Redundancy of controller in DCS?

Redundancy of the controller in DCS is used to improve effective availability of the controller and his ability of control , collect data, etc. Can be used in critical processes like power generation, steel factories, refineries, water and sewage systems where process shall not be interrupted. While in any case controllers are reliable devices, in this case redundancy can be used for firmware, user load files update and upgrade, without stopping plant or set of equipments. 

Where is the question? If you ask what is redundancy two CPU one duty and the other is standby or master and salve. if the master fails the slave shall take over without process interruption. 

Redundancy is a fail safe process which can be between two cards or more in order to avoid process parameters or control losses in a system and thus protects the equipment of the plant.

Usually there will be Primary and Secondary but in some systems it can be triple redundancy.

Now we have a lot of systems are used redundancy technology in all DCS levels such as in data cable , connectivity servers ,aspect Domain servers and CPU's

we have a necessary reasons to use redundancy reduce down time failure  and we can working normally if one of both is down

also we can distribute the tasks  between DCS components   

stand by controller that is helpful in failure of main controller and during online downloading program or modifying logic.