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SQL refers to Structured Query Language, a kind of language used to access, update and manipulate database. In SQL, ROLLBACK is a command that causes all data changes since the last START TRANSACTION or BEGIN to be discarded by the relational database management systems (RDBMS), so that the state of the data is "rolled back" to the way it was before those changes were made.
Commit Record notes a decision to commit a transaction. Abort Record notes a decision to abort and hence roll back a transaction. Checkpoint Record notes that a checkpoint has been made. These are used to speed up recovery. They record information that eliminates the need to read a long way into the log's past.
For systems without a commit/rollback mechanism available, one can undo a failed transaction with a compensating transaction, which will bring the system back to its initial state. Typically, this is only a workaround which has to be implemented manually and cannot guarantee that the system always ends in a consistent state.
If the coordinator fails permanently, some participants will never resolve their transactions: After a participant has sent an agreement message as a response to the commit-request message from the coordinator, it will block until a commit or rollback is received. A two-phase commit protocol cannot dependably recover from a failure of both the ...
A simplified finite state automaton showing possible DBMS after-failure (in red) states and the transitions (in black) that are necessary to return to a running system to achieve durability. In transaction-based systems, the mechanisms that assure durability are historically associated with the concept of reliability of systems, as proposed by ...
In terms of transactions, the opposite of commit is to discard the tentative changes of a transaction, a rollback. The transaction, commit and rollback concepts are key to the ACID property of databases. [1] A COMMIT statement in SQL ends a transaction within a relational database management system (RDBMS
All changes made after a savepoint has been declared can be undone by issuing a ROLLBACK TO SAVEPOINT name command. Issuing RELEASE SAVEPOINT name will cause the named savepoint to be discarded, but will not otherwise affect anything. Issuing the commands ROLLBACK or COMMIT will also discard any savepoints created since the start of the main ...
After the Redo phase, the database reflects the exact state at the crash. However the changes of uncommitted transactions have to be undone to restore the database to a consistent state. For that we run backwards through the log for each transaction in the TT (those runs can of course be combined into one) using the Previous Sequence Number ...