WebLogic Server also provides JDBC multipools for achieving load balancing or high availability capabilities with database connections in single-server configurations. Multipools are a "pool of pools" that provide a configurable algorithm for choosing which pool to provide a connection for a given request. Currently, WebLogic Server provides algorithms to support either high availability or load balancing behavior for database connections.
To register a connection pool in the JNDI naming tree, define a DataSource object for it. Java client applications can then get a connection from the pool by performing a JNDI lookup on the DataSource name.
Server-side Java classes use the WebLogic JDBC pool driver, which is a generic JDBC driver that calls through to the vendor-specific JDBC driver. This mechanism makes application code more portable, even if you change the brand of database used in the backend tier.
The client-side JDBC driver is the WebLogic JDBC/RMI driver, which is an RMI interface to the pool driver. Use this driver the same way you use any standard JDBC driver. When the JDBC/RMI driver is used, Java programs can access JDBC in a manner consistent with other WebLogic Server distributed objects, and they can keep database data structures in the middle tier.
WebLogic EJB and WebLogic JMS rely on connections from a JDBC connection pool to load and save persistent objects. By using EJB and JMS, you can often get a more useful abstraction than you can get by using JDBC directly in an application. For example, using an enterprise bean to represent a dataful object allows you to change the underlying store later without modifying JDBC code. If you use persistent JMS messages instead of coding database operations with JDBC, it will be easier to adapt your application to a third-party messaging system later.
The Java Transaction API (JTA) is the standard interface for managing transactions in Java applications. By using transactions, you can protect the integrity of the data in your databases and manage access to that data by concurrent applications or application instances. Once a transaction begins, all transactional operations must commit successfully or all of them must be rolled back.
WebLogic Server supports transactions that include EJB, JMS, JCA, and JDBC operations. Distributed transactions, coordinated with two-phase commit, can span multiple databases that are accessed with XA-compliant JDBC drivers, such as BEA WebLogic jDriver for Oracle/XA.
The EJB specification defines bean-managed and container-managed transactions.When an enterprise bean is deployed with container-managed transactions, WebLogic Server coordinates the transaction automatically. If an enterprise bean is deployed with bean-managed transactions, the EJB programmer must provide transaction code.
Application code based on the JMS or JDBC API can initiate a transaction, or participate in a transaction started earlier. A single transaction context is associated with the WebLogic Server thread executing an application; all transactional operations performed on the thread participate in the current transaction.
J2EE Connector Architecture
The J2EE Connector Architecture adds simplified Enterprise Information System (EIS) integration to the J2EE platform. It provides a Java solution to the problem of connectivity between the multitude of application servers and EISes. By using the Connector Architecture, it is no longer necessary for EIS vendors to customize their product for each application server. By conforming to the J2EE Connector Architecture, BEA WebLogic Server does not require added custom code in order to extend its support connectivity to a new EIS.
The J2EE Connector Architecture is implemented both in WebLogic Server and in an EIS-specific resource adapter. A resource adapter is a system library specific to an EIS and provides connectivity to the EIS. A resource adapter is analogous to a JDBC driver. The interface between a resource adapter and the EIS is specific to the underlying EIS, and can be a native interface.
The J2EE Connector Architecture comprises the system-level contracts between WebLogic Server and a given resource adaptor, a common interface for clients to access the adaptor, and interfaces for packaging and deploying resource adaptors to J2EE applications. See Programming the J2EE Connector Architecture for more information.
The J2EE messaging technologies provide standard APIs that WebLogic Server applications can use to communicate with one another as well as with non-WebLogic Server applications. The messaging services include the following APIs:
The following sections describe these APIs in detail.
Java Messaging Service (JMS) enables applications to communicate with one another by exchanging messages. A message is a request, report, and/or event that contains the information needed to coordinate communication between different applications. A message provides a level of abstraction, allowing you to separate details about the destination system from the application code.
WebLogic JMS implements two messaging models: point-to-point (PTP) and publish/subscribe (pub/sub). The PTP model allows any number of senders to send messages to a Queue. Each message in the Queue is delivered to a single reader. The pub/sub model allows any number of senders to send messages to a Topic. Each message on the Topic is sent to every reader with a subscription to the Topic. Messages can be delivered to readers synchronously or asynchronously; the particular messaging mode can be controlled either using the Administration Console or via the method used to send messages in the JMS application.
JMS messages can be persistent or non-persistent. Persistent messages are stored in a database and are not lost if WebLogic Server restarts. Non-persistent messages are lost if WebLogic Server is restarted. Persistent messages sent to a Topic can be retained until all interested subscribers have received them.
JMS supports several message types that are useful for different types of applications. The message body can contain arbitrary text, byte streams, Java primitive data types, name/value pairs, serializable Java objects, or XML content.
WebLogic Server includes the Sun JavaMail reference implementation. JavaMail allows an application to create e-mail messages and send them through an SMTP server on the network.
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