A data ingestion service has an availability objective of 99.5 percent over a 28-day window. At the end of the window, measured availability is 99.8 percent. How much of the error budget remains unspent?
Show answer and explanation
Answer: A
The error budget equals 100 percent minus the availability objective, so here it is 0.5 percentage points of allowed unavailability. The service was unavailable for 0.2 percent of the window, which is 100 minus 99.8. Subtracting that consumption leaves 0.3 percentage points unspent. Consumption is measured as actual unavailability, so a window that beats its target spends less than the full allowance and is never charged for the extra headroom.
Why the other options are wrong
- B. Beating the objective means unavailability stayed below the allowed level, so budget is left over rather than spent. Treating performance above target as consumption reverses how error budgets are meant to work.
- C. The 0.5 point figure is the total allowance for the window, not the amount left. Since 0.2 points of unavailability already happened, the full allowance is not available for the rest of the window.
- D. The 0.2 figure is the consumed portion, not the remainder. Remaining budget is the total allowance minus actual unavailability, so reporting consumption as what is left is a common calculation mistake.