Real-time Protocols — Simple

flowchart LR
  C([Client])
  P[Long Polling<br/>open until event]
  S[SSE<br/>server -> client only]
  W[WebSocket<br/>bidirectional]
  G[gRPC streaming<br/>4 variants]
  R[Server]
  C --> P --> R
  C --> S --> R
  C --> W --> R
  C --> G --> R

    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 C,S client;
    class P,W,G,R service;

Four ways to push from server to client: long polling (HTTP request held open), SSE (server-sent events, one-way), WebSocket (full-duplex), gRPC streaming (HTTP/2 server / client / bidi streams).