convex-realtime

SKILL.md

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

Automatic Subscriptions - useQuery creates a subscription that updates automatically

Smart Caching - Query results are cached and shared across components

Consistency - All subscriptions see a consistent view of the database

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";
import { v } from "convex/values";

export const list = query({
  args: { channelId: v.id("channels") },
  returns: v.array(v.object({
    _id: v.id("messages"),
    _creationTime: v.number(),
    content: v.string(),
    authorId: v.id("users"),
    authorName: v.string(),
  })),
  handler: async (ctx, args) => {
    const messages = await ctx.db
      .query("messages")
      .withIndex("by_channel", (q) => q.eq("channelId", args.channelId))
      .order("desc")
      .take(100);

    // Enrich with author names
    return Promise.all(
      messages.map(async (msg) => {
        const author = await ctx.db.get(msg.authorId);
        return {
          ...msg,
          authorName: author?.name ?? "Unknown",
        };
      })
    );
  },
});

export const send = mutation({
  args: {
    channelId: v.id("channels"),
    authorId: v.id("users"),
    content: v.string(),
  },
  returns: v.id("messages"),
  handler: async (ctx, args) => {
    return await ctx.db.insert("messages", {
      channelId: args.channelId,
      authorId: args.authorId,
      content: args.content,
    });
  },
});

// ChatRoom.tsx
import { useQuery, useMutation } from "convex/react";
import { api } from "../convex/_generated/api";
import { useState, useRef, useEffect } from "react";

function ChatRoom({ channelId, userId }: Props) {
  const messages = useQuery(api.messages.list, { channelId });
  const sendMessage = useMutation(api.messages.send);
  const [input, setInput] = useState("");
  const messagesEndRef = useRef<HTMLDivElement>(null);

  // Auto-scroll to bottom on new messages
  useEffect(() => {
    messagesEndRef.current?.scrollIntoView({ behavior: "smooth" });
  }, [messages]);

  const handleSend = async (e: React.FormEvent) => {
    e.preventDefault();
    if (!input.trim()) return;

    await sendMessage({
      channelId,
      authorId: userId,
      content: input.trim(),
    });
    setInput("");
  };

  return (
    <div className="chat-room">
      <div className="messages">
        {messages?.map((msg) => (
          <div key={msg._id} className="message">
            <strong>{msg.authorName}:</strong> {msg.content}
          </div>
        ))}
        <div ref={messagesEndRef} />
      </div>
      
      <form onSubmit={handleSend}>
        <input
          value={input}
          onChange={(e) => setInput(e.target.value)}
          placeholder="Type a message..."
        />
        <button type="submit">Send</button>
      </form>
    </div>
  );
}

Best Practices

Never run npx convex deploy unless explicitly instructed

Never run any git commands unless explicitly instructed

Use "skip" for conditional queries instead of conditionally calling hooks

Implement optimistic updates for better perceived performance

Use usePaginatedQuery for large datasets

Handle undefined state (loading) explicitly

Avoid unnecessary re-renders by memoizing derived data

Common Pitfalls

Conditional hook calls - Use "skip" instead of if statements

Not handling loading state - Always check for undefined

Missing optimistic update rollback - Optimistic updates auto-rollback on error

Over-fetching with pagination - Use appropriate page sizes

Ignoring subscription cleanup - React handles this automatically

References

Convex Documentation: https://docs.convex.dev/

Convex LLMs.txt: https://docs.convex.dev/llms.txt

React Client: https://docs.convex.dev/client/react

Optimistic Updates: https://docs.convex.dev/client/react/optimistic-updates

Pagination: https://docs.convex.dev/database/pagination