Convex Realtime

Convex Realtime

Build reactive applications with Convex's real-time subscriptions, optimistic updates, intelligent caching, and cursor-based pagination.

Documentation Sources

Before implementing, do not assume; fetch the latest documentation:

• Primary: https://docs.convex.dev/client/react
• Optimistic Updates: https://docs.convex.dev/client/react/optimistic-updates
• Pagination: https://docs.convex.dev/database/pagination
• For broader context: https://docs.convex.dev/llms.txt

Instructions

How Convex Realtime Works

1. Automatic Subscriptions - useQuery creates a subscription that updates automatically
2. Smart Caching - Query results are cached and shared across components
3. Consistency - All subscriptions see a consistent view of the database
4. Efficient Updates - Only re-renders when relevant data changes

Basic Subscriptions

// React component with real-time data
import { useQuery } from "convex/react";
import { api } from "../convex/_generated/api";

function TaskList({ userId }: { userId: Id<"users"> }) {
  // Automatically subscribes and updates in real-time
  const tasks = useQuery(api.tasks.list, { userId });

  if (tasks === undefined) {
    return <div>Loading...</div>;
  }

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

Conditional Queries

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

function UserProfile({ userId }: { userId: Id<"users"> | null }) {
  // Skip query when userId is null
  const user = useQuery(
    api.users.get,
    userId ? { userId } : "skip"
  );

  if (userId === null) {
    return <div>Select a user</div>;
  }

  if (user === undefined) {
    return <div>Loading...</div>;
  }

  return <div>{user.name}</div>;
}

Mutations with Real-time Updates

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

function TaskManager({ userId }: { userId: Id<"users"> }) {
  const tasks = useQuery(api.tasks.list, { userId });
  const createTask = useMutation(api.tasks.create);
  const toggleTask = useMutation(api.tasks.toggle);

  const handleCreate = async (title: string) => {
    // Mutation triggers automatic re-render when data changes
    await createTask({ title, userId });
  };

  const handleToggle = async (taskId: Id<"tasks">) => {
    await toggleTask({ taskId });
  };

  return (
    <div>
      <button onClick={() => handleCreate("New Task")}>Add Task</button>
      <ul>
        {tasks?.map((task) => (
          <li key={task._id} onClick={() => handleToggle(task._id)}>
            {task.completed ? "✓" : "○"} {task.title}
          </li>
        ))}
      </ul>
    </div>
  );
}

Optimistic Updates

Show changes immediately before server confirmation:

import { useMutation, useQuery } from "convex/react";
import { api } from "../convex/_generated/api";
import { Id } from "../convex/_generated/dataModel";

function TaskItem({ task }: { task: Task }) {
  const toggleTask = useMutation(api.tasks.toggle).withOptimisticUpdate(
    (localStore, args) => {
      const { taskId } = args;
      const currentValue = localStore.getQuery(api.tasks.get, { taskId });
      
      if (currentValue !== undefined) {
        localStore.setQuery(api.tasks.get, { taskId }, {
          ...currentValue,
          completed: !currentValue.completed,
        });
      }
    }
  );

  return (
    <div onClick={() => toggleTask({ taskId: task._id })}>
      {task.completed ? "✓" : "○"} {task.title}
    </div>
  );
}

Optimistic Updates for Lists

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

function useCreateTask(userId: Id<"users">) {
  return useMutation(api.tasks.create).withOptimisticUpdate(
    (localStore, args) => {
      const { title, userId } = args;
      const currentTasks = localStore.getQuery(api.tasks.list, { userId });
      
      if (currentTasks !== undefined) {
        // Add optimistic task to the list
        const optimisticTask = {
          _id: crypto.randomUUID() as Id<"tasks">,
          _creationTime: Date.now(),
          title,
          userId,
          completed: false,
        };
        
        localStore.setQuery(api.tasks.list, { userId }, [
          optimisticTask,
          ...currentTasks,
        ]);
      }
    }
  );
}

Cursor-Based Pagination

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

export const listPaginated = query({
  args: {
    channelId: v.id("channels"),
    paginationOpts: paginationOptsValidator,
  },
  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
import { usePaginatedQuery } from "convex/react";
import { api } from "../convex/_generated/api";

function MessageList({ channelId }: { channelId: Id<"channels"> }) {
  const { results, status, loadMore } = usePaginatedQuery(
    api.messages.listPaginated,
    { channelId },
    { initialNumItems: 20 }
  );

  return (
    <div>
      {results.map((message) => (
        <div key={message._id}>{message.content}</div>
      ))}
      
      {status === "CanLoadMore" && (
        <button onClick={() => loadMore(20)}>Load More</button>
      )}
      
      {status === "LoadingMore" && <div>Loading...</div>}
      
      {status === "Exhausted" && <div>No more messages</div>}
    </div>
  );
}

Infinite Scroll Pattern

import { usePaginatedQuery } from "convex/react";
import { useEffect, useRef } from "react";
import { api } from "../convex/_generated/api";

function InfiniteMessageList({ channelId }: { channelId: Id<"channels"> }) {
  const { results, status, loadMore } = usePaginatedQuery(
    api.messages.listPaginated,
    { channelId },
    { initialNumItems: 20 }
  );
  
  const observerRef = useRef<IntersectionObserver>();
  const loadMoreRef = useRef<HTMLDivElement>(null);

  useEffect(() => {
    if (observerRef.current) {
      observerRef.current.disconnect();
    }

    observerRef.current = new IntersectionObserver((entries) => {
      if (entries[0].isIntersecting && status === "CanLoadMore") {
        loadMore(20);
      }
    });

    if (loadMoreRef.current) {
      observerRef.current.observe(loadMoreRef.current);
    }

    return () => observerRef.current?.disconnect();
  }, [status, loadMore]);

  return (
    <div>
      {results.map((message) => (
        <div key={message._id}>{message.content}</div>
      ))}
      <div ref={loadMoreRef} style={{ height: 1 }} />
      {status === "LoadingMore" && <div>Loading...</div>}
    </div>
  );
}

Multiple Subscriptions

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

function Dashboard({ userId }: { userId: Id<"users"> }) {
  // Multiple subscriptions update independently
  const user = useQuery(api.users.get, { userId });
  const tasks = useQuery(api.tasks.list, { userId });
  const notifications = useQuery(api.notifications.unread, { userId });

  const isLoading = user === undefined || 
                    tasks === undefined || 
                    notifications === undefined;

  if (isLoading) {
    return <div>Loading...</div>;
  }

  return (
    <div>
      <h1>Welcome, {user.name}</h1>
      <p>You have {tasks.length} tasks</p>
      <p>{notifications.length} unread notifications</p>
    </div>
  );
}

Examples

Real-time Chat Application

// convex/messages.ts
import { query, mutation } from "./_generated/server";