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The first stage of distributed data access, as defined by IBM, is a remote request, shown in Figure 23-9. In this stage, the PC user may issue a SQL statement that queries or updates data in a single remote database. Each individual SQL statement operates as its own transaction, similar to the autocommit mode provided by many interactive SQL programs. The user can issue a sequence of SQL statements for various databases, but the DBMS doesn t support multistatement transactions. Remote requests are very useful when a PC user needs to query corporate data. Usually, the required data is located within a single database, such as a database of order-processing or manufacturing data. Using a remote request, the PC program can retrieve the remote data for processing by a PC spreadsheet, graphics program, or desktop publishing package. The remote request capability is not powerful enough for most transaction-processing applications. For example, consider a PC-based order-entry application that accesses a corporate database. To process a new order, the PC program must check inventory levels, add the order to the database, decrease the inventory totals, and adjust customer and sales totals, involving perhaps half a dozen different SQL statements. As explained in 11, database integrity can be corrupted if these statements do not execute as a single transaction. However, the remote request stage does not support multistatement transactions, so it cannot support this application.
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FIGURE 23-9 Distributed data access: remote requests
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Remote system DBMS
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FIGURE 23-10
Distributed data access; remote transactions
Remote Transactions
The second stage of distributed data access, as defined by IBM, is a remote transaction (called a remote unit of work by IBM), shown in Figure 23-10. Remote transactions extend the remote request stage to include multistatement transaction support. The PC user can issue a series of SQL statements that query or update data in a remote database and then commit or roll back the entire series of statements as a single transaction. The DBMS guarantees that the entire transaction will succeed or fail as a unit, as it does for transactions on a local database. However, all of the SQL statements that make up the transaction must reference a single remote database. Remote transactions open the door for distributed transaction-processing applications. For example, in an order-processing application, a PC-based order-entry program can now perform a sequence of queries, updates, and inserts in the inventory database to process a new order. The program ends the statement sequence with a COMMIT or ROLLBACK for the transaction. Remote transaction capability typically requires a DBMS (or at least transactionprocessing logic) on the PC as well as on the system where the database is. The transaction logic of the DBMS must be extended across the network to ensure that the local and remote systems always have the same opinion about whether a transaction has been committed. However, the actual responsibility for maintaining database integrity remains with the remote DBMS. Remote transaction capability is often the highest level of distributed database access provided by database gateways that link one vendor s DBMS to other DBMS brands. For example, most of the independent enterprise database vendors (Sybase, Oracle, Informix) provide gateways from their UNIX- or Linux-based DBMS systems to IBM s mainframe DB2 implementation. Some gateway products go beyond the bounds of remote transactions, allowing a user to join, in a single query, tables from a local database with tables from a remote database managed by a different brand of DBMS. However, these gateways do not (and cannot, without support from the remote DBMS) provide the underlying transaction logic required to support the higher stages of distributed access as defined by IBM. The gateway can ensure the integrity of the local and remote databases individually, but it cannot guarantee that a transaction will not be committed in one and rolled back in the other.
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