Answer / rajkumar

Systems, predominantly computing and computer-based systems,
which tolerate undesired changes in their internal structure
or external environment. Such changes, generally referred to
as faults, may occur at various times during the evolution
of a system, beginning with its specification and proceeding
through its utilization. Faults that occur during
specification, design, implementation, or modification are
called design faults; those occurring during utilization are
referred to as operational faults, The use of fault
tolerance techniques is based on the premise that a complex
system, no matter how carefully designed and validated, is
likely to contain residual design faults and to encounter
unpreventable operational faults.

Generally, fault tolerance techniques attempt to prevent
lower-level errors (caused by faults) from propagating into
system failures. By using various types of structural and
informational redundancy, such techniques either mask a
fault (no errors are propagated to the faulty subsystem's
output) or detect a fault (via an error) and then effect a
recovery process which, if successful, prevents a system
failure. In the case of a permanent internal fault, the
recovery process usually includes some form of structural
reconfiguration (for example, replacement of a faulty
subsystem with a spare or use of an alternate program) which
prevents the fault from causing further errors. Typically, a
fault-tolerant system design will incorporate a mix of fault
tolerance techniques which complement the techniques used
for fault prevention

Answer / gnb

It's advance version of Redundant.

Redundant: If one processor having one backup processor and
primary fils then secondary will take action and control
the process.

Fault tolerant: Here two processors available and two will
be active but one will be the master and backup will be
slave processor. Here we can avoid the switching time.

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