What reasons make it impossible to achieve absolute reliability and why is 99.99% stability sufficient in most cases?

Absolute reliability is impossible due to many factors:

• Hardware failures: equipment breakdowns, power outages, component wear.
• Software bugs: errors, logical mistakes, incompatibilities.
• External influences: network failures, human factors, natural disasters.
• System complexity: as complexity grows, the likelihood of errors and failures increases.

Why 99.99% ("four nines") is sufficient:

• It means about 52 minutes of downtime per year, which is acceptable for most business applications.
• Achieving higher reliability requires exponentially greater costs and efforts.
• The balance between cost and level of reliability is optimal at this level.

Thus, in real-world conditions, very high but not absolute reliability is aimed for, considering economic feasibility and technical limitations.