Database Recovery

Course: Database

Subject: database

Date: 07-06-2025

Topic: DatabaseRecoveryTechniques

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Database Recovery

purpose of database recovery

• To bring the database to the last consistent state, which existed before the failure.
• To preserve transaction properties (Atomicity, Consistency, Isolation, Durability)

Types of Failure

• There are three types of failure

1. Transaction error: error that happens due to user errors (not always though)

• Transactions may fail because of incorrect input, deadlock, incorrect synchronization.

2. System failure: System may fail because of addressing error, application error, operating system fault, RAM failure, etc.
3. Media failure: Disk head crash, power disruption, etc.

Transaction Log

• In order to recover a database we need to have the history of its transactions. in other words.
• For recovery from any type of failure data values prior to modification (BFIM - BeFore Image) and the new value after modification (AFIM – AFter Image) are required.
• These values and other information is stored in a sequential file called Transaction log.

Data Updates

• Immediate Update: As soon as a data item is modified in cache, the disk copy is updated.
• Deferred Update: All modified data in the cache is written either after a transaction ends its execution or after a fixed number of transactions have completed their execution.
• Shadow update: The modified version of a data item does not overwrite its disk copy but is written at a separate disk location.
• In-place update: The disk version of the data item is overwritten by the cache version.

Data Caching

• Data items to be modified are first stored into database cache by the Cache Manager (CM) and after modification they are flushed (written) to the disk.
• The flushing is controlled by Modified and Pin-Unpin bits.
  • Pin-Unpin: Instructs the operating system not to flush the data item.
  • Modified: Indicates the AFIM of the data item.

Transaction Roll-back (Undo) and Roll-Forward (Redo)

• To maintain atomicity, a transaction’s operations are redone or undone.
  • Undo: Restore all BFIMs on to disk (Remove all AFIMs).
  • Redo: Restore all AFIMs on to disk.
• Database recovery is achieved either by performing only Undo or only Redo or by a combination of the two. These operations are recorded in the log as they happen.

Write-Ahead Logging

• When in-place update (immediate or deferred) is used then log is necessary for recovery and it must be available to recovery manager. This is achieved by Write-Ahead Logging (WAL) protocol. WAL states that

  • For Undo: Before a data item’s AFIM is flushed to the database disk (overwriting the BFIM) its BFIM must be written to the log and the log must be saved on a stable store (log disk).
  • For Redo: Before a transaction executes its commit operation, all its AFIMs must be written to the log and the log must be saved on a stable store.

Check pointing

• Save the current state to reduce recovery time.
• The following steps defines a checkpoint operation:

1. Suspend execution of transactions temporarily.
2. Force write modified buffer data to disk.
3. Write a [checkpoint] record to the log, save the log to disk.
4. Resume normal transaction execution. Steal/No-Steal and Force/No-Force

• Possible ways for flushing database cache to database disk:

1. Steal: Cache can be flushed before transaction commits.
2. No-Steal: Cache cannot be flushed before transaction commit.
3. Force: Cache is immediately flushed (forced) to disk.
4. No-Force: Cache is deferred until transaction commits

• These give rise to four different ways for handling recovery:
  • Steal/No-Force (Undo/Redo)
  • Steal/Force (Undo/No-redo)
  • No-Steal/No-Force (Redo/No-undo)
  • No-Steal/Force (No-undo/No-redo)

Recovery schemes

1. Deferred update(no undo/ redo)

• After reboot from a failure the log is used to redo all the transactions affected by this failure. No undo is required because no AFIM is flushed to the disk before a transaction commits.
• deferred update in a single user system :There is no concurrent data sharing in a single user system.
• Deferred update with concurrent users: - This environment requires some concurrency control mechanism to guarantee isolation property of transactions. In a system recovery transactions which were recorded in the log after the last checkpoint were redone.The recovery manager may scan some of the transactions recorded before the checkpoint to get the AFIMs.
• Deferred Update with concurrent users
• Two tables are required for implementing this protocol:
  • Active table: All active transactions are entered in this table.
  • Commit table: Transactions to be committed are entered in this table.
  • During recovery,all transactions of the commit table are redone and all transactions of active tables are ignored since none of their AFIMs reached the database.It is possible that a commit table transaction may be redone twice but this does not create any inconsistency because of a redone is idempotent, that is, one redone for an AFIM is equivalent to multiple redone for the same AFIM.

2. Immediate update: Undo/No-redo Algorithm

• In this algorithm AFIMs of a transaction are flushed to the database disk under WAL before it commits.
• For this reason the recovery manager undoes all transactions during recovery.
• No transaction is redone.
• It is possible that a transaction might have completed execution and ready to commit but this transaction is also undone. Undo/ Redo Algorithm for a (single user environment)
• Recovery schemes of this category apply undo and also redo for recovery.
• In a single-user environment no concurrency control is required but a log is maintained under WAL.
• Note that at any time there will be one transaction in the system and it will be either in the commit table or in the active table.
• The recovery manager performs:
  • Undo of a transaction if it is in the active table.
  • Redo of a transaction if it is in the commit table. Undo/Redo Algorithm (Concurrent execution)
• Recovery schemes of this category applies undo and also redo to recover the database from failure.
• In concurrent execution environment a concurrency control is required and log is maintained under WAL.
• Commit table records transactions to be committed and active table records active transactions. To minimize the work of the recovery manager checkpointing is used.
  • The recovery performs:
• Undo of a transaction if it is in the active table.
• Redo of a transaction if it is in the commit table.

3. Shadow paging:

• The AFIM does not overwrite its BFIM but recorded at another place on the disk.Thus, at any time a data item has AFIM and BFIM (Shadow copy of the data item) at two different places on the disk.

4. WAL(write ahead logging):

   • All changes are logged before being applied to the database
   • Enables undo / redo
   • It identify the standing transaction before any transaction with out a commit/abort records is considered as uncommitted and will be undone.

5. Check-pointing:

6. ARIES algorithm: is based on WAL .

• 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.

• 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.

7. 2 Phase commit:

Recovery in Multi database System