As with all planning,
it??™s much better to consider all options, even if you quickly dismiss them, than to be
caught off guard when an unexpected event occurs.
System Availability Versus Component Availability
A complete system is composed of hardware, software, and networking components
operating as a technology stack. Ensuring the availability of individual components
doesn??™t necessarily guarantee system availability. Different strategies and solutions
exist for achieving high availability for each of the system components. Figure 11-2
illustrates the technology stack used to deliver a potential system.
Figure 11-1. Causes of unplanned downtime
Unplanned Downtime
Software Failures Hardware Failures Human Error Disaster
Operating System
Database
Middleware
Application
Network
System
CPU
Memory
Power Supply
Bus
Peripherals
Disk
Controllers
Network
Power
Operator Error
User Error
DBA Error
System Administrator
Error
Sabotage
Fire
Flood
Earthquake
Power Failure
Chemical Spill
Bombing
System Failure | 257
As this figure shows, a variety of physical and logical layers must cooperate to deliver
an application.
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