Saturday, February 7, 2009

Oracle: Types of database failures

There are different reasons for a database to fail .Some of them may result a partial failure ,some may become complete disaster.

Oracle database is enhanced with different types of recovery solutions for different types of failures.Some failure types can be recovered without dba's interaction. means oracle server will take care . on the other hands ,some requires serious dba's interaction. that's what a dba is paid for.............

let us check the database failure types:

1) Statement level failure
2) Network failure
3) User process failure
4) Instance failure
5) Media failure
6) Users issuing unwanted sql statements

a brief discussion:

1) Statement level failure:

a) the statement is not in proper syntax
solution: check the syntax and correct it

b) The user executing a statement does not have proper privilege to do so. for example:
select * from a.test;
here user is trying to access a table named test from a user's schema . if you do not have select privilege on test table , the statement can fail;
solution: give proper privillege to the user.

c) A huge insert or update which requires more space in destination tablespace to complete , compare to how much space already available.
solution: increase the size of tablespace/datafile/autoextend the tablespace or datafile

A statement can fail because of programmatic logical errors
solution: refer to development team for correction

e) trying to enter data to an oracle table violating the integrity of a table. means you are trying to enter invalid data
solution: check for the constraints enabled on that table and correct the data as it allows

2) network failure
May happen because of network interface card(NIC) failure or network connection (physical or logical) failure or oracle net listener failure
solution: configure multiple nic cards on your database server.
have backup physical links from the database server.
you may have backup listener also

3) user process failure:
user process may fail because of an abnormal termination of user process(shutdown of user's machine, failure of running oracle client software , user may disconnect by clicking on cross sign to close an applications.
solution: does not require dba interaction as oracle has background process called pmon to release the resources,locks held by that session and rollback the uncommitted changes by that session
4) instance failure:
a) hardware failure
b) power failure
c) background process like pmon shuts down the database because of an error situation
check the alert log file, trace files generated by different background processes,
start the database by using startup command from sql*plus prompt
while the database startup process, oracle will automatically perform instance recovery
(known as crash recovery also) , that is to put the database in consistant state
by performing roll forward the committed changes and roll back uncommitted changes
so , no interaction required from dba

5)media failure

any failure of reading from or writing to disks are known as media failure Normally this is what a dba is paid for .So it requires a complete dba interaction. this may happen because of hard disk crash,disk controller fails, a head crash of hard disk modify,delete the oracle files (datafiles,control files,redo files) manually or by any other applications then oracle.

solution: There are different solutions for these depending on what type of media failure are in picture. you may need to restore your complete database, or only control files or datafiles or redo files , may be archived log also.

NOTE: This also depends on how is your backup and recovery strategy to recover from a
disaster like this.and how quickly you can do the same

6) users issuing unwanted sql statements:

users may drop a table unwantedly
users can modify(insert/delete/update) data inside a table unwantedly or may need a
rollback after commit
solution: use oracle flashback feature to restore a table to a previous point in time or
to restore the dropped table
You can use log miner tool to determine the changes
If the changes are not yet committed then you can issue simple rollback command
to rollback the changes

have a nice day...........
Our greatest glory is not in never falling but in rising every time we fall.


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