visual basic barcode generator Sockets and Data Sharing Server 1 Application A Server 2 Application C in Java

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Sockets and Data Sharing Server 1 Application A Server 2 Application C
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Figure 3.3 Schematic representation of multiple applications-sharing data through a common database. Note that communications always occur over the network. The applications sharing the data could be running on the same computer or separate computers.
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the console. The names and phone numbers are stored as two columns in a table named employee. The code for writing to the database is similar.
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Listing 3-6
/* Sample Java code for reading from the database */ import java.io.*; import java.sql.*; public class TestClass { public void getEmployees (){ Connection con = null; Statement stmt = null; ResultSet rs = null; try { Class.forName ("sun.jdbc.odbc.JdbcOdbcDriver"); con= DriverManager.getConnection ("jdbc:odbc.somedb", "user", "password"); stmt = con.getStatement (); rs = stmt.executeQuery ("SELECT NAME, PHONE FROM EMPLOYEES"); while (rs.next())
Three
System.out.println (rs.getString("name") + " +rs.getString("phone"); } catch ( ClassNotFoundException e) { System.out.println ( e.getMessage()); } catch (SQLException e ) { System.out.println (e.getMessage () ); } finally { con.close(); } }
Although data sharing via a common database is an improvement over file-based data sharing in some respects, there are still some disadvantages to this method. The greatest disadvantage of the common database approach is, just as in the case of file-based data sharing, the data is not shared in real time. This is because when an application writes data to the database, other applications are not informed of the changes. Thus, even though the data is available to other applications for reading, right after an application writes to the database, other applications are not aware of the changes and therefore cannot take advantage of the updates in real time. Another serious problem of the common database approach is that it is very difficult, and sometime impossible, to come up with a suitable design for the common database. Defining a unified database schema that can meet the needs of multiple applications is very difficult. Furthermore, for the application programmers, the resulting database schema is difficult to work with. There are also severe political difficulties in designing a unified schema. If a critical application is likely to suffer delays due to working with a unified schema, often there is pressure to separate the databases. The problem of designing a unified database schema is exacerbated if externally packaged applications are part of the system of applications that need to be integrated. Most packaged applications have their own database schema, and these schemas will not work with any other schema. Even if there is room for changing the database schema of a given packaged application, it is likely to be limited to an extent that is not suitable for general integration. In addition, software vendors reserve the right to change the schema with every new release of the software. When vendors change the schema of their packaged applications, interfacing applications encounter a ripple effect that causes
Sockets and Data Sharing
development and maintenance issues or worse, production issues if schema changes are not advertised properly. The use of a common database is also not suitable when a number of applications need to be integrated and is therefore not a scalable solution to the problem of application integration. This is because if a fair amount of applications frequently read and write to the common database, the database can be a bottleneck as each application locks others out of the database. This integration method is also not a suitable solution if the applications are distributed across multiple locations. This is because accessing a single, common database across a wide area network (WAN) is typically too slow to be practical. Sockets In order to avoid the problem of stale data, a real-time connection between applications is needed. This is called connectivity. The most rudimentary way to establish a connection between two applications is through sockets. A socket is a communications connection point (endpoint) that you can name and address in a network. The processes that use a socket can reside on the same system or on different systems on different networks. Sockets are useful for both standalone and network applications. Socket APIs are the network standard for TCP/IP. A wide range of operating systems support socket APIs. Sockets allow one application to listen at a given port on a given machine for the incoming data, while another application can write to the same socket using the IP address and port address of the first application. The listening application can read the data as soon as the second application writes the data. Thus, the data is shared in real time and the problem of stale data is eliminated. The listening application is usually called a server whereas the other application is called a client. Because the overhead associated with applications that communicate through sockets is very low, direct socket programming leads to a very efficient way of communication. It is also interesting to note that most of the modern methods of communications (such as MOM/messages) as well as other methods (such as distributed objects) rely on socket programming under the hood. Listing 3-7 illustrates typical C language code on the server side. Note that the code listing contains the system header files (<sys/socket.h>, <sys/types.h>, and <netinet/in.h>) as the include files. This inclusion allows all the systems-related files to be compiled along with the code shown in Listing 3-7, and it allows the code to make system-level calls related to the sockets.
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