Unit N: Distributed Computing

1
Unit N Distributed Computing

• Areas to be covered
• Java database connectivity
• Common object request broker architecture

2
1. Java Database Connectivity

• A mechanism that allows Java programs to access
  relational databases
• A Java interface for SQL that supports full
  access to relational databases, including use of
  DDL, DML and Query commands
• Using JDBC involves
• Establishing a connection with a database by
  means of a DriverManager and Connection classes
• Using Statement objects to pass SQL commands to a
  DBMS
• Processing the results of an SQL query which are
  captured as ResultSet objects

3
Java Database Connectivity

• The interfaces and classes of JDBC are contained
  in the java.sql package
• DriverManager manages the loading of JDBC drivers
  and supports new database connections
• Connection represents database connections
• Statement represents SQL statements to be passed
  to the DBMS by an established connection
• ResultSet provides access to the results of a
  query

4
JDBC Driver

• The DriverManager class is used for loading and
  managing JDBC drivers
• A JDBC driver must be loaded before any other
  activities can take place
• The JDBC-ODBC bridge driver provides JDBC access
  via most ODBC drivers
• A driver is loaded as follows
• Class.forName(JDBCDriverName)
• Class.forName(sun.jdbc.odbc.JdbcOdbcDriver)

5
Connecting to Databases

• JDBC uses URLs to locate databases
• For a database on the local machine
• jdbcsubprotocolsubname
• For a database on a network host
• jdbcsubprotocol//host port/subname
• subprotocol specifies a driver or a database
  connectivity mechanism that may be supported by
  one or more drivers
• subname identifies a database

6
Connection Class

• Instances of the Connection class represent open
  database connections
• Connection objects are created by using a static
  method getConnection( ) of the DriverManager
  class
• The getConnection( ) method takes a JDBC URL, a
  user ID and a password as parameters

7
Connection Class

• In the following example, the ODBC subprotocol is
  used to access a database named myDatabase
• String url jdbcodbcmyDatabase
• Connection conn DriverManager.getConnection(url,
  myID, myPassword)
• Once the connection has been established, a
  Connection object is returned
• It can be used to query the database

8
Connection Class

• Methods
• createStatement() returns a new Statement object
  for executing SQL statements
• commit() makes permanent any database changes
  that have been made since the last commit. A
  call to commit also releases any database locks
  held by this Connection object
• close() releases the DBMS and any other JDBC
  resources currently being used by this Connection
  object

9
Statement Class

• Instances of the Statement class are used to
  compose SQL commands and send them to a DBMS
• Statement objects can be obtained from an opened
  database connection
• Statement stmt conn.createStatement()

10
Statement Class

• Methods
• executeQuery(sql) executes the statement sql that
  retrieves data from the database and returns a
  ResultSet object
• executeUpdate(sql) executes the statement sql
  that updates the database and returns an integer
  indicating the number of rows affected
• close() releases the DBMS and any other JDBC
  resources currently being used by this Statement
  object
• The parameter sql is a string that represents an
  SQL command

11
Statement Class

• To create a table called Beetles
• String createString CREATE TABLE Beetles (name
  VARCHAR(35), address VARCHAR(30),
• stmt.executeUpdate(createString)
• To retrieve the name and email columns of the
  table
• ResultSet rset stmt.executeQuery(SELECT name,
  email FROM Beetles)

12
ResultSet Class

• The results of a query are stored as a set of
  rows and columns in a ResultSet object.
• Methods
• next() advances to the nest row of the result
  set returns true if successful or false when
  there are no more rows in the result set
• getType(i) returns the value of the ith column.
  Type is the data type of the column
• getType(name) retrieves the falues of a column by
  its name
• void close() releases the DBMS and any other JDBC
  resources currently being used by this ResultSet
  object

13
ResultSet Class

• The rows of the ResultSet object can be accessed
  by using a combination of the next() method and
  the getType() method
• The next() method moves the cursor to the next
  row of a ResultSet object and makes that the
  current row
• Initially the cursor is positioned just above the
  first row so that invoking next() makes the first
  row the current row

14
ResultSet Class

• There are 2 ways to identify the columns in the
  current row
• By the column name, or
• getString(name) returns the name column as a
  String object
• By the column index
• getString(1) returns the first column of the
  result set as a String object

15
ResultSet Class

• The following loop iterates through the rows of
  the ResultSet object, displaying the name and
  email fields
• while (rset.next())
• String name rset.getStrin(name)
• String email rset.getString(email)
• System.out.println(name , email)

16
Examples

• Class.forName(sun.jdbc.odbc.JdbcOdbcDriver)
• // loads the JDBC-ODBC driver
• String url jdbcodbcOrdersDriver
• Connection conn DriverManager.getConnection(url,
  rPas,mp9)
• // connects to the Orders database
• Statement stmt conn.createStatement()
• // creates a Statement object

17
Examples

• String createString CREATE TABLE Beetles (name
  VARCHAR(35), address VARCHAR(30), )
• stmt.executeUpdate(createString)
• //builds a new Beetles table
• String insertString INSERT INTO Beetles VALUES
  (Michael Owen, 123 Elmwood Street,
  )
• stmt.executeUpdate(insertString)
• //insert a new row in the Beetles table

18
Examples

• ResultSet rset stmt.executeQuery(SELECT name,
  email FROM Beetles)
• // execute a query
• While (rset.next())
• System.out.print( rset.getString(name)
  , )
• System.out.print(rset.getString(2) ,
  )..
• // print columns as iterate through rows

19
Examples

• conn.commit()
• stmt.close()
• rset.close()
• conn.close()
• //close database connection

20
2. CORBA

• The majority of existing applications are not
  written in Java
• To interface Java applications with non-Java
  applications in distributed computing
  environments we use a mechanism called CORBA
• Common Object Request Broker Architecture, an
  open distributed object computing infrastructure
  being standardized by the Object Management Group

21
CORBA

• Simplifies the development of distributed
  applications by providing a unified view of all
  distributed systems
• The CORBA application framework provides
  interoperability between objects in heterogeneous
  distributed environments
• The basic architecture contains server, service
  contract, client, stub and skeleton components

22
CORBA

• Server is an object that provides services to
  objects residing on remote hosts
• Service contract is an interface that defines the
  services provided by the server
• Client is an object that uses services provided
  by objects residing on remote hosts
• Stub is an object that resides on the same host
  as the client and serves as a proxy of the remote
  server
• Skeleton is an object that resides on the same
  host as the server. It receives requests from the
  stubs and dispatches the requests to the server

23
CORBA

• The Interface Definition Language (IDL) is
  central to CORBA
• The service contract interface of a CORBA server
  is defined in a language-neutral fashion using
  IDL
• The IDL compilers translate the interfaces
  written in IDL to various implementation
  languages and generate stubs and skeletons in
  various implementation languages

24
CORBA

• The Object Request Broker (ORB) defines the
  mechanism and interfaces that enable objects to
  make requests and receive responses in a
  distributed environment
• It provides an infrastructure allowing objects to
  communicate independently of specific platforms
  and implementation languages

25
CORBA

• A CORBA application consists of a set of
  collaborating objects
• When a client requests a service, the ORB will
  locate a server that implements the requested
  service. It will also send the arguments and
  call information to the server, invoking the
  method on the server and send the results back to
  the client
• The client need not be concerned with the server
  objects location, programming language or
  operating system