What is composite availability?
Composite availability is the combined uptime of a system that depends on multiple components working together. When a request must pass through several services — an API gateway, an application server, a database, a cache — the system is only available when all components are available simultaneously.
The formula is: Composite availability = A₁ × A₂ × A₃ × ... × Aₙ where each A is expressed as a decimal (99.9% = 0.999).
Why composite availability is always lower
Each dependency in series reduces overall availability. Two services each at 99.9% produce a combined availability of 99.8% — not 99.9%. Three services at 99.9% give 99.7%. The more dependencies, the lower the floor.
| Components (all at 99.9%) | Composite availability | Annual downtime |
|---|---|---|
| 1 | 99.900% | 8h 45m |
| 2 | 99.800% | 17h 31m |
| 3 | 99.700% | 26h 16m |
| 5 | 99.501% | 43h 46m |
| 10 | 99.004% | 87h 12m |
Parallel vs series components
The formula above applies to components in series — where all must be available for the system to work. Components in parallel (redundant systems where only one needs to be available) have higher composite availability: Composite = 1 − (1−A₁) × (1−A₂).
For example, two load balancers each at 99.9% in parallel give 99.9999% combined availability — because both would need to fail simultaneously for the system to go down.
Practical implications
Understanding composite availability helps you identify where to invest in reliability. If your system has 10 components and you want to achieve 99.9% end-to-end availability, each component needs to be at approximately 99.99% individually. This clarifies which dependencies need redundancy and which SLAs to negotiate with third-party providers.