implementing-realtime-sync

Use Server-Sent Events (SSE) with an async generator that yields each token as an event. FastAPI's sse-starlette EventSourceResponse pairs with the browser's native EventSource API, which handles reconnection automatically.

Use SSE for one-way server-to-client streams like notifications, live feeds, and LLM tokens since it reconnects automatically. Use WebSocket for bidirectional communication like chat, games, and collaborative editing.

Yjs is best for text editing with a mature ecosystem supporting ProseMirror, Monaco, and Quill, plus built-in awareness. Automerge suits JSON-like data structures and offers time-travel history, with a Rust-first implementation.

Use Redis pub/sub to broadcast messages across backend instances so any server can deliver events to connected clients. You also need a WebSocket-compatible load balancer with sticky sessions or a Redis backplane.

Combine Yjs with y-indexeddb for local persistence and y-websocket for server sync. Changes apply instantly to IndexedDB, queue while offline, and merge conflict-free via CRDTs when the connection is restored.

Connections drop without heartbeat ping/pong keepalive messages or when proxies buffer traffic. Implement exponential backoff with jitter for reconnection and send heartbeats every 30-60 seconds to keep connections alive.