These three concepts are the cornerstones of building software systems that people can trust and engineers can manage.

1. Reliability

A reliable system is one that continues to work correctly (performing the right function at the desired level of performance) even when things go wrong. Things that go wrong are called faults, and systems that anticipate and cope with faults are called fault-tolerant.

• Hardware Faults: Hard disks crashing, RAM becoming faulty, power outages.
• Software Faults: Bugs, runaway processes that consume resources, slow dependencies.
• Human Errors: Misconfigurations, accidental deletion of data.

How to build reliable systems?

• Redundancy: Having multiple copies of components.
• Isolation: Ensuring a failure in one component doesn't take down others.
• Monitoring & Alerting: Knowing when something is wrong immediately.

2. Availability

Availability is the percentage of time a system is operational and reachable by users.

• High Availability (HA): Systems designed to be available almost all the time.
• Measuring Availability: Expressed in "nines".
  • 99% ("two nines"): 3.65 days of downtime per year.
  • 99.9% ("three nines"): 8.77 hours of downtime per year.
  • 99.99% ("four nines"): 52.6 minutes of downtime per year.
  • 99.999% ("five nines"): 5.26 minutes of downtime per year.

Reliability vs Availability: A system can be reliable but not available (e.g., it works perfectly but is unreachable due to a network cut), or available but not reliable (e.g., you can reach it, but it returns errors).

3. Maintainability

Maintainability is the ease with which a system can be understood, modified, and operated.

• Operability: Making it easy for operations teams to keep the system running smoothly.
• Simplicity: Managing complexity so that new engineers can understand the system.
• Evolvability: Making it easy for engineers to make changes to the system in the future.