Atom Store

The problem AtomStore solves

Atoms registered with createAtom live in a module-level registry (atoms object in atom.ts). In a Node process serving multiple concurrent requests, every request shares the same atoms. Request A writing userAtom.update({ name: "Alice" }) will corrupt Request B’s view of the same atom.

AtomStore gives each request its own isolated clone of every atom template. The template atom itself is never mutated.

Concepts

Term	Meaning
Template atom	The atom created with createAtom at module level. Holds the default. Never mutated directly in SSR.
Scoped clone	A per-store copy of a template atom. Created lazily on first store.use(template). Keyed by the template’s original key internally; owns its own state and event subscriptions.
Snapshot	A plain Record<string, unknown> — the current values of every scoped atom in the store. Used to pass server state to the client.

Basic SSR pattern

// state.ts — shared module (template atoms)
import { createAtom } from "@mongez/atom";

export const userAtom    = createAtom({ key: "user",    default: { name: "Anon", loggedIn: false } });
export const cartAtom    = createAtom({ key: "cart",    default: [] });
export const localeAtom  = createAtom({ key: "locale",  default: "en" });

// server.ts — per-request handler
import { createAtomStore } from "@mongez/atom";
import { userAtom, cartAtom, localeAtom } from "./state";

async function handleRequest(req, res) {
  const store = createAtomStore();

  try {
    // Get request-scoped clones. The template atoms are untouched.
    const user   = store.use(userAtom);
    const locale = store.use(localeAtom);

    user.update({ name: req.user.name, loggedIn: true });
    locale.update(req.headers["accept-language"]?.slice(0, 2) ?? "en");

    // Render your app — pass `store` via context so components call
    // store.use(atom) instead of reading the template directly.
    const html = renderApp(store);

    // Serialize current store state to embed in the HTML response.
    const snapshot = store.snapshot();

    res.send(buildHtml(html, snapshot));
  } finally {
    // CRITICAL: destroy the store after every request to release
    // event-bus subscriptions and allow GC.
    store.destroy();
  }
}

API reference

createAtomStore / new AtomStore

import { createAtomStore, AtomStore } from "@mongez/atom";

const store = createAtomStore();   // convenience factory, same as new AtomStore()

store.use(template)

Lazily creates a scoped clone of template. Second call with the same template returns the existing clone.

const scopedUser = store.use(userAtom);
// scopedUser is a full Atom<User> — update, merge, onChange, etc. all work.
scopedUser.update({ name: "Alice", loggedIn: true });

// The template is untouched:
userAtom.value;   // { name: "Anon", loggedIn: false }

store.get(key)

Look up an already-created scoped atom by its original key. Returns undefined if use() has not been called for that key yet.

store.get("user")?.value;   // { name: "Alice", loggedIn: true }
store.get("never-used");    // undefined

store.has(key)

store.has("user");   // true after store.use(userAtom)

store.list()

All scoped atoms currently in this store (in insertion order).

store.list();   // [scopedUserAtom, scopedLocaleAtom]

store.hydrate(snapshot)

Apply a snapshot of initial values. Atoms already in the store are updated immediately; atoms not yet registered have their values queued and applied on the next use(template) call.

// Server receives client-provided initial values (e.g. SSR rehydration from cookies)
store.hydrate({
  "locale": "fr",
  "ui.theme": "dark",
});

// Later, when the atom is accessed:
const locale = store.use(localeAtom);
locale.value;   // "fr"  (the queued value was applied)

store.snapshot()

Serialize current scoped atom values to a plain object. Use for embedding in SSR HTML or passing as initialValues to <AtomStoreProvider>.

const snapshot = store.snapshot();
// { "user": { name: "Alice", loggedIn: true }, "locale": "en" }

// Embed in HTML for client hydration:
const html = `<script>window.__ATOM_SNAPSHOT__ = ${JSON.stringify(snapshot)}</script>`;

store.destroy()

Destroys every scoped clone and clears the store. Must be called at the end of each request to prevent event-bus subscription leaks.

store.destroy();

Next.js App Router example

lib/atom-store.ts

import { createAtomStore } from "@mongez/atom";
import { cache } from "react";

// React's `cache()` gives each Server Component tree its own store instance.
export const getRequestStore = cache(() => createAtomStore());

// app/page.tsx (Server Component)
import { getRequestStore } from "@/lib/atom-store";
import { userAtom } from "@/state";
import { cookies } from "next/headers";

export default async function Page() {
  const store = getRequestStore();
  const user  = store.use(userAtom);

  const session = cookies().get("session")?.value;
  if (session) {
    user.update(JSON.parse(session));
  }

  // Pass snapshot to client component for hydration:
  const snapshot = store.snapshot();
  return <ClientRoot initialValues={snapshot} />;
}

// components/ClientRoot.tsx  ("use client")
// In @mongez/react-atom:
import { AtomStoreProvider } from "@mongez/react-atom";

export function ClientRoot({ initialValues, children }) {
  return (
    <AtomStoreProvider initialValues={initialValues}>
      {children}
    </AtomStoreProvider>
  );
}

Multiple stores (multi-tenant / parallel rendering)

You can create as many stores as needed. Each is fully independent:

const storeA = createAtomStore();
const storeB = createAtomStore();

storeA.use(userAtom).update({ name: "Alice", loggedIn: true });
storeB.use(userAtom).update({ name: "Bob",   loggedIn: true });

storeA.get("user")?.value;   // { name: "Alice" }
storeB.get("user")?.value;   // { name: "Bob" }
userAtom.value;              // { name: "Anon" }  — template untouched

Key pitfalls

• Always call store.destroy() after each request. Scoped clones subscribe to the @mongez/events bus. Failing to destroy creates an event-listener leak that grows with traffic.
• store.use(template) is lazy. If a component reads userAtom.value directly (template, not store.use(userAtom)) on the server, it gets the global default, not the request-scoped value. All server-side reads must go through store.use().
• store.hydrate() queues values for atoms not yet used. If you call hydrate before any use(), values are staged. They are applied the first time the corresponding use(template) is called. Order of operations does not matter.
• Clone keys are internal. A scoped clone’s key is ${original.key}.clone.N. The store indexes it by the original key (template.key), so store.get("user") works even though the clone’s .key property is different.
• Do not use persist: true with scoped atoms. The built-in localStorage adapter no-ops on the server, but a custom async adapter on a scoped atom would write per-request data to a shared store keyed by the clone’s suffixed key. If you need SSR-safe persistence, apply it to the template and use a cookie adapter that reads from req.cookies.