Real-Time Systems in React

// Every message over the wire carries an id for deduplication
// and a timestamp for ordering

export interface BaseMessage {
  id: string;       // Unique per message, used to deduplicate on reconnect
  type: string;
  ts: string;       // ISO 8601, server-assigned
}

// Server -> Client messages
export interface ChatMessage extends BaseMessage {
  type: "chat:message";
  payload: { roomId: string; userId: string; text: string; };
}

export interface PresenceUpdate extends BaseMessage {
  type: "presence:update";
  payload: { userId: string; status: "online" | "away" | "offline"; };
}

export interface CursorMove extends BaseMessage {
  type: "cursor:move";
  payload: { userId: string; x: number; y: number; };
}

export interface PatchEvent extends BaseMessage {
  type: "doc:patch";
  payload: { docId: string; patch: unknown; version: number; };
}

export interface ServerAck extends BaseMessage {
  type: "ack";
  payload: { messageId: string; };
}

export interface ServerError extends BaseMessage {
  type: "error";
  payload: { code: string; message: string; };
}

// Client -> Server messages
export interface SendChat {
  type: "chat:send";
  payload: { roomId: string; text: string; };
}

export interface JoinRoom {
  type: "room:join";
  payload: { roomId: string; };
}

// Union types for type-safe dispatch and handling
export type ServerMessage =
  | ChatMessage
  | PresenceUpdate
  | CursorMove
  | PatchEvent
  | ServerAck
  | ServerError;

export type ClientMessage = SendChat | JoinRoom;

import { useEffect, useRef, useCallback, useReducer } from "react";
import type { ServerMessage, ClientMessage } from "@/types/ws-protocol";

type ConnectionStatus =
  | "idle"
  | "connecting"
  | "connected"
  | "reconnecting"
  | "disconnected";

interface ConnectionState {
  status: ConnectionStatus;
  attempt: number;
  lastError: string | null;
}

type ConnectionAction =
  | { type: "CONNECT" }
  | { type: "CONNECTED" }
  | { type: "DISCONNECTED"; error?: string }
  | { type: "RECONNECT" }
  | { type: "GIVE_UP" };

function reducer(state: ConnectionState, action: ConnectionAction): ConnectionState {
  switch (action.type) {
    case "CONNECT":
      return { ...state, status: "connecting", lastError: null };
    case "CONNECTED":
      return { status: "connected", attempt: 0, lastError: null };
    case "DISCONNECTED":
      return { ...state, status: "disconnected", lastError: action.error ?? null };
    case "RECONNECT":
      return { ...state, status: "reconnecting", attempt: state.attempt + 1 };
    case "GIVE_UP":
      return { ...state, status: "disconnected" };
    default:
      return state;
  }
}

const MAX_RETRIES = 8;
const BASE_DELAY_MS = 500;
const MAX_DELAY_MS = 30_000;

export function useWebSocketConnection(
  url: string,
  onMessage: (msg: ServerMessage) => void
) {
  const [state, dispatch] = useReducer(reducer, {
    status: "idle",
    attempt: 0,
    lastError: null,
  });

  const wsRef = useRef<WebSocket | null>(null);
  const seenIds = useRef(new Set<string>());
  const retryTimer = useRef<ReturnType<typeof setTimeout>>();

  const connect = useCallback(() => {
    dispatch({ type: "CONNECT" });
    const ws = new WebSocket(url);
    wsRef.current = ws;

    ws.onopen = () => {
      dispatch({ type: "CONNECTED" });
    };

    ws.onmessage = (event) => {
      try {
        const msg = JSON.parse(event.data) as ServerMessage;
        // Deduplicate by message id to handle replay on reconnect
        if (seenIds.current.has(msg.id)) return;
        seenIds.current.add(msg.id);
        // Trim the set to avoid unbounded memory growth
        if (seenIds.current.size > 10_000) {
          const oldest = [...seenIds.current].slice(0, 5_000);
          oldest.forEach((id) => seenIds.current.delete(id));
        }
        onMessage(msg);
      } catch {
        // Malformed message: log and continue
      }
    };

    ws.onclose = (event) => {
      dispatch({ type: "DISCONNECTED", error: event.reason });
      // Do not retry on clean close (code 1000) or auth failure (4001)
      if (event.code === 1000 || event.code === 4001) return;

      const attempt = state.attempt + 1;
      if (attempt >= MAX_RETRIES) {
        dispatch({ type: "GIVE_UP" });
        return;
      }

      // Exponential backoff with jitter
      const delay = Math.min(BASE_DELAY_MS * 2 ** attempt, MAX_DELAY_MS);
      const jitter = Math.random() * delay * 0.2; // 20% jitter
      dispatch({ type: "RECONNECT" });
      retryTimer.current = setTimeout(connect, delay + jitter);
    };

    ws.onerror = () => {
      // onerror always fires before onclose; handle in onclose
    };
  }, [url, onMessage, state.attempt]);

  const send = useCallback((msg: ClientMessage) => {
    if (wsRef.current?.readyState === WebSocket.OPEN) {
      wsRef.current.send(JSON.stringify(msg));
    }
  }, []);

  useEffect(() => {
    connect();
    return () => {
      clearTimeout(retryTimer.current);
      wsRef.current?.close(1000, "Component unmounted");
    };
  }, []);

  return { status: state.status, lastError: state.lastError, send };
}

import { useEffect } from "react";
import { useQueryClient } from "@tanstack/react-query";
import { useWebSocketConnection } from "./useWebSocketConnection";
import type { ServerMessage, ChatMessage } from "@/types/ws-protocol";

export function useRealtimeRoom(roomId: string) {
  const queryClient = useQueryClient();

  const handleMessage = useCallback((msg: ServerMessage) => {
    switch (msg.type) {
      case "chat:message":
        // Append new message to the cached list
        queryClient.setQueryData<ChatMessage[]>(
          ["chat", roomId, "messages"],
          (old) => (old ? [...old, msg] : [msg])
        );
        break;
      case "presence:update":
        // Update a single user&apos;s presence in the presence map
        queryClient.setQueryData<Record<string, string>>(
          ["chat", roomId, "presence"],
          (old) => ({ ...old, [msg.payload.userId]: msg.payload.status })
        );
        break;
      case "error":
        // Surface server-side errors to the appropriate query
        console.error("[WS Error]", msg.payload.code, msg.payload.message);
        break;
    }
  }, [roomId, queryClient]);

  const { status, send } = useWebSocketConnection(
    `${process.env.NEXT_PUBLIC_WS_URL}/rooms/${roomId}`,
    handleMessage
  );

  return { connectionStatus: status, send };
}

import { auth } from "@/lib/auth";
import { eventBus } from "@/lib/event-bus";
import { NextRequest } from "next/server";

export const runtime = "nodejs"; // Edge runtime does not support long-lived responses

export async function GET(
  req: NextRequest,
  { params }: { params: { channel: string } }
) {
  const session = await auth();
  if (!session) {
    return new Response("Unauthorized", { status: 401 });
  }

  const { channel } = params;
  const encoder = new TextEncoder();

  const stream = new ReadableStream({
    start(controller) {
      // Helper to send a named SSE event
      function sendEvent(eventName: string, data: unknown) {
        const payload =
          `event: ${eventName}
` +
          `data: ${JSON.stringify(data)}
` +
          `id: ${Date.now()}

`;
        controller.enqueue(encoder.encode(payload));
      }

      // Send a heartbeat comment every 15s to prevent proxy timeouts
      const heartbeat = setInterval(() => {
        controller.enqueue(encoder.encode(": heartbeat

"));
      }, 15_000);

      // Subscribe to the event bus for this channel
      const unsubscribe = eventBus.subscribe(channel, (event) => {
        sendEvent(event.type, event.payload);
      });

      // Clean up when the client disconnects
      req.signal.addEventListener("abort", () => {
        clearInterval(heartbeat);
        unsubscribe();
        controller.close();
      });

      // Send connection confirmation
      sendEvent("connected", { channel, userId: session.user.id });
    },
  });

  return new Response(stream, {
    headers: {
      "Content-Type": "text/event-stream",
      "Cache-Control": "no-cache, no-transform",
      "X-Accel-Buffering": "no", // Disable nginx buffering
      Connection: "keep-alive",
    },
  });
}

import { useEffect, useRef } from "react";

type SSEHandler<T> = (data: T) => void;
type SSEHandlers = Record<string, SSEHandler<unknown>>;

interface UseSSEOptions {
  withCredentials?: boolean;
  onConnect?: () => void;
  onDisconnect?: () => void;
  onError?: (error: Event) => void;
}

export function useSSE(url: string, handlers: SSEHandlers, opts: UseSSEOptions = {}) {
  const handlersRef = useRef(handlers);
  handlersRef.current = handlers; // Always up-to-date without re-creating the effect

  useEffect(() => {
    let es: EventSource;
    let retryCount = 0;
    let stopped = false;

    function connect() {
      es = new EventSource(url, { withCredentials: opts.withCredentials ?? true });

      es.onopen = () => {
        retryCount = 0;
        opts.onConnect?.();
      };

      // Register all named event handlers
      for (const [eventName, handler] of Object.entries(handlersRef.current)) {
        es.addEventListener(eventName, (event: MessageEvent) => {
          try {
            handler(JSON.parse(event.data));
          } catch {
            handler(event.data);
          }
        });
      }

      es.onerror = (error) => {
        opts.onError?.(error);
        es.close();
        opts.onDisconnect?.();
        // EventSource auto-reconnects, but we layer our own backoff for hard failures
        if (!stopped && retryCount < 10) {
          const delay = Math.min(500 * 2 ** retryCount, 30_000);
          retryCount++;
          setTimeout(connect, delay);
        }
      };
    }

    connect();

    return () => {
      stopped = true;
      es?.close();
    };
  }, [url]); // Only reconnect if URL changes
}

// Usage:
// useSSE("/api/events/notifications", {
//   notification: (data) => addNotification(data as Notification),
//   "notifications:clear": () => clearAll(),
// });

import { useQuery } from "@tanstack/react-query";
import { useEffect, useRef } from "react";

type TerminalStatus = "delivered" | "failed" | "cancelled" | "refunded";
const TERMINAL: Set<string> = new Set(["delivered", "failed", "cancelled", "refunded"]);

interface OrderStatusResponse {
  status: string;
  updatedAt: string;
}

export function useOrderStatus(orderId: string) {
  const isHidden = useRef(false);

  // Track tab visibility to reduce polling when the tab is backgrounded
  useEffect(() => {
    function handleVisibility() {
      isHidden.current = document.visibilityState === "hidden";
    }
    document.addEventListener("visibilitychange", handleVisibility);
    return () => document.removeEventListener("visibilitychange", handleVisibility);
  }, []);

  return useQuery({
    queryKey: ["orders", orderId, "status"],
    queryFn: () =>
      fetch(`/api/orders/${orderId}/status`).then((r) => r.json()) as Promise<OrderStatusResponse>,

    refetchInterval: (query) => {
      // Stop polling when the order reaches a terminal state
      const status = query.state.data?.status;
      if (status && TERMINAL.has(status)) return false;

      // Poll less frequently when the tab is hidden
      return isHidden.current ? 60_000 : 10_000;
    },

    // Re-fetch immediately when the tab regains focus
    refetchOnWindowFocus: true,

    // Keep showing stale data while the refetch is in flight
    staleTime: 0,
  });
}

import { NextRequest, NextResponse } from "next/server";
import { getOrderStatus } from "@/lib/orders";
import { createETag } from "@/lib/etag";

export async function GET(
  req: NextRequest,
  { params }: { params: { id: string } }
) {
  const order = await getOrderStatus(params.id);
  const etag = createETag(order);

  // Conditional request: return 304 if data has not changed
  // This avoids transferring the payload on every poll tick
  if (req.headers.get("If-None-Match") === etag) {
    return new Response(null, { status: 304 });
  }

  return NextResponse.json(order, {
    headers: {
      ETag: etag,
      "Cache-Control": "no-cache", // Revalidate on every request, but use ETag
    },
  });
}

export class WebSocketHeartbeat {
  private pingTimer: ReturnType<typeof setInterval> | null = null;
  private pongTimeout: ReturnType<typeof setTimeout> | null = null;
  private readonly PING_INTERVAL = 25_000; // Ping every 25s
  private readonly PONG_TIMEOUT = 5_000;   // Consider dead if no pong in 5s

  constructor(
    private ws: WebSocket,
    private onDead: () => void
  ) {}

  start() {
    this.pingTimer = setInterval(() => {
      if (this.ws.readyState !== WebSocket.OPEN) return;
      this.ws.send(JSON.stringify({ type: "ping" }));

      this.pongTimeout = setTimeout(() => {
        // No pong received: close and trigger reconnect
        this.ws.close(4000, "Ping timeout");
        this.onDead();
      }, this.PONG_TIMEOUT);
    }, this.PING_INTERVAL);
  }

  receivedPong() {
    if (this.pongTimeout) {
      clearTimeout(this.pongTimeout);
      this.pongTimeout = null;
    }
  }

  stop() {
    if (this.pingTimer) clearInterval(this.pingTimer);
    if (this.pongTimeout) clearTimeout(this.pongTimeout);
  }
}

import { useEffect, useRef } from "react";

type PresenceStatus = "online" | "away" | "offline";

interface UsePresenceOptions {
  sendStatus: (status: PresenceStatus) => void;
  awayAfterMs?: number;
}

export function usePresence({ sendStatus, awayAfterMs = 120_000 }: UsePresenceOptions) {
  const awayTimer = useRef<ReturnType<typeof setTimeout>>();
  const currentStatus = useRef<PresenceStatus>("online");

  function setStatus(status: PresenceStatus) {
    if (currentStatus.current === status) return;
    currentStatus.current = status;
    sendStatus(status);
  }

  function resetAwayTimer() {
    clearTimeout(awayTimer.current);
    if (currentStatus.current !== "online") setStatus("online");
    awayTimer.current = setTimeout(() => setStatus("away"), awayAfterMs);
  }

  useEffect(() => {
    const events = ["mousemove", "keydown", "touchstart", "scroll", "click"];
    events.forEach((e) => window.addEventListener(e, resetAwayTimer, { passive: true }));

    function handleVisibility() {
      if (document.visibilityState === "hidden") {
        setStatus("away");
      } else {
        resetAwayTimer();
      }
    }
    document.addEventListener("visibilitychange", handleVisibility);

    setStatus("online");
    resetAwayTimer();

    return () => {
      clearTimeout(awayTimer.current);
      events.forEach((e) => window.removeEventListener(e, resetAwayTimer));
      document.removeEventListener("visibilitychange", handleVisibility);
    };
  }, []);
}

interface PendingMutation<T> {
  operationId: string;
  entityId: string;
  optimisticValue: T;
  timestamp: number;
}

// A simple in-memory store for pending mutations
// In a more complex app, this would live in Zustand or a context
const pending = new Map<string, PendingMutation<unknown>>();

export const optimisticQueue = {
  add<T>(entityId: string, optimisticValue: T): string {
    const operationId = crypto.randomUUID();
    pending.set(operationId, {
      operationId,
      entityId,
      optimisticValue,
      timestamp: Date.now(),
    });
    return operationId;
  },

  remove(operationId: string) {
    pending.delete(operationId);
  },

  hasPendingFor(entityId: string): boolean {
    for (const m of pending.values()) {
      if (m.entityId === entityId) return true;
    }
    return false;
  },

  // Call this when a real-time event arrives for an entity that has pending mutations.
  // Returns true if the event was deferred (should not be applied yet).
  shouldDefer(entityId: string): boolean {
    return this.hasPendingFor(entityId);
  },
};

import { useMutation, useQueryClient } from "@tanstack/react-query";
import { optimisticQueue } from "@/lib/optimistic-queue";
import type { Task } from "@/types/api";

export function useCompleteTask(taskId: string) {
  const queryClient = useQueryClient();

  return useMutation({
    mutationFn: () =>
      fetch(`/api/tasks/${taskId}/complete`, { method: "POST" }).then((r) => r.json()),

    onMutate: async () => {
      await queryClient.cancelQueries({ queryKey: ["tasks", taskId] });
      const snapshot = queryClient.getQueryData<Task>(["tasks", taskId]);

      const operationId = optimisticQueue.add(taskId, { status: "done" });

      queryClient.setQueryData<Task>(["tasks", taskId], (old) =>
        old ? { ...old, status: "done" } : old
      );

      return { snapshot, operationId };
    },

    onError: (_err, _vars, context) => {
      if (context?.snapshot) {
        queryClient.setQueryData(["tasks", taskId], context.snapshot);
      }
      if (context?.operationId) {
        optimisticQueue.remove(context.operationId);
      }
    },

    onSuccess: (_data, _vars, context) => {
      if (context?.operationId) {
        optimisticQueue.remove(context.operationId);
      }
    },

    onSettled: () => {
      queryClient.invalidateQueries({ queryKey: ["tasks", taskId] });
    },
  });
}

// In the WebSocket message handler, defer events for entities with pending mutations:
// if (optimisticQueue.shouldDefer(msg.payload.taskId)) {
//   deferredEvents.push(msg);
//   return;
// }
// After the mutation settles, replay deferred events.

import { QueryClient } from "@tanstack/react-query";
import type { ServerMessage } from "@/types/ws-protocol";

// A single function that routes incoming real-time events to the correct query keys.
// Register this as the onMessage handler for your WebSocket or SSE connection.

export function createRealtimeSyncHandler(queryClient: QueryClient) {
  return function handleServerMessage(msg: ServerMessage) {
    switch (msg.type) {
      case "chat:message":
        queryClient.setQueryData<ChatMessage[]>(
          ["rooms", msg.payload.roomId, "messages"],
          (old) => {
            if (!old) return [msg];
            // Prevent duplicates on reconnect replay
            if (old.some((m) => m.id === msg.id)) return old;
            return [...old, msg];
          }
        );
        break;

      case "presence:update":
        queryClient.setQueryData<Record<string, string>>(
          ["presence", msg.payload.userId.split(":")[0]], // roomId from userId scope
          (old) => ({ ...old, [msg.payload.userId]: msg.payload.status })
        );
        break;

      case "doc:patch":
        // For document patches, invalidate and re-fetch rather than applying the patch
        // client-side to avoid implementing an OT engine in the frontend
        queryClient.invalidateQueries({ queryKey: ["docs", msg.payload.docId] });
        break;

      default:
        // Unhandled message types: log in development only
        if (process.env.NODE_ENV === "development") {
          console.warn("[RealtimeSync] Unhandled message type:", (msg as ServerMessage).type);
        }
    }
  };
}

import { create } from "zustand";
import { subscribeWithSelector } from "zustand/middleware";
import { throttle } from "lodash-es";

interface CursorPosition { x: number; y: number; }

interface CursorsState {
  // userId -> position
  cursors: Record<string, CursorPosition>;
  setCursor: (userId: string, pos: CursorPosition) => void;
  removeCursor: (userId: string) => void;
}

export const useCursorsStore = create<CursorsState>()(
  subscribeWithSelector((set) => ({
    cursors: {},
    setCursor: (userId, pos) =>
      set((state) => ({
        cursors: { ...state.cursors, [userId]: pos },
      })),
    removeCursor: (userId) =>
      set((state) => {
        const { [userId]: _, ...rest } = state.cursors;
        return { cursors: rest };
      }),
  }))
);

// Throttle incoming cursor events to 60fps maximum before writing to the store
export const handleCursorMove = throttle((userId: string, x: number, y: number) => {
  useCursorsStore.getState().setCursor(userId, { x, y });
}, 16); // ~60fps

// A component subscribes only to a specific user&apos;s cursor to minimise re-renders
export function useUserCursor(userId: string): CursorPosition | undefined {
  return useCursorsStore((state) => state.cursors[userId]);
}

// Attach a version to every mutable entity
export interface Versioned<T> {
  data: T;
  version: number;    // Monotonically increasing server-assigned version
  updatedAt: string;  // ISO 8601 for human-readable ordering
}

// When sending a mutation, include the version the client based the change on
export interface MutationWithVersion<T> {
  payload: T;
  baseVersion: number; // The version the client had when they started editing
}

// The server compares baseVersion to the current version:
// - If they match: no concurrent modification, apply the change
// - If they differ: conflict, return 409 with the current state for the client to resolve

// Client-side conflict handler
export async function submitWithConflictResolution<T>(
  url: string,
  mutation: MutationWithVersion<T>,
  onConflict: (serverVersion: Versioned<T>, clientChange: T) => T
): Promise<Versioned<T>> {
  const res = await fetch(url, {
    method: "PATCH",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify(mutation),
  });

  if (res.status === 409) {
    // Server returned the current state alongside the conflict
    const serverVersion: Versioned<T> = await res.json();
    // Ask the application layer to resolve the conflict
    const resolved = onConflict(serverVersion, mutation.payload);
    // Retry with the resolved value based on the server&apos;s current version
    return submitWithConflictResolution(
      url,
      { payload: resolved, baseVersion: serverVersion.version },
      onConflict
    );
  }

  if (!res.ok) throw new Error(`Mutation failed: ${res.status}`);
  return res.json();
}

import * as Y from "yjs";
import { WebsocketProvider } from "y-websocket";
import { IndexeddbPersistence } from "y-indexeddb";

export function createCollaborativeDoc(docId: string, userId: string) {
  // The Y.Doc is the shared CRDT document
  const doc = new Y.Doc();

  // IndexedDB persistence: the document survives page refreshes and offline periods
  const localProvider = new IndexeddbPersistence(`doc-${docId}`, doc);

  // WebSocket provider: synchronises with the server and other clients in real time
  const wsProvider = new WebsocketProvider(
    process.env.NEXT_PUBLIC_YJS_SERVER!,
    docId,
    doc,
    {
      connect: true,
      // Resync all content when reconnecting after a gap
      resyncInterval: 5_000,
    }
  );

  // The awareness protocol carries ephemeral per-user state (cursor, selection, name)
  const { awareness } = wsProvider;
  awareness.setLocalStateField("user", {
    id: userId,
    name: "Current User",
    color: generateUserColor(userId),
    cursor: null,
  });

  return {
    doc,
    wsProvider,
    localProvider,
    awareness,
    // Shared data types within the document
    text: doc.getText("content"),
    comments: doc.getArray("comments"),
    metadata: doc.getMap("metadata"),
    destroy() {
      awareness.setLocalState(null); // Signal offline to other users
      wsProvider.destroy();
      localProvider.destroy();
    },
  };
}

function generateUserColor(userId: string): string {
  // Deterministic colour from userId so the same user always gets the same colour
  const hash = userId.split("").reduce((acc, c) => acc + c.charCodeAt(0), 0);
  const hue = hash % 360;
  return `hsl(${hue}, 70%, 55%)`;
}

import { useEditor, EditorContent } from "@tiptap/react";
import StarterKit from "@tiptap/starter-kit";
import Collaboration from "@tiptap/extension-collaboration";
import CollaborationCursor from "@tiptap/extension-collaboration-cursor";
import { createCollaborativeDoc } from "@/lib/collaboration/doc-provider";
import { useEffect, useRef } from "react";

interface Props {
  docId: string;
  userId: string;
  userName: string;
}

export function CollaborativeEditor({ docId, userId, userName }: Props) {
  const collab = useRef(createCollaborativeDoc(docId, userId));

  useEffect(() => {
    return () => collab.current.destroy();
  }, [docId]);

  const editor = useEditor({
    extensions: [
      StarterKit.configure({ history: false }), // Yjs manages history
      Collaboration.configure({ document: collab.current.doc }),
      CollaborationCursor.configure({
        provider: collab.current.wsProvider,
        user: { name: userName, color: "#0ea5e9" },
        // Render each remote user's cursor with their name label
        render: (user) => {
          const cursor = document.createElement("span");
          cursor.classList.add("collaboration-cursor__caret");
          cursor.style.borderColor = user.color;
          const label = document.createElement("div");
          label.classList.add("collaboration-cursor__label");
          label.style.backgroundColor = user.color;
          label.textContent = user.name;
          cursor.appendChild(label);
          return cursor;
        },
      }),
    ],
    editorProps: {
      attributes: { class: "prose focus:outline-none" },
    },
  });

  return <EditorContent editor={editor} />;
}

import { QueryClient } from "@tanstack/react-query";

export function applyRealtimeCacheOperation(
  queryClient: QueryClient,
  event: {
    operation: "create" | "update" | "delete";
    entity: string;
    id: string;
    payload?: unknown;
  }
) {
  const detailKey = [event.entity, event.id];
  const listKey = [event.entity];

  switch (event.operation) {
    case "create":
      // Set the detail cache entry
      if (event.payload) {
        queryClient.setQueryData(detailKey, event.payload);
      }
      // Prepend to any active list queries without refetching
      queryClient.setQueriesData<{ data: unknown[]; meta: unknown }>(
        { queryKey: listKey, exact: false },
        (old) => {
          if (!old || !event.payload) return old;
          return { ...old, data: [event.payload, ...old.data] };
        }
      );
      break;

    case "update":
      if (event.payload) {
        // Update detail
        queryClient.setQueryData(detailKey, event.payload);
        // Patch matching items in all list caches
        queryClient.setQueriesData<{ data: unknown[]; meta: unknown }>(
          { queryKey: listKey, exact: false },
          (old) => {
            if (!old) return old;
            return {
              ...old,
              data: old.data.map((item) =>
                (item as { id: string }).id === event.id ? event.payload : item
              ),
            };
          }
        );
      }
      break;

    case "delete":
      queryClient.removeQueries({ queryKey: detailKey });
      queryClient.setQueriesData<{ data: unknown[]; meta: unknown }>(
        { queryKey: listKey, exact: false },
        (old) => {
          if (!old) return old;
          return {
            ...old,
            data: old.data.filter((item) => (item as { id: string }).id !== event.id),
          };
        }
      );
      break;
  }
}

import { openDB, IDBPDatabase } from "idb";

interface QueuedMutation {
  id: string;
  url: string;
  method: string;
  body: string;
  timestamp: number;
  retryCount: number;
}

const DB_NAME = "offline-mutations";
const STORE = "queue";

async function getDb(): Promise<IDBPDatabase> {
  return openDB(DB_NAME, 1, {
    upgrade(db) {
      db.createObjectStore(STORE, { keyPath: "id" });
    },
  });
}

export const mutationQueue = {
  async enqueue(url: string, method: string, body: unknown): Promise<string> {
    const db = await getDb();
    const id = crypto.randomUUID();
    await db.put(STORE, {
      id,
      url,
      method,
      body: JSON.stringify(body),
      timestamp: Date.now(),
      retryCount: 0,
    });
    return id;
  },

  async getAll(): Promise<QueuedMutation[]> {
    const db = await getDb();
    return db.getAll(STORE);
  },

  async remove(id: string): Promise<void> {
    const db = await getDb();
    await db.delete(STORE, id);
  },

  async incrementRetry(id: string): Promise<void> {
    const db = await getDb();
    const item = await db.get(STORE, id);
    if (item) {
      await db.put(STORE, { ...item, retryCount: item.retryCount + 1 });
    }
  },
};

import { mutationQueue } from "./mutation-queue";

type SyncResult =
  | { id: string; status: "ok" }
  | { id: string; status: "conflict"; serverData: unknown }
  | { id: string; status: "error"; reason: string };

export async function syncOfflineMutations(
  onConflict: (mutation: { url: string; body: unknown }, serverData: unknown) => unknown
): Promise<SyncResult[]> {
  const pending = await mutationQueue.getAll();
  if (pending.length === 0) return [];

  // Process in chronological order to preserve causal ordering
  const sorted = pending.sort((a, b) => a.timestamp - b.timestamp);
  const results: SyncResult[] = [];

  for (const mutation of sorted) {
    try {
      const res = await fetch(mutation.url, {
        method: mutation.method,
        headers: { "Content-Type": "application/json" },
        body: mutation.body,
      });

      if (res.ok) {
        await mutationQueue.remove(mutation.id);
        results.push({ id: mutation.id, status: "ok" });
        continue;
      }

      if (res.status === 409) {
        // Server reports a conflict: let the application layer resolve it
        const serverData = await res.json();
        const resolved = onConflict(
          { url: mutation.url, body: JSON.parse(mutation.body) },
          serverData
        );

        // Retry with the resolved payload
        const retryRes = await fetch(mutation.url, {
          method: mutation.method,
          headers: { "Content-Type": "application/json" },
          body: JSON.stringify(resolved),
        });

        if (retryRes.ok) {
          await mutationQueue.remove(mutation.id);
          results.push({ id: mutation.id, status: "conflict", serverData });
        } else {
          results.push({ id: mutation.id, status: "error", reason: `${retryRes.status}` });
        }
        continue;
      }

      await mutationQueue.incrementRetry(mutation.id);
      results.push({ id: mutation.id, status: "error", reason: `HTTP ${res.status}` });
    } catch (err) {
      await mutationQueue.incrementRetry(mutation.id);
      results.push({ id: mutation.id, status: "error", reason: String(err) });
    }
  }

  return results;
}

// Wire up the sync engine to the online event
// window.addEventListener("online", () => syncOfflineMutations(resolveConflict));

import { useMutation, useQueryClient } from "@tanstack/react-query";
import { mutationQueue } from "@/lib/offline/mutation-queue";

interface OfflineMutationOptions<TInput> {
  url: string;
  method?: string;
  onSuccess?: () => void;
  queryKeysToInvalidate?: unknown[][];
}

export function useOfflineMutation<TInput>(opts: OfflineMutationOptions<TInput>) {
  const queryClient = useQueryClient();

  return useMutation({
    mutationFn: async (input: TInput) => {
      if (!navigator.onLine) {
        // Offline: enqueue the mutation for later sync
        await mutationQueue.enqueue(opts.url, opts.method ?? "POST", input);
        // Return a synthetic response to allow optimistic updates to proceed
        return { queued: true };
      }

      const res = await fetch(opts.url, {
        method: opts.method ?? "POST",
        headers: { "Content-Type": "application/json" },
        body: JSON.stringify(input),
      });
      if (!res.ok) throw new Error(`${res.status}`);
      return res.json();
    },

    onSettled: () => {
      opts.queryKeysToInvalidate?.forEach((key) => {
        queryClient.invalidateQueries({ queryKey: key });
      });
      opts.onSuccess?.();
    },
  });
}

import { createClient } from "redis";

const publisher = createClient({ url: process.env.REDIS_URL });
const subscriber = createClient({ url: process.env.REDIS_URL });

await publisher.connect();
await subscriber.connect();

// In-memory registry of active WebSocket connections per room on this server instance
const localConnections = new Map<string, Set<WebSocket>>();

export function registerConnection(roomId: string, ws: WebSocket) {
  if (!localConnections.has(roomId)) {
    localConnections.set(roomId, new Set());
    // Subscribe to the Redis channel for this room when the first client joins
    subscriber.subscribe(`room:${roomId}`, (message) => {
      deliverToLocalClients(roomId, message);
    });
  }
  localConnections.get(roomId)!.add(ws);
}

export function deregisterConnection(roomId: string, ws: WebSocket) {
  const conns = localConnections.get(roomId);
  if (!conns) return;
  conns.delete(ws);
  if (conns.size === 0) {
    localConnections.delete(roomId);
    subscriber.unsubscribe(`room:${roomId}`);
  }
}

// Publish a message to all servers via Redis
export async function broadcastToRoom(roomId: string, message: unknown) {
  await publisher.publish(`room:${roomId}`, JSON.stringify(message));
}

// Deliver a message to all clients connected to this server instance in the room
function deliverToLocalClients(roomId: string, rawMessage: string) {
  const conns = localConnections.get(roomId);
  if (!conns) return;
  for (const ws of conns) {
    if (ws.readyState === WebSocket.OPEN) {
      ws.send(rawMessage);
    }
  }
}

// WRONG: applies every incoming message without checking for duplicates
ws.onmessage = (event) => {
  const msg = JSON.parse(event.data);
  setMessages((prev) => [...prev, msg]); // Duplicates appear on reconnect
};

// CORRECT: deduplicate by stable message id
const seenIds = new Set<string>();

ws.onmessage = (event) => {
  const msg = JSON.parse(event.data);
  if (seenIds.has(msg.id)) return; // Replay on reconnect: skip
  seenIds.add(msg.id);
  setMessages((prev) => [...prev, msg]);
};

// WRONG: each component opens its own WebSocket connection
function ChatPanel() {
  const ws = new WebSocket("wss://api.example.com/ws"); // New conn per mount
  // ...
}

// CORRECT: share a single connection, subscribe components to a context or store
// The connection is opened once at the app level.
// Components consume events via a context or a pub/sub within the app.

// Option 1: React context
const WSContext = createContext<WSContextValue | null>(null);

export function WSProvider({ children }: { children: React.ReactNode }) {
  const connection = useWebSocketConnection(WS_URL, handleMessage);
  return <WSContext.Provider value={connection}>{children}</WSContext.Provider>;
}

// Option 2: Zustand store connected to a single WS instance (see section 08)

// WRONG: sends every keystroke immediately
function handleInput(value: string) {
  ws.send(JSON.stringify({ type: "doc:change", payload: { value } }));
}

// CORRECT: throttle expensive events, debounce infrequent ones
import { throttle, debounce } from "lodash-es";

// Cursor moves: throttle to 30fps (plenty for visible smoothness)
const sendCursorMove = throttle((x: number, y: number) => {
  ws.send(JSON.stringify({ type: "cursor:move", payload: { x, y } }));
}, 33);

// Document changes: debounce to send at most once per 200ms idle period
const sendDocChange = debounce((content: string) => {
  ws.send(JSON.stringify({ type: "doc:change", payload: { content } }));
}, 200);

// Typing indicator: send once on start, send stop after 3s idle
const sendTypingStop = debounce(() => {
  ws.send(JSON.stringify({ type: "typing:stop" }));
}, 3_000);

function handleInput(value: string) {
  ws.send(JSON.stringify({ type: "typing:start" })); // Only sends when idle > 3s
  sendTypingStop();
  sendDocChange(value);
}

// In a Vite SPA, create the connection once at the module level or in a provider.
// The useWebSocketConnection hook from section 03 works identically here.
// The only change is the URL source:

const WS_URL = import.meta.env.VITE_WS_URL as string;
// Token from your in-memory auth store (never localStorage for real-time tokens)
const AUTH_TOKEN = authStore.getAccessToken();

// Authenticate over the WebSocket handshake via URL query param
// (only safe over WSS / TLS - the token is in the URL)
const AUTHENTICATED_URL = `${WS_URL}?token=${encodeURIComponent(AUTH_TOKEN)}`;

// Or, authenticate via first message after connecting (preferred):
// ws.onopen = () => ws.send(JSON.stringify({ type: "auth", token: AUTH_TOKEN }));

export const wsConnection = new WebSocketConnection(AUTHENTICATED_URL, handleMessage);

import Ably from "ably";

// Obtain a token from your BFF rather than embedding the Ably API key in the browser
async function getAblyToken(): Promise<Ably.TokenRequest> {
  const res = await fetch("/api/realtime/token", { credentials: "include" });
  return res.json();
}

export const ablyClient = new Ably.Realtime({
  authCallback: async (_tokenParams, callback) => {
    try {
      const tokenRequest = await getAblyToken();
      callback(null, tokenRequest);
    } catch (err) {
      callback(err as Error, null);
    }
  },
});

// Subscribe to a channel with typed messages
export function subscribeToRoom(
  roomId: string,
  onMessage: (msg: unknown) => void
) {
  const channel = ablyClient.channels.get(`rooms:${roomId}`);
  channel.subscribe("message", (msg) => onMessage(msg.data));
  return () => channel.unsubscribe();
}

// Publish a message to a room
export async function publishToRoom(roomId: string, data: unknown) {
  const channel = ablyClient.channels.get(`rooms:${roomId}`);
  await channel.publish("message", data);
}

import { useEffect } from "react";
import { useQueryClient } from "@tanstack/react-query";
import { subscribeToRoom } from "@/lib/realtime-ably";
import { applyRealtimeCacheOperation } from "@/lib/realtime-cache";
import { useAuthStore } from "@/stores/auth";

export function RealtimeProvider({ children }: { children: React.ReactNode }) {
  const queryClient = useQueryClient();
  const { user } = useAuthStore();

  useEffect(() => {
    if (!user) return;

    // Subscribe to the user's personal channel for notifications
    const unsubscribePersonal = subscribeToRoom(
      `user:${user.id}`,
      (event) => {
        if (typeof event === "object" && event !== null && "operation" in event) {
          applyRealtimeCacheOperation(
            queryClient,
            event as Parameters<typeof applyRealtimeCacheOperation>[1]
          );
        }
      }
    );

    return () => {
      unsubscribePersonal();
    };
  }, [user?.id, queryClient]);

  return <>{children}</>;
}