In a traditional client/server program, your application code runs on a client system; that is, a system other than the database server. Database calls are transmitted from the client system to the database server. Data is transmitted from the client to the server for insert and update operations and returned from the server to the client for query operations. The data is processed on the client system. Client/server programs are typically written by using precompilers, whereas SQL statements are embedded within the code of another language such as C, C++, or COBOL.

You can develop application logic that resides entirely inside the database by using triggers that are executed automatically when changes occur in the database or stored subprograms that are invoked explicitly. Off-loading the work from your application lets you reuse code that performs verification and cleanup and control database operations from a variety of clients. For example, by making stored subprograms invocable through a web server, you can construct a web-based user interface that performs the same functions as a client/server application.

Client/server computing is often referred to as a two-tier model: your application communicates directly with the database server. In the three-tier model, a separate application server processes the requests. The application server might be a basic web server, or might perform advanced functions like caching and load-balancing. Increasing the processing power of this middle tier lets you lessen the resources needed by client systems, resulting in a thin client configuration in which the client system might need only a web browser or other means of sending requests over the TCP/IP or HTTP protocols.

The user interface is what your application displays to end users. It depends on the technology behind the application and the needs of the users themselves. Experienced users can enter SQL statements that are passed on to the database. Novice users can be shown a graphical user interface that uses the graphics libraries of the client system (such as Windows or X-Windows). Any traditional user interface can also be provided in a web browser through HTML and Java.

In traditional client/server applications, the application can keep a record of user actions and use this information over the course of one or more sessions. For example, past choices can be presented in a menu so that they not be entered again. When the application can save information in this way, the application is considered stateful.

Web or thin-client applications that are stateless are easier to develop. Stateless applications gather all the required information, process it using the database, and then start over with the next user. This is a popular way to process single-screen requests such as customer registration.

There are many ways to add stateful action to web applications that are stateless by default. For example, an entry form on one web page can pass information to subsequent web pages, enabling you to construct a wizard-like interface that remembers user choices through several different steps. You can use cookies to store small items of information about the client system, and retrieve them when the user returns to a website. You can use servlets to keep a database session open and store variables between requests from the same client.

Oracle JVM, the Java Virtual Machine provided with the Oracle Database, is compliant with the J2SE version 1.5.x specification and supports the database session architecture.

Any database session can activate a dedicated JVM. All sessions share the same JVM code and statics; however, private states for any given session are held, and subsequently garbage collected, in an individual session space.

This design provides these benefits:

• Java applications have the same session isolation and data integrity as SQL operations.

• You need not run Java in a separate process for data integrity.

• Oracle JVM is a robust JVM with a small memory footprint.

• The JVM has the same linear Symmetric Multiprocessing (SMP) scalability as the database and can support thousands of concurrent Java sessions.

Oracle JVM works consistently with every platform supported by Oracle Database. Java applications that you develop with Oracle JVM can easily be ported to any supported platform.

Oracle JVM includes a deployment-time native compiler that enables Java code to be compiled once, stored in executable form, shared among users, and invoked more quickly and efficiently.

Security features of the database are also available with Oracle JVM. Java classes must be loaded in a database schema (by using Oracle JDeveloper, a third-party IDE, SQL*Plus, or the loadjava utility) before they can be called. Java class calls are secured and controlled through database authentication and authorization, Java 2 security, and invoker's rights (IR) or definer's rights (DR).

Effective with Oracle Database 12c Release 1 (12.1.0.1), Oracle JVM provides complete support for the latest Java Standard Edition. Compatibility with latest Java standards increases application portability and enables direct execution of client-side Java classes in the database.

Java Database Connectivity (JDBC) is an Applications Programming Interface (API) that enables Java to send SQL statements to an object-relational database such as Oracle Database.

Oracle Database includes these extensions to the JDBC 1.22 standard:

• Support for Oracle data types

• Performance enhancement by row prefetching

• Performance enhancement by execution batching

• Specification of query column types to save round trips

• Control of DatabaseMetaData calls

Oracle Database supports all APIs from the JDBC 2.0 standard, including the core APIs, optional packages, and numerous extensions. Some highlights include datasources, JTA, and distributed transactions.

Oracle Database supports these features from the JDBC 3.0 standard:

• Support for JDK 1.5.

• Toggling between local and global transactions.

• Transaction savepoints.

• Reuse of prepared statements by connection pools.

SQLJ is an ANSI SQL-1999 standard for embedding SQL statements in Java source code. SQLJ provides a simpler alternative to JDBC for client-side SQL data access from Java.

A SQLJ source file contains Java source with embedded SQL statements. Oracle SQLJ supports dynamic and static SQL. Support for dynamic SQL is an Oracle extension to the SQLJ standard.

The Oracle SQLJ translator performs these tasks:

• Translates SQLJ source to Java code with calls to the SQLJ runtime driver. The SQLJ translator converts the source code to pure Java source code and can check the syntax and semantics of static SQL statements against a database schema and verify the type compatibility of host variables with SQL types.

• Compiles the generated Java code with the Java compiler.

• (Optional) Creates profiles for the target database. SQLJ generates "profile" files with customization specific to Oracle Database.

SQLJ is integrated with JDeveloper. Source-level debugging support for SQLJ is available in JDeveloper.

This is an example of a simple SQLJ executable statement, which returns one value because employee_id is unique in the employee table:

String name;
#sql  { SELECT first_name INTO :name FROM employees WHERE employee_id=112 };
System.out.println("Name is " + name + ", employee number = " + employee_id);

Each host variable (or qualified name or complex Java host expression) included in a SQL expression is preceded by a colon (:). Other SQLJ statements declare Java types. For example, you can declare an iterator (a construct related to a database cursor) for queries that retrieve many values, as follows:

#sql iterator EmpIter (String EmpNam, int EmpNumb);

JDBC code and SQLJ code interoperate, enabling dynamic SQL statements in JDBC to be used with both static and dynamic SQL statements in SQLJ. A SQLJ iterator class corresponds to the JDBC result set.

Some differences between JDBC and SQLJ are:

• JDBC provides a complete dynamic SQL interface from Java to databases. It gives developers full control over database operations. SQLJ simplifies Java database programming to improve development productivity.

• JDBC provides fine-grained control of the execution of dynamic SQL from Java, whereas SQLJ provides a higher-level binding to SQL operations in a specific database schema.

• SQLJ source code is more concise than equivalent JDBC source code.

• SQLJ uses database connections to type-check static SQL code. JDBC, being a completely dynamic API, does not.

• SQLJ provides strong typing of query outputs and return parameters and provides type-checking on calls. JDBC passes values to and from SQL without compile-time type checking.

• SQLJ programs enable direct embedding of Java bind expressions within SQL statements. JDBC requires a separate get or set statement for each bind variable and specifies the binding by position number.

• SQLJ provides simplified rules for calling SQL stored subprograms.

  For example, the following four examples show, on successive lines, how to call a stored procedure or a stored function using either JDBC escape syntax or Oracle JDBC syntax:

  prepStmt.prepareCall("{call fun(?,?)}");       //stored proc. JDBC esc.
  prepStmt.prepareCall("{? = call fun(?,?)}");   //stored func. JDBC esc.
  prepStmt.prepareCall("begin fun(:1,:2);end;"); //stored proc. Oracle
  prepStmt.prepareCall("begin :1 := fun(:2,:3);end;"); //stored func. Oracle
  

  The SQLJ equivalent is:

  #sql {call fun(param_list) };  //Stored procedure
  // Declare x
  ...
  #sql x = {VALUES(fun(param_list)) };  // Stored function
  // where VALUES is the SQL construct
  

These benefits are common to SQLJ and JDBC:

• SQLJ source files can contain JDBC calls. SQLJ and JDBC are interoperable.

• PL/SQL and Java stored subprograms can be used interchangeably.

Java stored subprograms enable you to implement programs that run in the database server and are independent of programs that run in the middle tier. Structuring applications in this way reduces complexity and increases reuse, security, performance, and scalability.

For example, you can create a Java stored subprogram that performs operations that require data persistence and a separate program to perform presentation or business logic operations.

Java stored subprograms interface with SQL using an execution model similar to that of PL/SQL.

Web services represent a distributed computing paradigm for Java application development that is an alternative to earlier Java protocols such as JDBC, and which enable applications to interact through the XML and web protocols. For example, an electronics parts vendor can provide a web-based programmatic interface to its suppliers for inventory management. The vendor can invoke a web service as part of a program and automatically order stock based on the data returned.

The key technologies used in web services are:

• Web Services Description Language (WSDL), which is a standard format for creating an XML document. WSDL describes what a web service can do, where it resides, and how to invoke it. Specifically, it describes the operations and parameters, including parameter types, provided by a web service. Also, a WSDL document describes the location, the transport protocol, and the invocation style for the web service.

• Simple Object Access Protocol (SOAP) messaging, which is an XML-based message protocol used by web services. SOAP does not prescribe a specific transport mechanism such as HTTP, FTP, SMTP, or JMS; however, most web services accept messages that use HTTP or HTTPS.

• Universal Description, Discovery, and Integration (UDDI) business registry, which is a directory that lists web services on the internet. The UDDI registry is often compared to a telephone directory, listing unique identifiers (white pages), business categories (yellow pages), and instructions for binding to a service protocol (green pages).

Web services can use a variety of techniques and protocols. For example:

• Dispatching can occur in a synchronous (typical) or asynchronous manner.

• You can invoke a web service in an RPC-style operation in which arguments are sent and a response returned, or in a message style such as a one-way SOAP document exchange.

• You can use different encoding rules: literal or encoded.

You can invoke a web service statically, when you might know everything about it beforehand, or dynamically, in which case you can discover its operations and transport endpoints while using it.

Oracle Database can function as either a web service provider or as a web service consumer. When used as a provider, the database enables sharing and disconnected access to stored subprograms, data, metadata, and other database resources such as the queuing and messaging systems.

As a web service provider, Oracle Database provides a disconnected and heterogeneous environment that:

• Exposes stored subprograms independently of the language in which the subprograms are written

• Exposes SQL Queries and XQuery

Both PL/SQL and Java provide packages and libraries.

Both PL/SQL and Java have object-oriented features:

• Both have inheritance.

• PL/SQL has type evolution, the ability to change methods and attributes of a type while preserving subtypes and table data that use the type.

• Java has polymorphism and component models for developing distributed systems.

As an extension of SQL, PL/SQL supports all SQL data types, data encapsulation, information hiding, overloading, and exception-handling. Therefore:

• SQL data types are easier to use in PL/SQL than in Java.

• SQL operations are faster with PL/SQL than with Java, especially when a large amount of data is involved, when mostly database access is done, or when bulk operations are used.

  Some advanced PL/SQL capabilities are unavailable for Java in Oracle9i (for example, autonomous transactions and the dblink facility for remote databases).

Code development is usually faster in PL/SQL than in Java; however, this really depends upon the development tool or development environment you are using.

PL/SQL is preferred when your data logic is SQL intensive. That is, the data processing or data validation requirements of your application are high.

Also, there is a large user base with Oracle-supplied packages and third party libraries that can draw upon for development.

Java is used for open distributed applications, and many Java-based development tools are available throughout the industry. Java has native mechanisms that are unavailable in PL/SQL. For example, Java has built-in security mechanisms, an automatic Garbage Collector, type safety mechanisms, byte-code verifier, and Java 2 security. Also, Java provides built-in rapid development features, such as, built-in automatic bounds checking on arrays, built-in network access classes, and APIs that contain many useful and ready-to-use classes. Java has a vast set of class libraries, tools, and third-party class libraries that can be reused in the database. Java has a richer type system than PL/SQL. Java can use CORBA (which can have many different computer languages in its clients) and Enterprise Java Beans. PL/SQL packages can be invoked from CORBA or Enterprise Java Beans clients. You can run XML tools, the Internet File System, or JavaMail from Java.

If you application must interact with ERP systems, RMI servers, Java/J2EE, and web services, Java is preferred because none of these things can be accomplished with PL/SQL. Java is also preferred if you must develop part of your application in the middle-tier because your business logic is complex or compute intensive with little to moderate direct SQL access, you are implementing a middle-tier-driven presentation logic, your application requires transparent Java persistence, or your application requires container-managed infrastructure services. Thus, when needing to partition your application between the database tier and middle tier, migrate that part of your application as needed to the middle tier and use Java/J2EE.

For the Pro*C/C++ precompiler, the host language is either C or C++. Some features of the Pro*C/C++ precompiler are:

• You can write multithreaded programs if your platform supports a threads package. Concurrent connections are supported in either single-threaded or multithreaded applications.

• You can improve performance by embedding PL/SQL blocks. These blocks can invoke subprograms in Java or PL/SQL that are written by you or provided in Oracle Database packages.

• Using precompiler options, you can check the syntax and semantics of your SQL or PL/SQL statements during precompilation, and at runtime.

• You can invoke stored PL/SQL and Java subprograms. Modules written in COBOL or in C can be invoked from Pro*C/C++. External C subprograms in shared libraries can be invoked by your program.

• You can conditionally precompile sections of your code so that they can run in different environments.

• You can use arrays, or structures, or arrays of structures as host and indicator variables in your code to improve performance.

• You can deal with errors and warnings so that data integrity is guaranteed. As a programmer, you control how errors are handled.

• Your program can convert between internal data types and C language data types.

• The Oracle Call Interface (OCI) and Oracle C++ Call Interface (OCCI), lower-level C and C++ interfaces, are available for use in your precompiler source.

• Pro*C/C++ supports dynamic SQL, a technique that enables users to input variable values and statement syntax.

• Pro*C/C++ can use special SQL statements to manipulate tables containing user-defined object types. An Object Type Translator (OTT) maps the ADTs and named collection types in your database to structures and headers that you include in your source.

• Three kinds of collection types: associative arrays, nested tables and VARRAY, are supported with a set of SQL statements that give you a high degree of control over data.

• Large Objects are accessed by another set of SQL statements.

• A new ANSI SQL standard for dynamic SQL is supported for new applications, so that you can run SQL statements with a varying number of host variables. An older technique for dynamic SQL is still usable by pre-existing applications.

• Globalization support lets you use multibyte characters and UCS2 Unicode data.

• Using scrollable cursors, you can move backward and forward through a result set. For example, you can fetch the last row of the result set, or jump forward or backward to an absolute or relative position within the result set.

• A connection pool is a group of physical connections to a database that can be shared by several named connections. Enabling the connection pool option can help optimize the performance of Pro*C/C++ application. The connection pool option is not enabled by default.

Example 20-1 is a code fragment from a C source program that queries the table employees in the schema hr.

...
#define  UNAME_LEN   10
...
int   emp_number;
/* Define a host structure for the output values of a SELECT statement. */
/* No declare section needed if precompiler option MODE=ORACLE          */
struct {
    VARCHAR  last_name[UNAME_LEN];
    float    salary;
    float    commission_pct;
} emprec;
/* Define an indicator structure to correspond to the host output structure. */
struct {
    short emp_name_ind;
    short sal_ind;
    short comm_ind;
} emprec_ind;
...
/* Select columns last_name, salary, and commission_pct given the user's input 
/* for employee_id. */
    EXEC SQL SELECT last_name, salary, commission_pct
        INTO :emprec INDICATOR :emprec_ind
        FROM employees
        WHERE employee_id = :emp_number;
...

For the Pro*COBOL precompiler, the host language is COBOL. Some features of the Pro*COBOL precompiler are:

• You can invoke stored PL/SQL or Java subprograms. You can improve performance by embedding PL/SQL blocks. These blocks can invoke PL/SQL subprograms written by you or provided in Oracle Database packages.

• Precompiler options enable you to define how cursors, errors, syntax-checking, file formats, and so on, are handled.

• Using precompiler options, you can check the syntax and semantics of your SQL or PL/SQL statements during precompilation, and at runtime.

• You can conditionally precompile sections of your code so that they can run in different environments.

• Use tables, or group items, or tables of group items as host and indicator variables in your code to improve performance.

• You can program how errors and warnings are handled, so that data integrity is guaranteed.

• Pro*COBOL supports dynamic SQL, a technique that enables users to input variable values and statement syntax.

  See Also:

  Pro*COBOL Programmer's Guide for complete information about the Pro*COBOL precompiler

Example 20-2 is a code fragment from a COBOL source program that queries the table employees in the schema hr.

...
 WORKING-STORAGE SECTION.
*
* DEFINE HOST INPUT AND OUTPUT HOST AND INDICATOR VARIABLES.
* NO DECLARE SECTION NEEDED IF MODE=ORACLE.
*
 01  EMP-REC-VARS.
     05  EMP-NAME    PIC X(10) VARYING.
     05  EMP-NUMBER  PIC S9(4) COMP VALUE ZERO.
     05  SALARY      PIC S9(5)V99 COMP-3 VALUE ZERO.
     05  COMMISSION  PIC S9(5)V99 COMP-3 VALUE ZERO.
     05  COMM-IND    PIC S9(4) COMP VALUE ZERO.
...
 PROCEDURE DIVISION.
...
     EXEC SQL
         SELECT last_name, salary, commission_pct
         INTO :EMP-NAME, :SALARY, :COMMISSION:COMM-IND
         FROM employees
         WHERE employee_id = :EMP-NUMBER
     END-EXEC.
...

OCI and OCCI provide significant advantages over other methods of accessing Oracle Database:

• More fine-grained control over all aspects of the application design.

• High degree of control over program execution.

• Use of familiar 3GL programming techniques and application development tools such as browsers and debuggers.

• Support of dynamic SQL, method 4.

• Availability on the broadest range of platforms of all the Oracle Database programmatic interfaces.

• Dynamic bind and define using callbacks.

• Describe functionality to expose layers of server metadata.

• Asynchronous event notification for registered client applications.

• Enhanced array data manipulation language (DML) capability for arrays.

• Ability to associate a commit request with a statement execution to reduce round trips.

• Optimization for queries using transparent prefetch buffers to reduce round trips.

• Thread safety, so you do not have to implement mutual exclusion (mutex) locks on OCI and OCCI handles.

• The server connection in nonblocking mode means that control returns to the OCI code when a call is still running or cannot complete.

Both OCI and OCCI have four kinds of functions:

OCI and OCCI enable you to develop applications that combine the nonprocedural data access power of SQL with the procedural capabilities of most programming languages, including C and C++. Procedural and nonprocedural languages have these characteristics:

• In a nonprocedural language program, the set of data to be operated on is specified, but what operations are performed and how the operations are to be carried out is not specified. The nonprocedural nature of SQL makes it an easy language to learn and to use to perform database transactions. It is also the standard language used to access and manipulate data in modern relational and object-relational database systems.

• In a procedural language program, the execution of most statements depends on previous or subsequent statements and on control structures, such as loops or conditional branches, which are unavailable in SQL. The procedural nature of these languages makes them more complex than SQL, but it also makes them very flexible and powerful.

The combination of both nonprocedural and procedural language elements in an OCI or OCCI program provides easy access to Oracle Database in a structured programming environment.

OCI and OCCI support all SQL data definition, data manipulation, query, and transaction control facilities that are available through Oracle Database. For example, an OCI or OCCI program can run a query against Oracle Database. The queries can require the program to supply data to the database using input (bind) variables, as follows:

SELECT name FROM employees WHERE empno = :empnumber

In the preceding SQL statement, :empnumber is a placeholder for a value to be supplied by the application.

Alternatively, you can use PL/SQL, Oracle's procedural extension to SQL. The applications you develop can be more powerful and flexible than applications written in SQL alone. OCI and OCCI also provide facilities for accessing and manipulating objects in Oracle Database.