Database failure and recovery 1

 Database Failures
 Recovery Schemes:
Shadow Paging and Log-based Recovery,
Recovery wth Concurrent transactions.

There can be a case in database system like any computer system
when database failure happens. When data base fails then its
difficult for users or machines to access the data from data base and
if data base failure is not dealt efficiently then huge loss for
organization can happen because entire business process will stop.
database failure can be defined as inability of the system to provide
required functionality correctly. Database failure can be resulted due
to a variety of reasons such as disk-crash, power failure, software
error or even sabotage.

To see wherever the failure has occurred, we
tend to generalize a failure into numerous
classes, as follows:
 Transaction failure
 System crash
 Disk failure

A transaction needs to abort once it fails to
execute or once it reaches to any further
extent from wherever it can’t go to any extent
further. This is often known as transaction
failure wherever solely many transactions or
processes are hurt. The reasons for transaction
failure are:
 Logical errors
 System errors

 Logical errors: Where a transaction cannot complete as a
result of its code error or an internal error condition.
 System errors: Wherever the information system itself
terminates an energetic transaction as a result of the
DBMS isn’t able to execute it, or it’s to prevent due to
some system condition. to Illustrate, just in case of
situation or resource inconvenience, the system aborts an
active transaction.

 There are issues − external to the system − that
will cause the system to prevent abruptly and
cause the system to crash. For instance,
interruptions in power supply might cause the
failure of underlying hardware or software
package failure. Examples might include OS
errors.

 In early days of technology evolution, it had
been a typical drawback wherever hard-disk
drives or storage drives accustomed to failing
oftentimes. Disk failures include the
formation of dangerous sectors,
unreachability to the disk, disk crash or the
other failure, that destroys all or a section of
disk storage.

Database recovery is the process of
restoring the database to a correct
(consistent) state in the event of a failure. In
other words, it is the process of restoring
the database to the most recent consistent
state that existed shortly before the time of
system failure.

 The failure may be the result of a system crash
due to hardware or software errors, a media
failure such as head crash, or a software error in
the application such as a logical error in the
program that is accessing the database.
Recovery restores a database form a given state,
usually inconsistent, to a previously consistent
state.
 The number of recovery techniques that are
used are based on the atomicity property of
transactions.

The storage of data usually includes four types of media with an increasing amount of reliability:
the main memory, the magnetic disk, the magnetic tape, and the optical disk. Many different
forms of failure can affect database processing and/or transaction, and each of them has to be
dealt with differently. Some data failures can affect the main memory only, while others involve
non-volatile or secondary storage also. Among the sources of failure are:
 Due to hardware or software errors, the system crashes, which ultimately resulting in loss of main
memory.
 Failures of media, such as head crashes or unreadable media that results in the loss of portions of
secondary storage.
 There can be application software errors, such as logical errors that are accessing the database
that can cause one or more transactions to abort or fail.
 Natural physical disasters can also occur, such as fires, floods, earthquakes, or power failures.
 Carelessness or unintentional destruction of data or directories by operators or users.
 Damage or intentional corruption or hampering of data (using malicious software or files)
hardware or software facilities.

Every DBMS should offer the following facilities to help
out with the recovery mechanism:
 Backup mechanism makes backup copies at a
specific interval for the database.
 Logging facilities keep tracing the current state of
transactions and any changes made to the database.
 Checkpoint facility allows updates to the database
for getting the latest patches to be made permanent
and keep secure from vulnerability.
 Recovery manager allows the database system for
restoring the database to a reliable and steady-state
after any failure occurs.

Database failure and recovery 1

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