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SQL: The Complete Reference
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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 the system where the database is located. 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-based DBMS systems to IBM s mainframe DB2 implementation. Some gateway products go beyond the bounds of
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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.
Distributed Transactions
The third stage of distributed data access, as defined by IBM, is a distributed transaction (a distributed unit of work in IBM parlance), shown in Figure 23-11. At this stage, each individual SQL statement still queries or updates a single database on a single remote computer system. However, the sequence of SQL statements within a transaction may access two or more databases located on different systems. When the transaction is committed or rolled back, the DBMS guarantees that all parts of the transaction on all of the systems involved in the transaction will be committed or rolled back. The DBMS specifically guarantees that there will not be a partial transaction, where the transaction is committed on one system and rolled back on another. Distributed transactions support the development of very sophisticated transactionprocessing applications. For example, in the corporate network of Figure 23-1, a PC order-processing application can query the inventory databases on two or three different distribution center servers to check the inventory of a scarce product and then update the databases to commit inventory from multiple locations to a customer s order. The DBMS ensures that other concurrent orders do not interfere with the remote access of the first transaction.
SQL: The Complete Reference
Figure 23-11.
Distributed data access: distributed transactions
Distributed transactions are much more difficult to provide than the first two stages of distributed data access. It s impossible to provide distributed transactions without the active cooperation of the individual DBMS systems involved in the transaction. For this reason, the DBMS brands that support distributed transactions almost always support them only for a homogeneous network of databases, all managed by the same DBMS brand (that is, an all-Oracle or all-Sybase network). A special transaction protocol, called the two-phase commit protocol, is used to implement distributed transactions and ensure that they provide the all-or-nothing requirement of a SQL transaction. The details of this protocol are described later in the section The Two-Phase Commit Protocol.
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