Availability
Also called: Uptime, Nines
The share of time a system works correctly, often written in nines: 99.9% allows about 43 minutes of downtime a month.
Definition
Availability is usually measured as successful requests or uptime over a period. Each extra nine cuts allowed downtime by ten. Services that depend on each other in series multiply their availability, so a chain is weaker than its weakest link, while redundant copies multiply their unavailability and make a component much more reliable.
In practice
Three dependencies at 99.9% each, called one after another, give at most about 99.7%.