Databases Reference
In-Depth Information
Oracle Active Data Guard and Zero Data Loss
Active Data Guard is an option to Oracle Database Enterprise Edition and deployed
where data loss must be completely avoided. It features a real-time apply, in which redo
data is applied at the standby Oracle Database as soon as it is received, and automatic
block repair. The DBA has a choice of three different data loss modes that can be speci‐
fied. Maximum protection mode guarantees that switching over to the standby database
will not result in any lost data as each log write must also be completed to a remote
logfile before transactions commit on the primary database and, in the event that this
does not occur, the primary database is shut down. Maximum performance mode pro‐
vides a high level of protection but will not impact the performance of the primary
database, as transactions commit as soon as they are written to the online redo log and
then written asynchronously to one or more standby databases. Maximum availability
mode ensures that transactions don't commit unless they are written to the online redo
log and one or more standby databases, but will fall back to maximum performance
mode if writing to a standby is not possible.
Active Data Guard also enables the standby Oracle database to be used for reporting
and, since the Oracle Database 12
c
release, you can also query the standby database with
DML to global temporary tables while the redo apply is active. The ability to offload
reporting requests provides flexibility for reporting and queries and can help perfor‐
mance on the primary server while making use of the standby server. Active Data Guard
for Oracle Database12
c
also supports scaling of reads across multiple standby databases
simultaneously.
The Far Synch capabilities introduced with Active Data Guard for Oracle Data‐
base12
c
enable zero-loss primary and standby configurations that are separated by dis‐
tances that can span continents. A lightweight database instance consisting of a control
file and archive logfile but no recovery or datafiles is deployed to a system outside of
the primary database. Redo is transported to this location synchronously, and then is
forwarded asynchronously to the distant standby database, the failover target. If the
primary database fails, failover is executed as usual to the failover with zero data loss.
Oracle GoldenGate and Replication
An alternative for guarding against site and computer failures that cause unplanned
downtime is to implement client and application failover leveraging log-based change
data capture and replication software, such as Oracle's GoldenGate. These types of sol‐
utions are typically used where there are heterogeneous server platforms and/or het‐
erogeneous database implementations.
This type of platform failover architecture requires no additional Oracle software be‐
yond GoldenGate. However, a solution of this type requires a great deal of additional
scripting and testing. For example, data that is in a queue to be replicated from a server
that goes down will not be available at the secondary site where GoldenGate replicates
