diff --git a/backend/src/services/store.ts b/backend/src/services/store.ts
new file mode 100644
index 0000000..07cf020
--- /dev/null
+++ b/backend/src/services/store.ts
@@ -0,0 +1,55 @@
+import fs from 'node:fs/promises';
+import path from 'node:path';
+import { env } from '../config/env.js';
+import type { DatabaseShape } from '../types.js';
+
+const emptyDb = (): DatabaseShape => ({
+  users: [],
+  refreshTokens: [],
+  files: [],
+  events: [],
+});
+
+export class JsonStore {
+  private dbPath: string;
+  private writeQueue: Promise<void> = Promise.resolve();
+
+  constructor(dataDir = env.dataDir) {
+    this.dbPath = path.join(dataDir, 'db.json');
+  }
+
+  async init() {
+    await fs.mkdir(path.dirname(this.dbPath), { recursive: true });
+    try {
+      await fs.access(this.dbPath);
+    } catch {
+      await fs.writeFile(this.dbPath, JSON.stringify(emptyDb(), null, 2));
+    }
+  }
+
+  async read(): Promise<DatabaseShape> {
+    await this.init();
+    const raw = await fs.readFile(this.dbPath, 'utf8');
+    return { ...emptyDb(), ...JSON.parse(raw) };
+  }
+
+  async write(db: DatabaseShape) {
+    await this.init();
+    this.writeQueue = this.writeQueue.then(async () => {
+      const tmpPath = `${this.dbPath}.tmp`;
+      await fs.writeFile(tmpPath, JSON.stringify(db, null, 2));
+      await fs.rename(tmpPath, this.dbPath);
+    });
+    await this.writeQueue;
+  }
+
+  async update<T>(mutator: (db: DatabaseShape) => T | Promise<T>): Promise<T> {
+    const db = await this.read();
+    const result = await mutator(db);
+    await this.write(db);
+    return result;
+  }
+}
+
+export const store = new JsonStore();
+