Algorithm for Recovery and Isolation Exploiting Semantics (ARIES) is based on the Write Ahead Log (WAL) protocol. Every update operation writes a log record which is one of the following :
- Undo-only log record:
Only the before image is logged. Thus, an undo operation can be done to retrieve the old data.
- Redo-only log record:
Only the after image is logged. Thus, a redo operation can be attempted.
- Undo-redo log record:
Both before images and after images are logged.
In it, every log record is assigned a unique and monotonically increasing log sequence number (LSN). Every data page has a page LSN field that is set to the LSN of the log record corresponding to the last update on the page. WAL requires that the log record corresponding to an update make it to stable storage before the data page corresponding to that update is written to disk. For performance reasons, each log write is not immediately forced to disk. A log tail is maintained in main memory to buffer log writes. The log tail is flushed to disk when it gets full. A transaction cannot be declared committed until the commit log record makes it to disk.
Once in a while the recovery subsystem writes a checkpoint record to the log. The checkpoint record contains the transaction table and the dirty page table. A master log record is maintained separately, in stable storage, to store the LSN of the latest checkpoint record that made it to disk. On restart, the recovery subsystem reads the master log record to find the checkpoint’s LSN, reads the checkpoint record, and starts recovery from there on.
The recovery process actually consists of 3 phases:
The recovery subsystem determines the earliest log record from which the next pass must start. It also scans the log forward from the checkpoint record to construct a snapshot of what the system looked like at the instant of the crash.
Starting at the earliest LSN, the log is read forward and each update redone.
The log is scanned backward and updates corresponding to loser transactions are undone.
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