Fault Tolerance in distributed system is achieved by Hardware Redundancy and as well as Software Recovery.
Software fault tolerance is the ability for software to detect and recover from a fault that is happening or has already happened in either the software or hardware in the system in which the software is running in order to provide service in accordance with the specification. Software fault tolerance is a necessary component in order to construct the next generation of highly available and reliable computing systems from embedded systems to data warehouse systems. Software fault tolerance is not a solution unto itself however, and it is important to realize that software fault tolerance is just one piece necessary to create the next generation of systems.
Hardware redundancy may be provided in one of the following ways:
- One for One Redundancy
- Load Sharing
- N + X Redundancy
One for One Redundancy
each hardware module has a redundant hardware module. The hardware module that performs the functions under normal conditions is called Active and the redundant unit is called Standby. The standby keeps monitoring the active unit at all times. It will takeover and become active if the active unit fails. Since standby has to takeover under fault conditions it has to keep itself synchronized with the active unit operations.
Since the probability of both the units failing at the same time is very low, this technique provides the highest level of availability. The main disadvantage here is that it doubles the hardware cost.
CAS redundancy scheme in the Xenon Switching System is a good example of one for one redundancy.
N + X Redundancy
if N hardware modules are required to perform system functions, the system is configured with N + X hardware modules; typically X is much smaller than N. Whenever any of the N modules fails, one of the X modules takes over its functions. Since health monitoring of N units by X units at all times is not practical, a higher level module monitors the health of N units. If one of the N units fails, it selects one of the X units ( It may be noted that one for one is a special case of N + X).
The advantage lies in reduced hardware cost of the system as only X units are required to backup N units. However, in case of multiple failures, this scheme provides lesser system availability.
XEN redundancy scheme in Xenon Switching System is a good example of N+ X redundancy.
Load Sharing
In this scheme, under zero fault conditions, all the hardware modules that are equipped to perform system functions, share the load. A higher level module performs the load distribution. It also maintains the health status of the hardware units. If one of the load sharing module fails, the higher level module starts distributing the load among the rest of the units. There is graceful degradation of performance with hardware failure.
Here, there is almost no extra hardware cost to provide the redundancy. The main disadvantage is that if a hardware failure happens during the busy hour, system will perform at a sub-optimal level until the failed module is replaced.
In the WebTaxi design, the Taxi Session processor uses load sharing to distribute the taxi session load.
Answer will be option C) Both A) and B) (Software Recovery and Hardware Redundancy)
