Deliverables

sequenceDiagram
  autonumber
  participant C as Client
  participant G as API Gateway
  participant O as Order Service
  participant P as Payment Gateway
  participant Q as Event Log

  C->>G: POST /orders (Idempotency-Key)
  G->>O: create order
  O->>O: persist as PENDING
  O->>P: authorise (deadline 3s)
  alt authorised
    P-->>O: approved + auth id
    O->>O: mark CONFIRMED
    O->>Q: OrderConfirmed
    O-->>G: 201 Created
  else declined
    P-->>O: declined
    O->>O: mark REJECTED
    O-->>G: 402 Payment Required
  else timeout
    P--xO: no response by deadline
    O->>O: keep PENDING
    O-->>G: 202 Accepted (poll for status)
    Note over O,P: reconciliation job settles<br/>PENDING against the provider
  end
  G-->>C: response

flowchart TB
  users(["Users"]) --> dns["Global DNS<br/><i>latency routing + health checks</i>"]

  subgraph r1["Region: primary"]
    direction TB
    lb1["Load Balancer<br/><i>public</i>"]
    subgraph az1["Zone A"]
      n1["Node Pool<br/><i>api ×3, worker ×2</i>"]
    end
    subgraph az2["Zone B"]
      n2["Node Pool<br/><i>api ×3, worker ×2</i>"]
    end
    subgraph az3["Zone C"]
      n3["Node Pool<br/><i>api ×2, worker ×1</i>"]
    end
    db1[("Primary DB<br/><i>synchronous replica in Zone B</i>")]
    lb1 --> n1
    lb1 --> n2
    lb1 --> n3
    n1 --> db1
    n2 --> db1
    n3 --> db1
  end

  subgraph r2["Region: secondary"]
    direction TB
    lb2["Load Balancer<br/><i>warm</i>"]
    n4["Node Pool<br/><i>api ×2, scaled down</i>"]
    db2[("Read Replica<br/><i>asynchronous — RPO 30s</i>")]
    lb2 --> n4 --> db2
  end

  dns --> lb1
  dns -.->|"failover only"| lb2
  db1 -.->|"async replication"| db2

flowchart TB
  u(["Users"]) --> gslb["Global traffic manager<br/><i>health + latency routing</i>"]

  subgraph ra["Region A — write primary"]
    direction TB
    a1["Edge + API"]
    a2["Services"]
    a3[("Primary DB<br/><i>accepts writes</i>")]
    a1 --> a2 --> a3
  end

  subgraph rb["Region B — read + standby"]
    direction TB
    b1["Edge + API"]
    b2["Services"]
    b3[("Replica<br/><i>read only · lag &lt; 2s</i>")]
    b1 --> b2 --> b3
  end

  gslb -->|"reads: nearest"| a1
  gslb -->|"reads: nearest"| b1
  gslb -->|"writes: always A"| a1
  b2 -.->|"write forwarding<br/>+45ms"| a2
  a3 ==>|"async replication"| b3
  arb{{"Failover arbitration<br/><i>manual promote · documented<br/>RPO &le; 2s · RTO 15 min</i>"}}
  arb -.-> a3
  arb -.-> b3