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classDiagram
    direction LR

    %% ── Python / SQLAlchemy (booking_system_backend/models.py) ──────────

    class User {
        +int user_id PK
        +str name
        +str email
    }

    class Flight {
        +int flight_id PK
        +str origin
        +str destination
        +str departure_time
        +str arrival_time
        +int base_price
        +int economy_seats_available
        +int business_seats_available
        +int galaxium_seats_available
    }

    class Booking {
        +int booking_id PK
        +int user_id FK
        +int flight_id FK
        +str status
        +str booking_time
        +str seat_class
        +int price_paid
    }

    class BookingStatus {
        <<enumeration>>
        BOOKED = "booked"
        CANCELLED = "cancelled"
        CANCELED = "cancelled"
        COMPLETED = "completed"
    }

    %% ── Java / JPA (holdservice/domain) ────────────────────────────────

    class Quote {
        +String quoteId PK
        +Integer flightId
        +String seatClass
        +Integer quantity
        +Integer travelerId
        +String travelerName
        +Long pricePerSeat
        +Long totalPrice
        +Instant expiresAt
        +QuoteStatus status
        +Instant createdAt
        #onCreate() void
    }

    class QuoteStatus {
        <<enumeration>>
        CREATED
    }

    class Hold {
        +String holdId PK
        +String quoteId FK
        +HoldStatus status
        +Instant reservedUntil
        +String externalBookingReference
        +String errorMessage
        +Instant createdAt
        +Instant updatedAt
        #onCreate() void
        #onUpdate() void
    }

    class HoldStatus {
        <<enumeration>>
        HELD
        EXPIRED
        CONFIRMED
        RELEASED
        CONFIRMATION_FAILED
    }

    class AuditEvent {
        +String eventId PK
        +String entityType
        +String entityId
        +String eventType
        +String details
        +Instant createdAt
        #onCreate() void
    }

    %% ── Relationships ───────────────────────────────────────────────────

    User        "1" --> "0..*" Booking      : places
    Flight      "1" --> "0..*" Booking      : booked on
    Booking          ..>        BookingStatus : status

    Quote       "1" --> "0..*" Hold         : generates
    Quote            ..>        QuoteStatus  : status
    Hold             ..>        HoldStatus   : status

    AuditEvent       ..>        Quote        : references entityId
    AuditEvent       ..>        Hold         : references entityId

sequenceDiagram
    autonumber
    participant U  as User (Browser)
    participant FE as Frontend<br/>(api.ts)
    participant PY as Python Backend<br/>(server.py)
    participant BS as BookingService<br/>(booking.py)
    participant QC as QuoteController<br/>(Java)
    participant QS as QuoteService<br/>(Java)
    participant HC as HoldController<br/>(Java)
    participant HS as HoldService<br/>(Java)
    participant DB as Python SQLite
    participant JDB as Java SQLite

    Note over U,JDB: ── Phase 1: Create Quote ──────────────────────────────────

    U->>FE: createQuote({ flightId, seatClass,<br/>quantity, travelerId, travelerName })
    FE->>PY: POST /quotes
    PY->>QC: POST /api/v1/quotes
    QC->>QS: createQuote(request)
    QS->>QS: generateQuoteId() → "Q-2025-000001"
    QS->>QS: pricingService.calculatePrice(flightId, seatClass)
    QS->>JDB: save Quote (status=CREATED, expiresAt=+24h)
    QS->>JDB: save AuditEvent (QUOTE / CREATED)
    QC-->>PY: 201 Quote
    Note right of PY: Returns {"error":"..."} HTTP 200<br/>if Java service unreachable
    PY-->>FE: Quote JSON
    FE->>FE: assertNotProxyError(response.data)
    FE-->>U: quoteId

    Note over U,JDB: ── Phase 2: Create Hold ───────────────────────────────────

    U->>FE: createHold(quoteId)
    FE->>PY: POST /quotes/{quoteId}/holds
    PY->>HC: POST /api/v1/quotes/{quoteId}/holds
    HC->>HS: createHold(quoteId)
    HS->>JDB: findById(quoteId)
    JDB-->>HS: Quote
    HS->>HS: check quote not expired
    HS->>HS: generateHoldId() → "H-2025-000001"
    HS->>JDB: save Hold (status=HELD,<br/>reservedUntil=+15min)
    HS->>JDB: save AuditEvent (HOLD / CREATED)
    HC-->>PY: 201 Hold
    PY-->>FE: Hold JSON
    FE->>FE: assertNotProxyError(response.data)
    FE-->>U: holdId

    Note over U,JDB: ── Phase 3: Confirm Hold → Create Booking ─────────────────

    U->>FE: confirmHold(holdId)
    FE->>PY: POST /holds/{holdId}/confirm
    PY->>HC: POST /api/v1/holds/{holdId}/confirm
    HC->>HS: confirmHold(holdId)
    HS->>JDB: findById(holdId)
    JDB-->>HS: Hold
    HS->>HS: check status == HELD
    HS->>HS: check reservedUntil not passed
    HS->>JDB: findById(hold.quoteId)
    JDB-->>HS: Quote

    HS->>PY: POST /internal/bookings/from-hold<br/>{ travelerId, travelerName, flightId, seatClass }
    PY->>BS: book_flight(db, user_id, name,<br/>flight_id, seat_class)
    BS->>DB: query Flight (check seats available)
    BS->>DB: query User (validate user_id + name match)
    BS->>DB: decrement {seat_class}_seats_available
    BS->>DB: insert Booking (status="booked")
    DB-->>BS: Booking row
    BS-->>PY: BookingOut

    alt booking succeeded
        PY-->>HS: 200 { booking_id, ... }
        HS->>JDB: update Hold (status=CONFIRMED,<br/>externalBookingReference=booking_id)
        HS->>JDB: save AuditEvent (HOLD / CONFIRMED)
        HC-->>PY: 200 Hold (CONFIRMED)
        PY-->>FE: Hold JSON
        FE->>FE: assertNotProxyError(response.data)
        FE-->>U: Booking confirmed ✓
    else booking failed (name mismatch / no seats / user not found)
        PY-->>HS: 400 { error, error_code, details }
        HS->>JDB: update Hold (status=CONFIRMATION_FAILED,<br/>errorMessage=...)
        HS->>JDB: save AuditEvent (HOLD / CONFIRMATION_FAILED)
        HC-->>PY: 400 Bad Request
        PY-->>FE: error response
        FE-->>U: Error shown to user ✗
    end

flowchart LR

    subgraph Traveller["👤 Traveller (Browser)"]
        T1[Browse available flights]
        T2[Search flights by origin / destination]
        T3[Filter flights by date, price, seat class,\nduration, route category, time period]
        T4[Register account]
        T5[Sign in with name and email]
        T6[Select seat class\neconomy / business / galaxium]
        T7[Get price quote]
        T8[Place seat hold - 15-minute timer]
        T9[Confirm hold and create booking]
        T10[Release hold]
        T11[View active bookings]
        T12[View past bookings]
        T13[Cancel booking]
        T14[View pending holds with countdown]
        T15[Dismiss expired hold]
    end

    subgraph AIAgent["🤖 AI Agent (MCP Client)"]
        A1[list_flights]
        A2[register_user]
        A3[get_user_id]
        A4[book_flight]
        A5[get_bookings]
        A6[cancel_booking]
    end

    subgraph JavaService["☕ Java Hold Service\n(Internal System)"]
        J1[Confirm hold via POST /internal/bookings/from-hold]
        J2[Auto-expire holds after timeout]
    end

    subgraph RestAPI["🐍 Python REST API\n(External Consumers / Swagger)"]
        R1[GET  /flights — list and filter flights]
        R2[POST /register — register user]
        R3[GET  /user — look up user by name and email]
        R4[POST /book — book a flight]
        R5[GET  /bookings/{user_id} — get user bookings]
        R6[POST /cancel/{booking_id} — cancel booking]
        R7[POST /quotes — create quote proxy]
        R8[GET  /quotes/{id} — get quote proxy]
        R9[POST /quotes/{id}/holds — create hold proxy]
        R10[GET  /holds/{id} — get hold proxy]
        R11[POST /holds/{id}/confirm — confirm hold proxy]
        R12[POST /holds/{id}/release — release hold proxy]
        R13[GET  / — health check]
    end

    Traveller   -->|uses frontend which calls| RestAPI
    AIAgent     -->|MCP over HTTP at /mcp| RestAPI
    JavaService -->|calls back via internal endpoint| RestAPI