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Patterns for building reactive apps including subscription management, optimistic updates, cache behavior, and paginated queries with cursor-based loading
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/paginationdon't have the plugin yet? install it then click "run inline in claude" again.