Software → System Design
Availability
The ability of a system to remain operational and responsive when needed.
Where it fits
Availability measures whether a system is usable when needed. It is central to system design, reliability engineering, operations, and product expectations.
Mental model
Availability is about successful service over time. Improving it usually requires redundancy, failover, monitoring, and careful dependency design.
Example
A service with 99.9% monthly availability can be unavailable for roughly 43 minutes per month before missing that target.
Common mistakes
- Confusing availability with durability.
- Ignoring dependency availability.
- Chasing more nines without understanding cost and complexity.
Related concepts
Reliability, failover, redundancy, SLO, SLA.