# DECISION-20260422-pumpingstation-simulations-harness

## Context
- Task/request: Provide a way to fluctuate inputs to the pumpingStation and observe the system's response over time, in a readable form suitable for post-hoc analysis (operator review, Grafana, or ad-hoc debugging).
- Impacted files/contracts: `nodes/pumpingStation/simulations/*`, `test/basic/*`.
- Why a decision is required now: Unit tests (`node --test`) verify individual functions in isolation. They can't ergonomically show "what does the level look like over 20 minutes of storm surge". That's a different artefact.

## Options
1. Extend unit tests to cover scenarios
- Benefits: Single testing surface.
- Risks: Unit tests are assertion-heavy and slow to read; scenario output (tables, events) gets lost in TAP.

2. Separate `simulations/` folder with a scenario runner (selected)
- Benefits: Scenarios read as narratives ("steady state", "storm surge", "safety dry-run"); output is human-friendly (ASCII table + events + expectation checks); JSONL per-tick log enables Grafana streaming or offline analysis.
- Risks: Second test surface to maintain.

3. Real-time Node-RED deployment + observe
- Benefits: Closest to production.
- Risks: Slow, requires infrastructure, irreproducible.

## Decision
- Selected option: Option 2.
- Decision owner: User
- Date: 2026-04-22
- Rationale: Unit tests answer "is this function correct?"; scenarios answer "how does the system behave under this input profile?". Two distinct questions — two distinct tools. The split also matches the .claude/rules/testing.md 3-tier convention (basic/integration/edge) which is for asserted behaviours, not scenario replay.

### Addendum (same-day rename)

Folder was initially named `eval/`. Renamed to `simulations/` in commit pumpingStation@3e13512 — `eval` and `test` are near-synonyms so the split implied a conceptual difference that doesn't really exist. `simulations/` is more honest about what's happening (scripted plant inputs driving a physics sim, recorded for analysis). Rationale above is unchanged; only the folder name is.

## Architecture

```
test/
  basic/ integration/ edge/   — node:test + assertions
simulations/
  run.js                      — scenario driver
  scenarios/*.js              — each exports { name, config, setup, inputs(t,ps), expectations }
  formatters/table.js         — ASCII summary
  logs/*.jsonl                — one-line-per-tick output
  README.md                   — usage + how to pipe into Grafana
```

Driver monkey-patches `Date.now()` so the volume integrator sees 1 second per tick regardless of wall-clock. Every tick records a state snapshot (level, volume, direction, netFlow, flowSource, demand, mode, safetyActive) to JSONL for streaming.

## Consequences
- Compatibility impact: None.
- Safety/security impact: None — read-only simulation.
- Data/operations impact: Running `node simulations/run.js --all` produces artefacts that can be checked into CI for regression (e.g. "did the storm scenario's max level rise compared to last release?"). The JSONL format is friendly to InfluxDB/Grafana for interactive review.

## Implementation Notes
- Required code/doc updates: Driver + three starter scenarios (`levelbased-steady`, `levelbased-storm`, `safety-dry-run-trip`) + README in `simulations/`.
- Validation evidence required: `node simulations/run.js --all` exits 0; manual inspection of JSONL confirms per-tick records make physical sense.

## Rollback / Migration
- Rollback strategy: Delete `simulations/`. Unit tests continue to work.
- Migration/deprecation plan: N/A.