|
In computer science, Algorithms for Recovery and Isolation Exploiting Semantics, or ARIES is a recovery algorithm designed to work with a no-force, steal database approach; it is used by IBM DB2, Microsoft SQL Server and many other database systems. Three main principles lie behind ARIES * Write ahead logging: Any change to an object is first recorded in the log, and the log must be written to stable storage before changes to the object are written to disk. * Repeating history during Redo: On restart after a crash, ARIES retraces the actions of a database before the crash and brings the system back to the exact state that it was in before the crash. Then it undoes the transactions still active at crash time. * Logging changes during Undo: Changes made to the database while undoing transactions are logged to ensure such an action isn't repeated in the event of repeated restarts. == Logging == For the ARIES algorithm to work a number of log records have to be created during the operation of the database. Log entries are sequentially ordered with Sequence Numbers. Usually the resulting logfile is stored on so-called "stable storage", that is a storage medium that is assumed to survive crashes and hardware failures. To gather the necessary information for the logging two data structures have to be maintained: the dirty page table (DPT) and the transaction table (TT). The dirty page table keeps record of all the pages that have been modified and not yet written back to disc and the first Sequence Number that caused that page to become dirty. The transaction table contains all transactions that are currently running and the Sequence Number of the last log entry they caused. We create log records of the form (Sequence Number, Transaction ID, Page ID, Redo, Undo, Previous Sequence Number). The Redo and Undo fields keep information about the changes this log record saves and how to undo them. The Previous Sequence Number is a reference to the previous log record that was created for this transaction. In the case of an aborted transaction, it's possible to traverse the log file in reverse order using the Previous Sequence Numbers, undoing all actions taken within the specific transaction. Every time a transaction begins or commits we write a "Begin Transaction" entry or an "End Of Log" entry for that transaction respectively. During a recovery or while undoing the actions of an aborted transaction a special kind of log record is written, the Compensation Log Record (CLR), to record that the action has already been undone. CLRs are of the form (Sequence Number, Transaction ID, Page ID, Redo, Previous Sequence Number, Next Undo Sequence Number). The Undo field is omitted because that information is already stored in the original log record for those actions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Algorithms for Recovery and Isolation Exploiting Semantics」の詳細全文を読む スポンサード リンク
|