Next.js

The routeCopy Anchor Link

One dynamic segment catches the four endpoints (push, pull, bootstrap and wait):

import { syncServer } from '@/lib/sync-server';

export const POST = syncServer.fetch;
// A realtime wait is held up to 30 s: let the function run longer than that.
export const maxDuration = 60;

The serverCopy Anchor Link

On your machine a SQLite file is enough. On Vercel, every instance must reach the same database, so use Postgres (Neon, Supabase, Vercel's own), and hand work after the response to after:

import { createSyncServer, nodeSqlite, sqliteStore } from 'ferrysync/server';
import { DatabaseSync } from 'node:sqlite';

import { schema } from './schema';

export const syncServer = createSyncServer({
  schema,
  store: sqliteStore(nodeSqlite(new DatabaseSync('sync.db'))),
  auth: async (request) => (await getSession(request))?.userId ?? null,
});

AuthCopy Anchor Link

auth gets the route's Request. Read your session from it:

• Better Auth: (await auth.api.getSession({ headers: request.headers }))?.user.id ?? null.
• Auth.js: (await auth())?.user?.id ?? null, since auth() reads the request's cookies.
• A bearer token: see Auth.

The clientCopy Anchor Link

The client points at the route, with no endpoint at the end:

import { createSync, http, indexedDbStorage } from 'ferrysync';

import { schema } from './schema';

export const sync = createSync({
  schema,
  storage: indexedDbStorage('my-app'),
  backend: http({ url: '/api/sync' }),
});

Components that read it are client components ('use client'). On the server, SyncProvider renders its fallback; in the browser, the local data opens and shows, with no request in between.

GotchasCopy Anchor Link

• The Node.js runtime, not Edge. The Postgres and SQLite drivers need Node's sockets and files. Route handlers run on Node.js by default; do not set runtime = 'edge' on this one.
• Keep the server in its own module (lib/sync-server.ts), imported by the route. Next.js creates it once per instance; in development, editing that module creates it again, which is fine.
• Several instances on Vercel each hold their own waits: a change wakes the devices waiting on the instance that applied it at once, and the others at their next look, within 15 seconds. To make it instant everywhere, pass a broadcast.
• Do not put the route behind a middleware that reads the body. fetch reads it.
• node:sqlite needs Node 22.13 or later. On an older Node, or on Vercel, use Postgres.