Feature Flag Service — Detailed

flowchart TB
  subgraph Admin
    UI
    API
    CLI
  end

  subgraph Core
    FSVC[Flag Service]
    RULES[Rule engine<br/>targeting + percentage rollout]
    SEG[Segments]
    ENV[Environments]
    APP[Per-application config]
    AUD[Audit log]
    APPR[Approval / 4-eyes]
  end

  subgraph Store
    KV[(Versioned KV)]
    HIST[(Change history)]
  end

  subgraph Distribute
    BUS[Push channel]
    POP[Edge POPs]
    LP[Long-poll / SSE]
    SDK_LOCAL([SDK local cache + fallback])
  end

  subgraph Clients
    APPCL[Apps]
    SERVER([Server-side SDKs])
    EDGE[Edge worker eval]
  end

  subgraph Safety
    KILL[Kill switch]
    DEFAULT[Default-OFF for new flags]
    LIMIT[Per-tenant limits]
  end

  Admin --> Core --> Store
  Core --> Distribute --> Clients
  Safety --- Core

    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 SDK_LOCAL,SERVER client;
    class EDGE edge;
    class FSVC,RULES,SEG,ENV,APP,APPR,BUS,POP,LP,APPCL,KILL,DEFAULT,LIMIT service;
    class KV,HIST datastore;
    class AUD obs;

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	Realtime protocols	WS / SSE / polling / gRPC streaming	realtime-protocols
LLD	Async models	futures / async-await / coroutines / actors	async-models