Engineering Quality, Testing & Operations

Automated Test Suite Overview

The test suite consists of 54 test files (52 backend, 2 frontend) delivering 96 passing automated tests across unit, integration, and chaos scenarios.

backend/tests/
├── test_atomic_stock_reservation.py       # Zero oversell under concurrency
├── test_atomic_checkout.py               # Two-phase order creation
├── test_virtual_waiting_room.py          # FIFO queue admission & tickets
├── test_queue_fairness.py                # Uniform queue draw distributions
├── test_payment_idempotency.py           # Duplicate payment request deduplication
├── test_reliability_load.py              # 100-thread concurrent reservation stress test
├── test_reliability_chaos.py             # Database recovery and cache drop resilience
└── test_timezone_activation.py           # DST boundary crossing and 1.0s tolerance

Concurrency Stress Test Analysis

In backend/tests/test_reliability_load.py, the system tests extreme contention:

• Scenario: 100 concurrent asynchronous tasks attempt to reserve stock for a SKU with only 10 available items.
• Assertion: Exactly 10 tasks succeed with 200 OK, and 90 tasks fail with 409 Conflict.
• Invariant Audit: Immediately following the run, the test asserts that available == 0, reserved == 10, and available + reserved + sold == total_allocated. Zero overselling is confirmed.

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Observability & Prometheus Telemetry

The service exposes a standard Prometheus scrape endpoint at GET /metrics (backend/src/observability/metrics.py):

• inventory_available_gauge{sku_id="..."}: Real-time available units.
• inventory_reserved_gauge{sku_id="..."}: Active reservation holds.
• inventory_sold_gauge{sku_id="..."}: Confirmed purchases.
• oversell_incidents_total{sku_id="...", severity="..."}: Critical anomaly counter.