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Part VI:
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4. System A waits for replies to its GET READY message. If all of the replies are YES, System A sends a COMMIT message to both System B and System C, and notes the decision in its transaction log. If any of the replies is NO, or if all of the replies are not received within some timeout period, System A sends a ROLLBACK message to both systems and notes that decision in its transaction log. 5. When the DBMS on System B or C receives the COMMIT or ROLLBACK message, it must do as it is told. The DBMS gave up the capability to decide the transaction s fate autonomously when it replied YES to the GET READY message in Step 3. The DBMS commits or rolls back its part of the transaction as requested, writes the COMMIT or ROLLBACK message in its transaction log, and returns an OK message to System A. 6. When System A has received all the OK messages, it knows the transaction has been committed or rolled back and returns the appropriate SQLCODE value to the program.
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System B
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UPDATE COMMIT GET READY NO Oops! (rolled back) ROLLBACK OK ROLLBACK ERROR!
FIGURE 23-14
The two-phase commit protocol
23:
SQL Networking and Distributed Databases
This protocol protects the distributed transaction against any single failure in System B, System C, or the communications network. These two examples illustrate how the protocol permits recovery from failures: Suppose a failure occurs on System C before it sends a YES message in Step 3. System A will not receive a YES reply and will broadcast a ROLLBACK message, causing System B to roll back the transaction. The DBMS recovery facility on System C will not find the YES message or a COMMIT message in the local transaction log, and it will roll back the transaction on System C as part of the recovery process. All parts of the transaction will have been rolled back at this point. Suppose a failure occurs on System C after it sends a YES message in Step 3. System A will decide whether to commit or roll back the distributed transaction based on the reply from System B. The DBMS recovery facility on System C will find the YES message in the local transaction log, but will not find a COMMIT or ROLLBACK message to mark the end of the transaction. The recovery facility then asks the coordinator (System A) what the final disposition of the transaction was and acts accordingly. Note that System A must maintain a record of its decision to commit or roll back the transaction until it receives the final okay from all of the participants, so that it can respond to the recovery facility in case of failure. The two-phase commit protocol guarantees the integrity of distributed transactions, but it generates a great deal of network traffic. If n systems are involved in the transaction, the coordinator must send and receive a total of 4n messages to successfully commit the transaction. Note that these messages are in addition to the messages that actually carry the SQL statements and query results among the systems. However, there s no way to avoid the message traffic if a distributed transaction is to provide database integrity in the face of system failures. Because of their heavy network overhead, distributed transactions can have a serious negative effect on database performance. For this reason, distributed databases must be carefully designed so that frequently accessed (or at least frequently updated) data is on a local system or on a single remote system. If possible, transactions that update two or more remote systems should be relatively rare.
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