Databases Reference
In-Depth Information
segment blocks are part of the redo for a given transaction. For example, suppose that
a user changed an employee name from “John” to “Jonathan.” As Oracle applies the redo
log, it will read the block containing the employee row into the cache and redo the name
change. As part of recovering the transaction, Oracle will also write the old name “John”
to an UNDO segment, as was done for the original transaction.
When the roll forward phase is finished, all the changes for committed and uncom‐
mitted transactions have been reproduced. The uncommitted transactions are in-flight
once again, just as they were at the time the crash occurred. These in-flight transactions
must be rolled back to return to a consistent state. This next phase is called
rollback
or
transaction recovery
.
Oracle opens the database after the roll forward phase of recovery and performs the
rollback of uncommitted transactions in the background in what is called
deferred roll‐
back
. This process reduces database downtime and helps to reduce the variability of
recovery times. If a user's transaction begins working in a database block that contains
some changes left behind by an uncommitted transaction, the user's transaction will
trigger a foreground rollback to undo the changes and will then proceed when rollback
is complete. This action is transparent to the user—they don't receive error messages or
have to resubmit the transaction.
Protecting Against System Failure
There are a variety of approaches you can take to help protect your system against the
ill effects of system crashes and other failures, including the following:
• Providing component redundancy
• Deploying Data Guard to provide an alternate site in case of primary site failure
• Deploying Real Application Clusters for database continuity in the event of failure
of an instance
• Deploying application continuity or Transparent Application Failover software
services
Component Redundancy
As basic protection, the various hardware components that make up the Oracle Data‐
base server itself must be fault-tolerant.
Fault-tolerance
, as the name implies, allows the
overall hardware system to continue to operate even if one of its components fails. This,
in turn, implies redundant components and the ability to detect component failure and
seamlessly integrate the failed component's replacement. The major system components
that should have redundancy include the following:
