ONDC, the whole design.

%% title: Parties talk directly; shared services stay thin
flowchart TD
  BUY@{ icon: "sd:users", label: "Buyers<br/>any app they already use" }
  BAP["Buyer apps · BAP<br/>catalogue cache · cart · collector"]:::core
  GW@{ icon: "sd:gateway", label: "Gateway<br/>multicasts search only" }
  REG@{ icon: "sd:id", label: "Registry<br/>subscribers · keys · domains" }
  BPP["Seller apps · BPP<br/>catalogues · quotes · orders"]:::hot
  IGM@{ icon: "sd:chat", label: "Issues · IGM<br/>48 h · 30 days · ODR" }
  LSP@{ icon: "sd:delivery", label: "Logistics apps<br/>bought by the seller app" }
  SOR@{ icon: "sd:store", label: "Sellers on record<br/>shops like Rehana's" }
  SA@{ icon: "sd:bank", label: "Settlement agency<br/>match · net · RTGS batch" }
  OBS@{ icon: "sd:dashboard", label: "Network observability<br/>masked logs · scorecards" }
  BUY ==> BAP
  BAP ==>|"search"| GW
  BAP ==>|"select · init · confirm"| BPP
  BAP --> IGM
  GW ==>|"by domain and city"| BPP
  GW -.->|"keys, cached"| REG
  BPP ==>|"cascaded logistics"| LSP
  BPP --> SOR
  BPP -->|"settle"| SA
  LSP -.->|"every NP pushes logs"| OBS
  class BUY client
  class GW,REG edge
  class IGM service
  class LSP external
  class SOR store
  class SA store
  class OBS service

%% title: Hold, then release: one confirm at the seller app
flowchart TD
  CONF["/confirm O-77<br/>network-unique order id"]:::service
  DUP{"Order id seen before?"}
  VAL{"Matches quote and terms?"}
  NACK["NACK 31002<br/>buyer app cancels 999"]:::bad
  HOLD@{ icon: "sd:queue", label: "Order HELD<br/>on_confirm in outbox" }
  ACKED{"Buyer app ACKs<br/>on_confirm in window?"}
  REL["Released to the shop<br/>Accepted"]:::core
  C998["Cancel 998<br/>push on_cancel"]:::bad
  CONF ==> DUP
  DUP -->|"yes: same answer"| HOLD
  DUP ==>|"no"| VAL
  VAL -->|"no"| NACK
  VAL ==>|"yes, ACK"| HOLD
  HOLD ==> ACKED
  ACKED ==>|"yes"| REL
  ACKED -->|"no"| C998
  class HOLD queue

%% title: The Pune thali, from cached search to pickup
sequenceDiagram
  autonumber
  participant B as Buyer app
  participant S as Seller app
  participant L as Logistics app
  Note over B: search answered from its catalogue cache
  B->>S: select thali
  S-->>B: ACK
  S->>L: logistics search Moradabad to Pune
  L->>S: on_search · options
  S->>B: on_select · item + delivery charge
  B->>S: init · billing, address, payment terms
  S->>B: on_init · cancellation and settlement terms
  B->>S: confirm O-77 · buyer has paid
  S-->>B: ACK · order held
  S->>L: logistics confirm
  S->>B: on_confirm Accepted
  B-->>S: ACK · released to the shop
  L->>S: on_status Order-picked-up
  S->>B: on_status Order-picked-up

%% title: A silent confirm ends the same way on both sides
sequenceDiagram
  autonumber
  participant B as Buyer app
  participant S as Seller app
  B->>S: confirm O-77
  alt no ACK
    B->>S: confirm O-77 again · same id, capped retries
    Note over B: still nothing · cancel with 999, refund
  else ACK but no on_confirm reaches the buyer app
    S-->>B: ACK · order held, not sent to shop
    S->>B: on_confirm · retried from the outbox
    Note over S: no ACK in the window · cancel with 998, push on_cancel
    Note over B: no on_confirm in its window · cancel with 999, refund
  else lost ACK of on_confirm
    S->>B: on_confirm O-77 resent
    B-->>S: ACK again · same order, nothing new
  end

%% title: Money moves only when both books match
sequenceDiagram
  autonumber
  participant C as Buyer app · collector
  participant R as Seller app · receiver
  participant A as Settlement agency
  C->>A: settle · inter-NP 1,000, collector 50, self 50
  R->>A: settle · inter-NP 1,000, collector 50, self 200, provider 800
  A->>A: dedupe, validate, match, balance check
  alt amounts match
    A->>C: on_settle SETTLED
    A->>R: on_settle SETTLED · net batch to RTGS, 800 to the seller
  else mismatch
    A->>R: on_settle NOT_SETTLED
    R->>C: recon · breakup
    C->>R: on_recon · agree or difference
    Note over C,R: agreed · settle next cycle · else an IGM issue
  end

%% title: Core entities, each held by its own participant
classDiagram
  class Participant {
    +subscriberId : string · domain name
    +role : buyer app, seller app, both
    +domains : list
    +cities : list
  }
  class SigningKey {
    +uniqueKeyId : string
    +publicKey : Ed25519
    +validUntil : time
  }
  class Order {
    +networkOrderId : string · set by the buyer app
    +transactionId : string
    +state : Created, Accepted, In-progress, Completed, Cancelled
    +quote : items + delivery + fees
  }
  class Fulfillment {
    +id : string
    +type : delivery, return, RTO
    +state : Pending to Order-delivered
  }
  class LogisticsOrder {
    +id : string
    +provider : string
    +pickupCode : string
    +deliveryCode : string
  }
  class SettlementLine {
    +orderId : string
    +interParticipant : amount
    +collector : amount
    +self : amount
    +provider : amount
    +status : SETTLED or NOT_SETTLED
  }
  class Issue {
    +id : string
    +type : issue, grievance, dispute
    +actions : append-only list
    +expectedResolution : time
  }
  Participant "1" --> "many" SigningKey : publishes
  Order "1" --> "many" Fulfillment : tracks
  Fulfillment "1" --> "0..1" LogisticsOrder : bought as
  Order "1" --> "many" SettlementLine : settled by
  Order "1" --> "many" Issue : complained about in

%% title: An order on both sides, or on neither
stateDiagram-v2
  [*] --> CONFIRM_SENT: buyer app names the order
  CONFIRM_SENT --> HELD_AT_SELLER: seller ACKs
  CONFIRM_SENT --> CANCELLED_999: NACK or silence after retries
  HELD_AT_SELLER --> ACCEPTED: on_confirm ACKed, released
  HELD_AT_SELLER --> CANCELLED_998: on_confirm NACKed or unanswered
  ACCEPTED --> IN_PROGRESS: packed, picked up
  IN_PROGRESS --> COMPLETED: delivered
  ACCEPTED --> CANCELLED: cancel or force cancel
  IN_PROGRESS --> CANCELLED: RTO initiated
  COMPLETED --> [*]
  CANCELLED_999 --> [*]
  CANCELLED_998 --> [*]
  CANCELLED --> [*]
  class COMPLETED ok
  class CANCELLED_999 bad
  class CANCELLED_998 bad
  class HELD_AT_SELLER queue