graph TB
    subgraph Client["Frontend — React 18 + Vite"]
        UI_TRAINS["Train Browser<br/>schedules, seat grid"]
        UI_BOOK["Booking Flow<br/>reserve → pay → ticket"]
        UI_DEMO["Concurrency Demo<br/>Alice vs Bob split-screen"]
        UI_ADMIN["Admin Dashboard<br/>stats, occupancy, audit"]
    end

    subgraph API["Backend — FastAPI (fully async)"]
        AUTH["Auth Module<br/>register, login, refresh, me<br/><i>JWT + bcrypt</i>"]
        TRAINS["Trains Module<br/>list trains, schedules<br/>seat availability"]
        BOOKING["Booking Engine<br/>reserve, pay, refund<br/><i>SELECT FOR UPDATE</i>"]
        PAYMENT["Payment Gateway<br/>mock, configurable<br/><i>idempotent charges</i>"]
        AUDIT["Audit Service<br/>atomic in-transaction<br/><i>append-only log</i>"]
        RATELIMIT["Rate Limiter<br/>sliding window<br/><i>sorted sets</i>"]
        DEMO["Demo Service<br/>asyncio.gather race"]
        ADMIN["Admin Service<br/>stats, occupancy"]
        CLEANUP["Cleanup Task<br/>60s interval<br/><i>SKIP LOCKED</i>"]
    end

    subgraph Data["Data Layer"]
        PG[("PostgreSQL 16<br/>8 tables<br/>row-level locks<br/>ACID transactions")]
        REDIS[("Redis 7<br/>seat cache (5s TTL)<br/>rate limit counters")]
    end

    UI_TRAINS -->|"GET /trains/**"| TRAINS
    UI_BOOK -->|"POST /bookings/**"| BOOKING
    UI_DEMO -->|"POST /demo/race-condition"| DEMO
    UI_ADMIN -->|"GET /admin/**"| ADMIN
    Client -->|"POST /auth/**"| AUTH

    AUTH --> PG
    TRAINS --> PG
    TRAINS -.->|"cache read/write"| REDIS
    BOOKING --> PG
    BOOKING --> PAYMENT
    BOOKING --> AUDIT
    RATELIMIT -.->|"sorted set ops"| REDIS
    CLEANUP --> PG
    CLEANUP --> AUDIT
    DEMO --> BOOKING
    ADMIN --> PG

    style PG fill:#336791,color:#fff,stroke:#1e3a5f
    style REDIS fill:#dc382d,color:#fff,stroke:#a12a23
    style BOOKING fill:#2b6cb0,color:#fff,stroke:#1e4e8c
    style CLEANUP fill:#744210,color:#fff,stroke:#5a3510
    style AUDIT fill:#276749,color:#fff,stroke:#1a4731

stateDiagram-v2
    [*] --> reserved : POST /bookings

    reserved --> confirmed : Payment succeeds
    reserved --> cancelled : Payment fails
    reserved --> cancelled : Reservation expires (5min)

    confirmed --> refunded : Refund requested

sequenceDiagram
    actor Alice
    actor Bob
    participant API as FastAPI
    participant DB as PostgreSQL

    Note over Alice, Bob: Both click "Book" on seat A3 at the same time

    Alice->>API: POST /bookings {seat: A3, key: uuid-1}
    Bob->>API: POST /bookings {seat: A3, key: uuid-2}

    API->>DB: BEGIN (Alice's txn)
    API->>DB: BEGIN (Bob's txn)

    API->>DB: SELECT ... WHERE seat=A3 FOR UPDATE (Alice)
    Note over DB: Alice acquires row lock

    API->>DB: SELECT ... WHERE seat=A3 FOR UPDATE (Bob)
    Note over DB: Bob BLOCKS — waiting for Alice's lock

    API->>DB: INSERT booking (Alice)
    API->>DB: INSERT audit_trail (Alice)
    API->>DB: COMMIT (Alice)
    Note over DB: Lock released

    API-->>Alice: 201 Created ✓

    Note over DB: Bob's SELECT unblocks<br/>Finds Alice's committed booking
    API-->>Bob: 409 Conflict ✗

    API->>DB: ROLLBACK (Bob)