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PART VI
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Part VI:
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In practice, therefore, distributed requests must be implemented selectively. Database administrators must decide which remote tables are to be made visible to local users and which will remain hidden. The cooperating DBMS copies must translate user-ids from one system to another, allowing each database to be administered autonomously while providing security for remote data access. Distributed requests that would consume too many DBMS or network resources must be detected and prohibited before they impact overall DBMS performance. Because of their complexity, distributed requests are not fully supported by any commercial SQL-based DBMS today, and it will be some time before even a majority of their features are available. One major step toward distributed processing across database brands has been the standardization of a distributed transaction protocol. The XA protocol, originally developed to coordinate among multiple transaction monitors, is being actively applied to distributed database transactions as well. A Java version of the same capability, called Java Transaction Protocol (JTP), provides a distributed transaction interface for Javabased applications and application servers. Today, most commercial DBMS products designed to be used in a network environment support XA and JTA interfaces.
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The Two-Phase Commit Protocol*
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A distributed DBMS must preserve the all-or-nothing quality of a SQL transaction if it is to provide distributed transactions. The user of the distributed DBMS expects that a committed transaction will be committed on all of the systems where data resides, and that a rolled back transaction will be rolled back on all of the systems as well. Further, failures in a network connection or in one of the systems should cause the DBMS to abort a transaction and roll it back, rather than leaving the transaction in a partially committed state. All commercial DBMS systems that support distributed transactions use a technique called two-phase commit to provide that support. You don t have to understand the twophase commit scheme to use distributed transactions. In fact, the whole point of the scheme is to support distributed transactions without your knowing it. However, understanding the mechanics of a two-phase commit can help you plan efficient database access. To understand why a special two-phase commit protocol is needed, consider the database in Figure 23-13. The user, located on System A, has updated a table on System B and a table on System C and now wants to commit the transaction. Suppose the DBMS software on System A tried to commit the transaction by simply sending a COMMIT message to System B and System C, and then waiting for their affirmative replies. This strategy works as long as Systems B and C can both successfully commit their part of the transaction. But what happens if a problem such as a disk failure or a deadlock condition prevents System C from committing as requested System B will commit its part of the transaction and send back an acknowledgment, System C will roll back its part of the transaction because of the error and send back an error message, and the user ends up with a partially committed, partially rolled back transaction. Note that System A can t change its mind at this point and ask System B to roll back the transaction. The transaction on System B has been committed, and other users may already have modified the data on System B based on the changes made by the transaction.
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23:
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System B
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System A INSERT
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System C INSERT
UPDATE COMMIT
UPDATE COMMIT (committed) INSERT COMMIT
INSERT
UPDATE COMMIT
UPDATE COMMIT Oops! NO ROLLBACK ERROR!
Error: transaction committed on System B, but rolled back on System C
FIGURE 23-13
Problems with a broadcast commit scheme
The two-phase commit protocol eliminates the problems of the simple strategy shown in Figure 23-13. Figure 23-14 illustrates the steps involved in a two-phase commit: 1. The program on System A issues a COMMIT for the current (distributed) transaction, which has updated tables on System B and System C. System A will act as the coordinator of the commit process, coordinating the activities of the DBMS software on Systems B and C. 2. System A sends a GET READY message to both System B and System C and notes the message in its own transaction log. 3. When the DBMS on System B or C receives the GET READY message, it must prepare to either commit or roll back the current transaction. If the DBMS can get into this ready to commit state, it replies YES to System A and notes that fact in its local transaction log; if it cannot get into this state, it replies NO.
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