The numbers that matter

The second hour of the review is about speed, and the first thing Sam does, with the ceremony of somebody who has waited all year for this, is ban the word. Fast is not a number. Fast is a feeling.

Fast, Sam says, means two different numbers, and race day needs them separately. performanceConcept · lights on your mapperformanceHow fast and how much: the umbrella word over latency, the time one operation takes, and throughput, how many operations fit in a unit of time. A performance problem means one or both numbers has gone bad. is the umbrella over both. Underneath it, latencyConcept · lights on your maplatencyHow long one operation takes, measured on the asker’s clock, tap to response. Measured in milliseconds, felt in patience. The round trips this story has timed since Act I, given their formal name. is the time one thing takes: the half-second at the mat is a latency, and so was the fifth of a second after Case’s tap. throughputConcept · lights on your mapthroughputHow many operations per unit of time a system can handle: requests per second, splits per minute. Latency is the speed of one; throughput is the volume of many, and a system can be good at either without the other. is how many things fit per second, and it is the other number entirely: a quarter-million timing events in a four-hour window is a throughput problem that never once cares how fast any single split travelled. One number for the one. One for the many. The trap, and the reason Sam polices the vocabulary with such pleasure, is that the two can be traded against each other without anyone in the room noticing it happened. Batch a hundred splits and send them together, and throughput soars while every individual split arrives later. Better number. Worse morning. Bandwidth, from Act I, was the width of one pipe. These two are the general case, and they are the two numbers on the wall Sunday morning.

The quiet axes

Two more axes close the meeting. maintainabilityConcept · lights on your mapmaintainabilityHow easily future engineers can understand, change, and fix the system. The least glamorous axis and the one veterans weight heaviest: most of a system’s life is maintenance, and cleverness illegible at 3 a.m. is a liability. gets one veteran sentence from Priya: whatever we build for Sunday, somebody debugs at 3 a.m. someday, so cleverness we cannot read exhausted is not cleverness. And securityConcept · lights on your mapsecurityA system’s resistance to misuse: who can get in, what they can reach, what happens when somebody hostile tries. In tradeoff meetings it is the voice saying that the convenient design opens a door. speaks once, in its tradeoff voice, when somebody proposes making the results export publicly linkable for speed and the room says, in unison, no. The full anatomy of that no is Act IV’s audit.

Nine axes, one meeting, every purchase deliberate. What the meeting decides to actually buy, more machines, and which machines, and how the buying happens by itself, fills the rest of Thursday.