Taiichi Ohno, developing what became the Toyota Production System, took the idea directly from American supermarkets: a shopper takes what they need off the shelf, and the shelf gets restocked to replace exactly what was taken — never scheduled centrally, never overstocked on faith. Apply that to a factory floor and you get a Lean supermarket: a small, deliberately-sized stock of parts sitting between two processes, replenished only when downstream actually withdraws from it.
Why not just let it flow?
In an ideal world every process would flow directly into the next with zero inventory between them (one-piece flow). In practice, processes rarely run at identical speeds, share equipment across product families, or sit far apart geographically — continuous flow isn't achievable everywhere. A supermarket is the deliberate compromise: instead of an uncontrolled pile of WIP building up wherever two mismatched processes meet, you cap it at a known quantity and make replenishment a signal, not a guess.
The pull signal: kanban
A supermarket only works because withdrawing from it triggers a signal — a kanban card, historically, though the mechanism doesn't have to be a physical card — that tells the upstream process "make one more, exactly one was just taken." No withdrawal, no signal, no new production. That's the entire definition of a pull system: upstream never produces except in direct response to downstream consumption. Push systems, by contrast, produce to a forecast or schedule regardless of whether downstream is ready, which is exactly how you end up with inventory nobody needs yet sitting next to a shortage of something else.
Sizing a supermarket
The size of a supermarket isn't arbitrary — it needs to cover the replenishment lead time (how long it takes upstream to make more once signaled) plus a safety margin for variation, and no more. Oversize it and you've just built a warehouse with extra steps; undersize it and downstream stalls waiting for replenishment. On a value stream map, an oversized inventory triangle between two supermarket-controlled steps is a visible tell that the buffer needs re-sizing, not celebrating.
On the map
PruneChart has a dedicated `supermarket` node distinct from a plain inventory triangle, plus withdrawal, production and signal kanban node types for drawing the actual pull loop — the withdrawal kanban travels from the supermarket to the upstream process, the production kanban authorizes upstream to build a replacement, and a signal kanban (usually for batch/changeover-heavy processes) triggers production only once stock drops to a reorder point.