Algorithms for Recovery and Isolation Exploiting Semantics, or ARIES is a recovery algorithm designed to work with a no-force, steal database approach.

ARIES uses a steal/no-force approach for writing, and it is based on three concepts:

1. Write-ahead logging

2. Repeating history during redo

   ARIES will retrace all actions of the database system prior to the crash to reconstruct the database state when the crash occurred. Transactions that were uncommitted at the time of the crash (active transactions) are undone.

3. Logging changes during undo It will prevent ARIES from repeating the completed undo operations if a failure occurs during recovery, which causes a restart of the recovery process.

Before describing this topic we need to elaborate some concepts:

1. Log sequence number

   It refers to a pointer used to identify the log records.

2. Dirty page table

   It refers to pages with their updated version placed in main memory and disk version of it is not updated.

   A table is maintained which is useful in reducing unnecessary redo operation.

3. Fuzzy checkpoints.

   A new type checkpoints i.e. fuzzy checkpoints has been derived that allowed to process new transactions alter the log has been updated without having to update the database.

The ARIES recovery procedure consists of three main steps:

1. Analysis

   The analysis step identifies the dirty (updated) pages in the buffer (Note 6), and the set of transactions active at the time of the crash. The appropriate point in the log where the REDO operation should start is also determined

2. REDO

   The REDO phase actually reapplies updates from the log to the database. Generally, the REDO operation is applied to only committed transactions. However, in ARIES, this is not the case. Certain information in the ARIES log will provide the start point for REDO, from which REDO operations are applied until the end of the log is reached. In addition, information stored by ARIES and in the data pages will allow ARIES to determine whether the operation to be redone has actually been applied to the database and hence need not be reapplied. Thus only the necessary REDO operations are applied during recovery.

3. UNDO

   During the UNDO phase, the log is scanned backwards and the operations of transactions that were active at the time of the crash are undone in reverse order. The information needed for ARIES to accomplish its recovery procedure includes the log, the Transaction Table, and the Dirty Page Table. In addition, check pointing is used. These two tables are maintained by the transaction manager and written to the log during check pointing.