Google Docs Collaboration, the whole design.

%% title: Collaborative editing on one page: one owner per document
flowchart TD
  WEB@{ icon: "sd:browser", label: "Browser + mobile editors<br/>local text · one change in flight · pending" } ==>|"changes, cursors"| GW@{ icon: "sd:gateway", label: "Session gateways<br/>WebSocket · route by document" }
  GW -.->|"role? owner server?"| CTL@{ icon: "sd:lock", label: "Permissions + router<br/>role per file · document → server" }
  GW ==> DS["Document server · one owner per document<br/>transform past newer revisions<br/>number · append · ack · broadcast"]:::hot
  DS --> PR@{ icon: "tech:redis", label: "Redis · presence<br/>cursor = position + revision · TTL" }
  DS ==>|"append N+1 if last = N"| LOG@{ icon: "sd:log", label: "Revision log<br/>every change · author · seq" }
  DS -.-> SNAP@{ icon: "sd:object-storage", label: "Snapshots<br/>every 1,000 revisions" }
  LOG --> VH["Version history<br/>groups · 40 named · restore"]:::service
  LOG --> CF@{ icon: "tech:kafka", label: "Kafka · change feed" }
  CF --> IDX@{ icon: "sd:search", label: "Search · comments · notifications" }
  class WEB client
  class GW edge
  class CTL service
  class PR cache
  class LOG,SNAP store
  class CF queue
  class IDX service

%% title: Inside the document server: one change
flowchart TD
  IN(["Change · client c7, seq 41<br/>made against revision 811"]):::client --> DUP{"seq 41 already applied<br/>for c7?"}:::service
  DUP -->|"yes"| ACK(["Ack again<br/>a resend after reconnect"]):::ok
  DUP -->|"no"| ROLE{"Can this session edit?"}:::service
  ROLE -->|"no"| REJ(["Reject VIEW_ONLY<br/>editor drops its changes"]):::bad
  ROLE -->|"yes"| TX["Transform past 812 … latest<br/>Insert 40 → Insert 24"]:::hot
  TX --> AP{"Append revision N+1<br/>if the log ends at N?"}:::core
  AP -->|"fails"| STALE(["Stale owner · stop serving"]):::bad
  AP -->|"ok"| OUT["Ack to author · broadcast to others<br/>editors shift cursors locally"]:::core

%% title: Two edits to one sentence, merged by one server
sequenceDiagram
  autonumber
  participant A as Ananya
  participant S as Document server
  participant L as Revision log
  participant K as Kabir
  Note over A,K: both at revision 811
  K->>S: rev 811 · Delete 20-35
  A->>S: rev 811 · Insert 40 "the"
  S->>L: Append 812 if last is 811
  S-->>K: ack 812
  S-)A: 812 · Delete 20-35
  A->>A: Transform past her pending insert, apply
  S->>S: Transform her insert past 812 · now 24
  S->>L: Append 813 if last is 812
  S-->>A: ack 813
  S-)K: 813 · Insert 24 "the"
  Note over A,K: both screens show the same sentence

%% title: Three days offline: merge by composition
sequenceDiagram
  autonumber
  participant K as Kabir's editor
  participant W as Background worker
  participant S as Document server
  participant L as Revision log
  K->>S: Hello · base revision 112,000 · 2,400 changes held
  S->>W: Compose revisions 112,001 to 172,000
  W->>L: Read 60,000 revisions
  alt base still inside the fine-grained window
    W-->>K: One composed change, bounded by the document
    K->>K: Save a version · compose his 2,400 · transform one vs one
    K->>S: One change against 172,000
    S->>S: Transform past the few newer revisions · append · ack
  else base older than the window
    W-->>K: Three-way merge from his base snapshot
  end

%% title: Core entities
classDiagram
  class Document {
    +docId : string
    +rev : int · latest revision
    +text : string · ≤ 1.02M characters
    +owner : server · from the router
  }
  class Revision {
    +docId : string
    +rev : int · N+1 only if last is N
    +ops : insert / delete / style list
    +author : user
    +clientSeq : client and seq · dedupe
    +at : timestamp
  }
  class Snapshot {
    +docId : string
    +rev : int · every 1,000
    +content : text and styles
  }
  class NamedVersion {
    +docId : string
    +rev : int
    +name : string · up to 40
  }
  class Presence {
    +session : string
    +anchor : int
    +head : int
    +rev : int · position is relative to it
    +ttl : 30 s
  }
  class CommentAnchor {
    +start : int · sticks left
    +end : int
    +orphaned : bool
  }
  Document "1" --> "many" Revision : history
  Document "1" --> "many" Snapshot : opens fast from
  Document "1" --> "many" NamedVersion : labelled
  Document "1" --> "many" Presence : live cursors
  Document "1" --> "many" CommentAnchor : comments

%% title: One local change, from keystroke to the log
stateDiagram-v2
  [*] --> PENDING: typed, applied locally
  PENDING --> PENDING: more typing, composed
  PENDING --> IN_FLIGHT: nothing else in flight, sent
  IN_FLIGHT --> IN_FLIGHT: remote revision, transformed
  IN_FLIGHT --> IN_FLIGHT: reconnect, resent with same seq
  IN_FLIGHT --> COMMITTED: ack with revision number
  IN_FLIGHT --> DROPPED: rejected, view only
  COMMITTED --> [*]
  DROPPED --> [*]
  class COMMITTED ok
  class DROPPED bad
  class IN_FLIGHT queue