Amazon — Detailed

flowchart TB
  subgraph Clients
    WEB([Web])
    MOB([Mobile])
  end

  subgraph Edge
    CDN
    GW[API Gateway]
  end

  subgraph Catalog[Catalog]
    CSVC[Catalog Service]
    CDB[(Catalog DB)]
    IMG[(Product images)]
    CACHE_C[Catalog cache]
  end

  subgraph Search
    SRCH[Search Service]
    IDX[(Inverted index)]
    RANK([Ranker - ML])
    AUTOC[Autocomplete]
  end

  subgraph Reco[Recommendations]
    SIM[Similar items]
    BB[Bought-also-bought]
    PRS[Personalized]
  end

  subgraph Cart[Cart & Checkout]
    CARTSVC[Cart Service]
    CARTDB[(Cart KV<br/>session)]
    CKT[Checkout]
    ADDR[Address book]
    TAX[Tax / Pricing]
    PROMO[Promo / coupon]
  end

  subgraph Order
    OSVC[Order Service]
    ODB[(Orders DB)]
    SAGA([Order Saga<br/>orchestrator])
    OUTB[[Outbox events]]
  end

  subgraph Inv[Inventory]
    INVSVC[Inventory Service]
    INVDB[(Inventory KV<br/>per SKU per warehouse)]
    RES[Soft reservation]
  end

  subgraph Pay[Payment]
    PAY[Payment Service]
    GTW((Stripe / Adyen / PayPal))
    REC([Reconciliation])
  end

  subgraph Ship[Fulfillment]
    WMS[Warehouse Mgmt]
    PICK[Pick / Pack / Ship]
    CARR[Carrier API<br/>UPS / FedEx]
    TRK[Tracking]
  end

  subgraph After[Returns / RMA]
    RMA[Returns service]
    REF[Refund]
  end

  subgraph Notif
    EMAIL[Email]
    PUSH[Push]
  end

  Clients --> CDN --> GW
  GW --> Catalog
  GW --> Search
  GW --> Reco
  GW --> Cart
  Cart --> Order
  Order --> Inv
  Order --> Pay
  Order --> Ship
  Order --> Notif
  Pay --> GTW
  Order --> SAGA --> OUTB
  OUTB --> Inv
  OUTB --> Ship
  After --> Order

    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 WEB,MOB client;
    class GW edge;
    class CSVC,CACHE_C,SRCH,AUTOC,SIM,BB,PRS,CARTSVC,CKT,ADDR,TAX,PROMO,OSVC,INVSVC,RES,PAY,PICK,CARR,TRK,RMA,REF,EMAIL,PUSH service;
    class CDB,IMG,IDX,CARTDB,ODB,INVDB,WMS datastore;
    class OUTB queue;
    class RANK,SAGA,REC compute;
    class GTW external;

Order flow as a saga

1. Create order (PENDING).
2. Reserve inventory (compensate: release).
3. Authorize payment (compensate: void).
4. Capture payment + confirm order.
5. Hand off to WMS for fulfillment.
6. On failure at any step → compensations in reverse.

Inventory consistency

• Per-SKU-per-warehouse counters with soft reservations.
• Read replica may show stale stock; final check at order placement.
• Optimistic concurrency on stock decrement.

Storage choices

• Catalog: read-mostly → SQL/NoSQL with heavy cache.
• Orders: SQL with strong consistency (financial).
• Inventory: KV with optimistic CAS.
• Cart: KV/session (eventual OK, low staleness).

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	API gateway / BFF	single ingress, auth, rate limit, routing	api-gateway
HLD	CAP / PACELC	C vs A under partition; L vs C otherwise	cap-pacelc
HLD	Leader/follower replication	sync/semi-sync/async replication, failover	replication-leader-follower
HLD	Distributed transactions	2PC, TCC, sagas, outbox/inbox	distributed-transactions
HLD	Search internals	inverted index, BM25, embeddings, ANN	search-internals
LLD	Concurrency primitives	mutex, semaphore, RW lock, atomic, CAS	concurrency-primitives