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Time Slices Error Budget Calculation Method

The Time Slices method of error budget calculation measures how many good minutes were achieved (minutes where a system is operating within defined boundaries) during a time window. Time slices are currently hard-coded to 1-minute evaluation intervals. A good minute is one where the Time Slice Allowance is not violated. This will be explained further later in this document.

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Target

No matter what budgeting method you choose, Target is used to calculate your error budget. If you want something to be reliable 95% of the time (or in other words, have at most a 5% failure rate over the defined time window), then your target should be set at 95%. This is the reliability target for the Service Level Objective (SLO).

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Time Slice Allowance

The Time Slice Allowance is used to evaluate each time slice. It can be considered a micro-objective. This is used to determine if a time slice should be considered good or bad and is a separate evaluation from the error budget. Each time slice is evaluated independently to determine whether it fell within the defined allowance. If so, then it is considered a good minute. If not, then it is considered a bad minute, and some of the error budget will be burned.

If you decide that a good minute is one with a 90% success rate, then your Time Slice Allowance should be set at 90%. Your Target will then be for 95% of minutes to have at minimum a 90% success rate.

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Use Case Example

Let’s say you are told that over a 24-hour time period a given SLI should have fewer than 10% slow responses (over 750 ms), 95% of the time. Another way of viewing this is that 90% of the data points must be good (in this case, with a value less than 750 ms), 95% of the time. You’ll need to use the Time Slices error budget calculation method for this SLO: you have been given a Time Slice Allowance of 90% - a maximum 10% error/failure rate per 1-minute time slice - and this must be achieved in 95% of the time slices in a 24-hour time period.

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