Availability requirements of four or five nines typically require a system that is fault-tolerant. A fault-tolerant system must be able to continue service even during a hardware or software failure. Typically, fault tolerance is achieved by redundancy in both hardware (such as CPUs, memory, and network devices) and in software providing key services.
A single point of failure is a hardware or software component that is part of a critical path but is not backed up by redundant components. The failure of this component results in the loss of service for the system. When designing a fault-tolerant system, you must identify and eliminate potential single points of failure.
Fault-tolerant systems can be expensive to implement and maintain. Make sure you understand the nature of the business requirements for availability and consider the strategies and costs of availability solutions that meet those requirements.