Availability pertains to the accessibility of a system while in use. When a system is available, it means it’s operational and can respond to requests. In contrast, reliability denotes how consistently the system operates without unexpected shutdowns or errors.

Key Metrics

• Availability: Measured as a percentage, often over a specific timeframe, it tracks the time a system is operational.
• Reliability: Measured as a probability of successful operation over a given period.

Sampling Scenario

Consider a system that issues requests at specific intervals, say every hour.

• If we report availability every hour, and the system is down for 15 minutes, the observed availability will be 75% for that hour.
• If instead, we monitor the reliability of the system, it will provide an overall picture of the system’s ability to stay up over time, considering any partial downtimes or recoveries.

Code Example: Reliability Metrics

Here is the Python code:

import statistics

# Times in hours the system was operational
operational_times = [1, 1, 1, 1, 0.75, 1, 1]

reliability = statistics.mean(operational_times)
print(f"System was operational {reliability * 100}% of the time.")