Load Is Three Numbers: the Average, the Peak, and the Rate at Which Both Grow
A brief that says the system serves a million requests a day has said almost nothing useful. A million a day is roughly twelve a second on average, but traffic is never flat: a consumer service can take a third of its daily volume in one hour, and a payroll system can take almost all of it on two days a month. The design has to survive the peak, not the average, so the peak is the number that sizes everything. The ratio between them is worth stating on its own, because a system with a peak five times its average is a different machine from one with a peak fifty times its average. The third number is growth: the rate at which both figures are rising, and the horizon over which the design must hold. A design sized exactly for today's peak is already obsolete, and one sized for a peak ten years out wastes money now. Load is also a requirement rather than an observation, because it is a statement about the future that somebody has to own and be accountable for. State it with the shape of the work too, since a read-heavy million and a write-heavy million are not the same load. After this Concept you can turn a single traffic figure into an average, a peak with its ratio, and a growth rate with a horizon, and say who owns each of the three.
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A brief that says the system serves a million requests a day has said almost nothing useful. A million a day is roughly twelve a second on average, but traffic is never flat: a consumer service can take a third of its daily volume in one hour, and a payroll system can take almost all of it on two days a month. The design has to survive the peak, not the average, so the peak is the number that sizes everything. The ratio between them is worth stating on its own, because a system with a peak five times its average is a different machine from one with a peak fifty times its average. The third number is growth: the rate at which both figures are rising, and the horizon over which the design must hold. A design sized exactly for today's peak is already obsolete, and one sized for a peak ten years out wastes money now. Load is also a requirement rather than an observation, because it is a statement about the future that somebody has to own and be accountable for. State it with the shape of the work too, since a read-heavy million and a write-heavy million are not the same load. After this Concept you can turn a single traffic figure into an average, a peak with its ratio, and a growth rate with a horizon, and say who owns each of the three.
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