Algorithms for Recovery and Isolation Exploiting Semantics

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.IBM Fellow Dr. C. Mohan is the primary inventor of the ARIES family of algorithms.

Three main principles lie behind ARIES

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 transaction implicitly begins with the first "Update" type of entry for the given TransactionID, and is committed with "End Of Log" entry for the transaction.

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.

The recovery works in three phases. The first phase, Analysis, computes all the necessary information from the logfile. The Redo phase restores the database to the exact state at the crash, including all the changes of uncommitted transactions that were running at that point in time. The Undo phase then undoes all uncommitted changes, leaving the database in a consistent state.

...
Wikipedia