Dark Factory — TDD Developer

Overview

Build every service test-first: RED → GREEN → REFACTOR, one case at a time. Each test is an executable validation rule. The Developer owns unit + integration tests (green before handoff); QA owns E2E + holdout.

The loop

Where the test list comes from (don't invent it)

• LOCAL validation rule → unit test: feed bad input at the edge, assert it is rejected.
• GLOBAL validation rule → integration/reconciliation test: assert the invariant + the authority tie-break.
• PO Test Scenario (State 0 → Trigger → State 1) → unit/integration test asserting the end state (happy/edge/failure).
• effect transform → idempotency test (replay → one action) + compensation test (failure → clean rollback).
• pure transform → plain input → output unit test.

Blind synthesis (anti-Goodhart)

Your own tests drive your build loop, but the acceptance evidence (PO Test Scenarios, the QA held-back acceptance suite) is withheld from you — you're verified against cases you never saw, which proves you built the spec, not your own test. Don't build to the holdout: if known_input: return known_answer is the lookup-degeneration failure.

GREEN discipline

Minimal code only; honour the exact contracts (no invented schemas); Log.Warn not Log.Error (panics in this stack's shared messaging library); decimal for money; config over code.

⚠️ That logging rule is an instance, not a universal. Some libraries escalate an error-level call into a panic or a process exit, so a defensive Log.Error in a handler takes the service down on the first bad message — the opposite of what the author meant. Others do nothing of the kind. Check what your own stack does and write the answer into your own guidance; the rule above is kept because a reader who follows it on a stack that behaves this way is saved, and one who does not know to ask is not.

Worked example — a LOCAL validation rule (Go + Python)

Rule: a vitals reading with no correlationId, or heart rate outside 0–300, is rejected.

// RED — failing test first (table-driven)
func TestIngestVitals_Validation(t *testing.T) {
    cases := []struct{ name string; in VitalsReading; wantErr error }{
        {"missing id", VitalsReading{HeartRate: 80}, ErrNoCorrelationID},
        {"hr too high", VitalsReading{CorrelationID: "abc", HeartRate: 350}, ErrHeartRateRange},
        {"valid", VitalsReading{CorrelationID: "abc", HeartRate: 80}, nil},
    }
    for _, c := range cases {
        t.Run(c.name, func(t *testing.T) {
            if _, err := IngestVitals(c.in); !errors.Is(err, c.wantErr) {
                t.Fatalf("got %v, want %v", err, c.wantErr)
            }
        })
    }
}
// GREEN — smallest code that passes
func IngestVitals(v VitalsReading) (Accepted, error) {
    if v.CorrelationID == "" { return Accepted{}, ErrNoCorrelationID }
    if v.HeartRate < 0 || v.HeartRate > 300 { return Accepted{}, ErrHeartRateRange }
    return Accepted{ID: v.CorrelationID}, nil
}

import pytest
from vitals import ingest_vitals, NoCorrelationID, HeartRateRange

@pytest.mark.parametrize("reading, expected", [
    ({"heart_rate": 80}, NoCorrelationID),
    ({"correlation_id": "abc", "heart_rate": 350}, HeartRateRange),
    ({"correlation_id": "abc", "heart_rate": 80}, None),
])
def test_ingest_vitals(reading, expected):
    if expected is None:
        assert ingest_vitals(reading)["id"] == "abc"
    else:
        with pytest.raises(expected):
            ingest_vitals(reading)

Exit gate

Every validation rule + scenario touching this service has a green test; every effect has idempotency + compensation tests; go test ./... -race / pytest all green.

Anti-patterns

Assertion-free tests · testing the framework/bus instead of your transform · code-before-test · tests bound to internals instead of the rule · skipping -race.