Zoom In / Zoom Out
'Checkout is slow', from the box on the architecture diagram to the flash cells. Every level says what the problem looks like there, what you would measure, and why you would go one level deeper — or stop. Zoom out past the top and the question changes shape.
Problem→Understand→Requirements→Constraints→Unknowns→Decomposition→Smallest Step→Model→Experiment→Observe→Debug→Learn→Iterate
The point of the zoom is not to reach the bottom. Most checkout problems are decided at the query planner, three levels down, and almost none go past the operating system. Zoom one level at a time, with a measurement at each, and stop where the problem is decided. The other direction matters as much: some of the most important performance problems are dissolved by asking whether the slow work should be happening at all.
Level 1 of 7
Level 1 · Software Architecture
Architecture: Frontend → Backend → Database
what it looks like here
The checkout page takes several seconds. Users see a spinner after clicking "Place order". Nobody yet knows which box is slow.
what you would investigate
Time the request from the browser: how long until the first byte of the response? Then split that between the network, the backend and whatever the backend waits on. One number per box.
go deeper — or stop
Go down if the backend's own time accounts for most of the wait. Stop here if the time is in the browser (a bundle, a render) or between browser and server — then the problem is a different domain and zooming into the backend teaches nothing.
How to read this page honestly
What the tool does, and what it deliberately refuses to do.
SIMPLIFIED
One symptom, one path down. A real slow checkout might be the browser bundle at level one and never need level two; a different query might stop at the index, or go past the SSD to a network-attached volume that is not shown here. The levels are the store's, in the order this particular symptom would descend, and the "go deeper or stop" line at each is the argument — not a rule that every problem has seven levels.
Take it further
Abstraction Levels →
From "user clicks Buy" to the SSD — the seven levels and why to solve at the highest useful one.
Zoom In, Zoom Out →When to go down a level, when to come back up, and how to know which you need.
Debugging Simulator →The same descent as an investigation: symptom, observation, hypothesis, experiment.