---
type: concept
tags: [parking, domain, business, occupancy]
sources: []
updated: 2026-06-15
status: open
---

# Capacity & Occupancy

How many vehicles are inside, how many spaces remain, and what happens when the lot is full.

## Occupancy is a projection (like everything else)

`occupancy = count(open [[parking-session|sessions]])` — an entry with no matching exit. It is a
**fold over the signed [[append-only-event-chain]]**, never a hand-maintained counter (a counter is
editable and drifts; the chain is the truth). Spaces-free = `capacity − occupancy`.

- **`capacity`** is admin-set per site (and per **zone/level** if the lot has sections — model a
  `zone` on capacity + on the entry so multi-level is a later addition, not a rewrite).
- Permit concurrency (`maxConcurrent`, see [[permit]]) is the same kind of fold, scoped to one
  permit's open sessions.

## Full → refuse entry + FULL sign

- When `occupancy ≥ capacity`, the entry flow **refuses** (no `vehicle_entry`, no barrier open) and
  can drive a **"FULL" sign** (a relay/output, via the device adapter layer).
- **Safety/policy nuance:** "full" blocks *entry* only — **exit always works** ([[fail-state-safety]]:
  exit fails open; never trap a vehicle). Permit holders may be allowed in past a "transient full"
  threshold (reserve spaces for subscribers) — an optional policy knob.
- **Counting drift is real:** tailgating (two cars, one entry) and missed reads make the live count
  diverge from physical reality. The count is the *system's* occupancy; periodic ground-truth (a
  loop count, or the [[opencv-anpr-service|vision]] count) reconciles it — surfaced as an anomaly,
  not silently corrected.

## Open

- Whether "FULL" is a hard block or a soft warning (operator can wave one in) — operator policy.
- Zone/level granularity at launch vs. single capacity number.
- Reserve-for-permits threshold.