Statement
failure: A single database operation (select, insert, update, or delete) fails.
User
process failure: A single database session fails.
Network
failure: Connectivity to the database is lost.
User
error: A user successfully completes an operation, but the operation (dropping
a table or entering bad data) is incorrect.
Instance
failure: The database instance shuts down unexpectedly.
Media
failure: One or more of the database files are lost (that is, the files have been
deleted or the disk has failed).
Statement
Failure
When a
single database operation fails, DBA involvement may be necessary to correct
errors with user privileges or database space allocation. DBAs may also need to
assist in trouble-shooting, even for problems that are not directly in their
task area. This can vary greatly from one organization to another. For example,
in organizations that use off-the-shelf applications (that is, organizations
that have no software developers), the DBA is the only point of contact and
must examine logic errors in
applications.
User
Process Failure
User
processes that abnormally disconnect from the instance may have uncommitted
work in progress that needs to be rolled back. The Process Monitor (PMON) background
process periodically polls server processes to ensure that their sessions are
still connected. If PMON finds a server process whose user is no longer
connected, PMON recovers from any ongoing transactions; it also rolls back
uncommitted changes and releases any locks that are held by the failed session.
A DBA’s
intervention should not be required to recover from user process failure, but
the administrator must watch for trends. One or two users disconnecting
abnormally is not a cause for concern. A small percentage of user process
failures may occur from time to time.
But
consistent and systemic failures indicate other problems. A large percentage of
abnormal disconnects may indicate a need for user training (which includes
teaching users to log out rather than just terminate their programs). It may
also be indicative of network or application problems.
Network
Failure
The best
solution to network failure is to provide redundant paths for network
connections. Backup listeners, network connections, and network interface cards
reduce the chance that network failures will affect system availability.
User
Error
The Oracle
database provides Oracle Flashback technology: a group of features that support
viewing past states of data—and winding data back and forth in time—without
requiring restoring the database from backup. With this technology, you help
users analyze and recover from errors. For users who have committed erroneous
changes, use the following to analyze the errors:
•
Flashback
Query: View
committed data as it existed at some point in the past. The SELECT command with
the AS OF clause references a time in the past through a time stamp or SCN.
•
Flashback
Version Query: View
committed historical data for a specific time interval. Use the VERSIONS BETWEEN
clause of the SELECT command (for performance reasons with existing indexes).
•
Flashback
Transaction Query:
View all database changes made at the transaction level
Possible
solutions to recover from user error:
•
Flashback
Transaction Backout:
Rolls back a specific transaction and dependent transactions
•
Flashback Table: Rewinds one or more
tables to their contents at a previous time without affecting other database
objects
•
Flashback Drop: Reverses the effects
of dropping a table by returning the dropped table from the recycle bin to the
database along with dependent objects such as indexes and triggers
Instance
Failure
Instance
failure occurs when the database instance is shut down before synchronizing all
database files. An instance failure can occur because of hardware or software
failure or through the use of the emergency SHUTDOWN ABORT and STARTUP FORCE
shutdown commands.
Administrator
involvement in recovering from instance failure is usually limited to
restarting the instance and working to prevent future occurrences.
Media Failur
Oracle
Corporation defines media failure
as any failure that results in the loss or corruption of one or more database
files (data, control, or redo log file).Recovering
from media failure requires that you restore and recover the missing files. To
ensure that your database can be recovered from media failure, follow the best
practices outlined in the next few pages.
