How to Prevent Double Bookings in Next.js (Race Conditions & DB Constraints)

#nextjs #postgres #webdev #programming

Most booking tutorials show you how to build a pretty calendar and save a reservation. Then they skip the one problem that actually matters in production.

What happens when two people book the same slot at the same time?

👉 Full guide here

Why "check then save" is a trap

The obvious approach:

const existing = await prisma.booking.findFirst({
  where: { resourceId, startTime },
});
if (existing) throw new Error("Slot taken");
await prisma.booking.create({ data: { resourceId, startTime } });

This works - until two requests arrive at the same moment. Both pass the check. Both create a booking. Race condition. Invisible in testing, guaranteed in production.

The fix isn't better app code — it's letting the database enforce the rule atomically.

Fix 1: Unique constraint (fixed slots)

model Booking {
  @@unique([resourceId, startTime])
}

Try to insert and catch the error:

try {
  await prisma.booking.create({ data: { resourceId, startTime } });
} catch (e) {
  if (e.code === "P2002") {
    return Response.json({ error: "Slot just taken" }, { status: 409 });
  }
}

Whoever commits first wins. Second request gets a clean 409. No race condition — uniqueness check and insert are one atomic op.

Fix 2: Overlapping bookings (the hard case)

Unique constraints only catch identical start times. A 60-min booking from 2:00 can still conflict with one from 2:30.

PostgreSQL exclusion constraint handles this:

CREATE EXTENSION IF NOT EXISTS btree_gist;

ALTER TABLE "Booking"
ADD CONSTRAINT no_overlapping_bookings
EXCLUDE USING gist (
  resource_id WITH =,
  tstzrange(start_time, end_time) WITH &&
);

Database now rejects any booking whose time range overlaps an existing one — no matter how many concurrent requests hit it.

Fix 3: Transactions for multi-step bookings

await prisma.$transaction(async (tx) => {
  await tx.booking.create({ data: { resourceId, startTime, endTime } });
  await tx.credit.update({
    where: { userId },
    data: { balance: { decrement: 1 } },
  });
});