Ticketmaster / BookMyShow — Detailed

flowchart TB
  subgraph Clients
    WEB
    MOB
  end

  subgraph Edge
    CDN
    LB
    WAF[WAF + Bot filter]
    QUEUE[[Virtual Queue / Waiting Room]]
  end

  subgraph Discover
    EVT[Events Catalog]
    SRCH[Search]
    EVDB[(Events DB)]
  end

  subgraph Seat[Seat selection]
    MAP[Seat Map renderer]
    SEATSVC[Seat Service]
    SEATDB[(Seats per show<br/>strong consistency)]
    HOLD[Hold Service<br/>TTL 7-10 min]
    LOCK[Redis SETNX EX lock per seat]
  end

  subgraph Buy
    CKT[Checkout]
    PAYSVC[Payment]
    PG((Payment Provider))
    IDEM[Idempotency keys]
    SAGA([Booking saga])
  end

  subgraph Issue
    TKT[Ticket Issuer]
    QR[QR / barcode]
    EMAIL[Email / SMS]
    WALLET[Apple/Google Wallet]
  end

  subgraph Anti[Anti-abuse]
    CAPTCHA
    BOT[Bot scoring]
    LIMIT([Per-user limits])
    BLACKLIST
  end

  subgraph After
    REFUND[Refund / cancel]
    RESALE[Resale / Transfer]
    SCAN([Venue scanner])
  end

  Clients --> CDN --> LB --> WAF --> QUEUE --> Discover
  Discover --> Seat --> HOLD --> LOCK
  HOLD --> Buy --> Issue
  Anti --- Clients
  After --- Issue

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class LIMIT client;
    class WAF edge;
    class EVT,SRCH,MAP,SEATSVC,HOLD,CKT,PAYSVC,IDEM,TKT,QR,EMAIL,WALLET,BOT,REFUND,RESALE service;
    class EVDB,SEATDB datastore;
    class LOCK cache;
    class QUEUE queue;
    class SAGA,SCAN compute;
    class PG external;

Seat reservation correctness

• Each seat is a unique row in seats(show_id, seat_id, state) with state machine: available → held → sold.
• Hold = atomic CAS update to held with held_until = now + TTL.
• Background sweeper releases expired holds.
• Selling = CAS held → sold only if owner_id matches and payment captured.

Waiting room

• High-demand events route users to a virtual queue (Cloudflare-style).
• Random fair-share lottery; users enter the buy flow in waves.
• Prevents thundering herd on seat APIs.

Anti-bot

• Heavy captcha + behavioral fingerprinting.
• Per-account / per-IP / per-payment-method limits.
• Account age and history matter for hot drops.

Glossary & fundamentals

Concepts referenced in this design. Each row links to its canonical page; the tag column shows whether it is a high-level (HLD) or low-level (LLD) concept.

Tag	Concept	What it is	Page
HLD	CDN	edge caching for static assets	cdn
HLD	CAP / PACELC	C vs A under partition; L vs C otherwise	cap-pacelc
HLD	Distributed transactions	2PC, TCC, sagas, outbox/inbox	distributed-transactions
HLD	Idempotency & retries	safe re-execution, backoff + jitter	idempotency-retries
HLD	Search internals	inverted index, BM25, embeddings, ANN	search-internals
LLD	State machines	FSM, HSM, transitions, guards	state-machines
LLD	REST API design	verbs, statuses, pagination, errors	rest-api-design
LLD	Behavioural patterns	Strategy, Observer, State, Command, Chain	behavioral-patterns
LLD	Concurrency primitives	mutex, semaphore, RW lock, atomic, CAS	concurrency-primitives