convex-realtime

name	convex-realtime
description	Realtime subscriptions and optimistic updates in Convex. Use when implementing live data updates, optimistic UI, pagination with realtime, presence indicators, typing indicators, or any feature requiring instant data synchronization.

Convex Realtime

Automatic Subscriptions

Queries in Convex automatically subscribe to updates:

// React component - automatically updates when data changes
function TaskList({ userId }: { userId: Id<"users"> }) {
  const tasks = useQuery(api.tasks.list, { userId });

  if (tasks === undefined) return <Loading />;

  return (
    <ul>
      {tasks.map((task) => (
        <li key={task._id}>{task.title}</li>
      ))}
    </ul>
  );
}

Optimistic Updates

Basic Optimistic Update

import { useMutation } from "convex/react";
import { api } from "../convex/_generated/api";

function AddTask() {
  const addTask = useMutation(api.tasks.create).withOptimisticUpdate(
    (localStore, args) => {
      const { title, userId } = args;

      // Get current tasks from local cache
      const currentTasks = localStore.getQuery(api.tasks.list, { userId });
      if (currentTasks === undefined) return;

      // Add optimistic task
      const optimisticTask = {
        _id: crypto.randomUUID() as Id<"tasks">,
        _creationTime: Date.now(),
        title,
        userId,
        completed: false,
      };

      // Update local cache immediately
      localStore.setQuery(api.tasks.list, { userId }, [
        optimisticTask,
        ...currentTasks,
      ]);
    }
  );

  return (
    <button onClick={() => addTask({ title: "New Task", userId })}>
      Add Task
    </button>
  );
}

Optimistic Delete

const deleteTask = useMutation(api.tasks.remove).withOptimisticUpdate(
  (localStore, args) => {
    const { taskId, userId } = args;

    const currentTasks = localStore.getQuery(api.tasks.list, { userId });
    if (currentTasks === undefined) return;

    // Remove task from local cache
    localStore.setQuery(
      api.tasks.list,
      { userId },
      currentTasks.filter((t) => t._id !== taskId)
    );
  }
);

Optimistic Toggle

const toggleTask = useMutation(api.tasks.toggle).withOptimisticUpdate(
  (localStore, args) => {
    const { taskId, userId } = args;

    const currentTasks = localStore.getQuery(api.tasks.list, { userId });
    if (currentTasks === undefined) return;

    // Toggle completed status locally
    localStore.setQuery(
      api.tasks.list,
      { userId },
      currentTasks.map((t) =>
        t._id === taskId ? { ...t, completed: !t.completed } : t
      )
    );
  }
);

Paginated Realtime

// convex/messages.ts
import { query } from "./_generated/server";
import { v } from "convex/values";
import { paginationOptsValidator } from "convex/server";

export const list = query({
  args: {
    channelId: v.id("channels"),
    paginationOpts: paginationOptsValidator,
  },
  returns: v.object({
    page: v.array(v.object({
      _id: v.id("messages"),
      _creationTime: v.number(),
      content: v.string(),
      authorId: v.id("users"),
    })),
    isDone: v.boolean(),
    continueCursor: v.string(),
  }),
  handler: async (ctx, args) => {
    return await ctx.db
      .query("messages")
      .withIndex("by_channel", (q) => q.eq("channelId", args.channelId))
      .order("desc")
      .paginate(args.paginationOpts);
  },
});

// React component with pagination
function MessageList({ channelId }: { channelId: Id<"channels"> }) {
  const { results, status, loadMore } = usePaginatedQuery(
    api.messages.list,
    { channelId },
    { initialNumItems: 25 }
  );

  return (
    <div>
      {results.map((message) => (
        <Message key={message._id} message={message} />
      ))}

      {status === "CanLoadMore" && (
        <button onClick={() => loadMore(25)}>Load More</button>
      )}

      {status === "LoadingMore" && <Loading />}
    </div>
  );
}

Presence Indicators

Schema

// convex/schema.ts
export default defineSchema({
  presence: defineTable({
    odcumentId: v.string(),
    odcumentType: v.string(),
    lastSeen: v.number(),
  })
    .index("by_user", ["userId"])
    .index("by_document", ["documentId", "documentType"]),
});

Update Presence

// convex/presence.ts
export const heartbeat = mutation({
  args: {
    documentId: v.string(),
    documentType: v.string(),
  },
  returns: v.null(),
  handler: async (ctx, args) => {
    const identity = await ctx.auth.getUserIdentity();
    if (!identity) return null;

    const existing = await ctx.db
      .query("presence")
      .withIndex("by_user", (q) => q.eq("userId", identity.subject))
      .filter((q) =>
        q.and(
          q.eq(q.field("documentId"), args.documentId),
          q.eq(q.field("documentType"), args.documentType)
        )
      )
      .unique();

    if (existing) {
      await ctx.db.patch(existing._id, { lastSeen: Date.now() });
    } else {
      await ctx.db.insert("presence", {
        userId: identity.subject,
        documentId: args.documentId,
        documentType: args.documentType,
        lastSeen: Date.now(),
      });
    }
    return null;
  },
});

export const getActive = query({
  args: {
    documentId: v.string(),
    documentType: v.string(),
  },
  returns: v.array(v.object({
    userId: v.string(),
    lastSeen: v.number(),
  })),
  handler: async (ctx, args) => {
    const fiveMinutesAgo = Date.now() - 5 * 60 * 1000;

    return await ctx.db
      .query("presence")
      .withIndex("by_document", (q) =>
        q.eq("documentId", args.documentId).eq("documentType", args.documentType)
      )
      .filter((q) => q.gt(q.field("lastSeen"), fiveMinutesAgo))
      .collect();
  },
});

Client Hook

function usePresence(documentId: string, documentType: string) {
  const heartbeat = useMutation(api.presence.heartbeat);
  const activeUsers = useQuery(api.presence.getActive, {
    documentId,
    documentType,
  });

  useEffect(() => {
    // Send heartbeat every 30 seconds
    const interval = setInterval(() => {
      heartbeat({ documentId, documentType });
    }, 30000);

    // Initial heartbeat
    heartbeat({ documentId, documentType });

    return () => clearInterval(interval);
  }, [documentId, documentType, heartbeat]);

  return activeUsers ?? [];
}

Typing Indicators

// convex/typing.ts
export const setTyping = mutation({
  args: { channelId: v.id("channels"), isTyping: v.boolean() },
  returns: v.null(),
  handler: async (ctx, args) => {
    const identity = await ctx.auth.getUserIdentity();
    if (!identity) return null;

    const existing = await ctx.db
      .query("typing")
      .withIndex("by_channel_user", (q) =>
        q.eq("channelId", args.channelId).eq("userId", identity.subject)
      )
      .unique();

    if (args.isTyping) {
      if (existing) {
        await ctx.db.patch(existing._id, { updatedAt: Date.now() });
      } else {
        await ctx.db.insert("typing", {
          channelId: args.channelId,
          userId: identity.subject,
          updatedAt: Date.now(),
        });
      }
    } else if (existing) {
      await ctx.db.delete(existing._id);
    }
    return null;
  },
});

export const getTyping = query({
  args: { channelId: v.id("channels") },
  returns: v.array(v.string()),
  handler: async (ctx, args) => {
    const identity = await ctx.auth.getUserIdentity();
    const tenSecondsAgo = Date.now() - 10000;

    const typing = await ctx.db
      .query("typing")
      .withIndex("by_channel", (q) => q.eq("channelId", args.channelId))
      .filter((q) => q.gt(q.field("updatedAt"), tenSecondsAgo))
      .collect();

    // Exclude current user
    return typing
      .filter((t) => t.userId !== identity?.subject)
      .map((t) => t.userId);
  },
});

Conditional Queries

function UserProfile({ userId }: { userId: Id<"users"> | null }) {
  // Query only runs when userId is not null
  const user = useQuery(
    api.users.get,
    userId ? { userId } : "skip"
  );

  if (userId === null) return <GuestView />;
  if (user === undefined) return <Loading />;

  return <ProfileView user={user} />;
}

Common Pitfalls

• Stale optimistic updates - Always verify server state matches expected
• Over-subscribing - Only subscribe to data you need
• Missing loading states - Handle undefined (loading) vs null (not found)
• Presence cleanup - Add scheduled job to clean old presence records

References

• Reactivity: https://docs.convex.dev/client/react
• Optimistic Updates: https://docs.convex.dev/client/react/optimistic-updates
• Pagination: https://docs.convex.dev/database/pagination