javascript barcode scanner The Start and End of a Transaction in Software

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The Start and End of a Transaction
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A session begins a transaction the moment it issues any DML The transaction continues through any number of further DML commands until the session issues either a COMMIT or a ROLLBACK statement Only committed changes will be made permanent and become visible to other sessions It is impossible to nest transactions The SQL standard does not allow a user to start one transaction and then start another before terminating the first This can be done with PL/SQL (Oracle s proprietary thirdgeneration language), but not with industry-standard SQL The explicit transaction control statements are COMMIT, ROLLBACK, and SAVEPOINT There are also circumstances other than a user-issued COMMIT or ROLLBACK that will implicitly terminate a transaction: Issuing a DDL or DCL statement Exiting from the user tool (SQL*Plus or SQL Developer or anything else) If the client session dies If the system crashes If a user issues a DDL (CREATE, ALTER, or DROP) or DCL (GRANT or REVOKE) command, the transaction in progress (if any) will be committed: it will be made permanent and become visible to all other users This is because the DDL and DCL commands are themselves transactions As it is not possible to nest transactions in SQL, if the user already has a transaction running, the statements the user has run will be committed along with the statements that make up the DDL or DCL command If you start a transaction by issuing a DML command and then exit from the tool you are using without explicitly issuing either a COMMIT or a ROLLBACK, the transaction will terminate but whether it terminates with a COMMIT or a ROLLBACK is entirely dependent on how the tool is written Many tools will have different
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8: DML and Concurrency
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behavior, depending on how the tool is exited (For instance, in the Microsoft Windows environment, it is common to be able to terminate a program either by selecting the File | Exit options from a menu on the top left of the window, or by clicking an X in the top-right corner The programmers who wrote the tool may well have coded different logic into these functions) In either case, it will be a controlled exit, so the programmers should issue either a COMMIT or a ROLLBACK, but the choice is up to them If a client s session fails for some reason, the database will always roll back the transaction Such failure could be for a number of reasons: the user process can die or be killed at the operating system level, the network connection to the database server may go down, or the machine where the client tool is running can crash In any of these cases, there is no orderly issue of a COMMIT or ROLLBACK statement, and it is up to the database to detect what has happened The behavior is that the session is killed, and an active transaction is rolled back The behavior is the same if the failure is on the server side If the database server crashes for any reason, when it next starts up all transactions from any sessions that were in progress will be rolled back
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PART II
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Transaction Control: COMMIT, ROLLBACK, SAVEPOINT, SELECT FOR UPDATE
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Oracle s implementation of the relational database paradigm begins a transaction implicitly with the first DML statement The transaction continues until a COMMIT or ROLLBACK statement The SAVEPOINT command is not part of the SQL standard and is really just an easy way for programmers to back out some statements, in reverse order It need not be considered separately, as it does not terminate a transaction
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COMMIT
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Commit processing is where many people (and even some experienced DBAs) show an incomplete, or indeed completely inaccurate, understanding of the Oracle architecture When you say COMMIT, all that happens physically is that LGWR flushes the log buffer to disk DBWn does absolutely nothing This is one of the most important performance features of the Oracle database EXAM TIP What does DBWn do when you issue a COMMIT command Answer: absolutely nothing To make a transaction durable, all that is necessary is that the changes that make up the transaction are on disk: there is no need whatsoever for the actual table data to be on disk, in the datafiles If the changes are on disk, in the form of multiplexed redo log files, then in the event of damage to the database the transaction can be reinstantiated by restoring the datafiles from a backup taken before the damage occurred and applying the changes from the logs This process is covered in detail in later chapters for now, just hang on to the fact that a COMMIT involves nothing more than flushing the log buffer to disk, and flagging the transaction as complete This is why a transaction involving millions of updates in thousands of files over many minutes or hours can
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