HA(High Availability) :

Key Principles of High Availability

The following are the key principles of High Availability:

Eliminate any single point of failure: Adding redundancy, so that the failure of any one part of the system does not lead to the collapse of the entire system.
Reliable crossover: In a redundant system, the crossover point itself becomes a single point of failure. Fault-tolerant systems must provide a reliable crossover or automatic switchover mechanism to avoid failure.
Detection of failures: If the above two principles are proactively monitored, then a user may never see a system failure.

HA를 구현하기 위한 방법중에서 Oracle은 RAC지원

RAC(Real Application Cluser)

Single-instance Oracle databases have a one-to-one relationship between the Oracle database and the instance. Oracle RAC environments, however, have a one-to-many relationship between the database and instances. An Oracle RAC database can have up to 100 instances,Foot 1 all of which access one database. All database instances must use the same interconnect, which can also be used by Oracle Clusterware.