Race Conditions Trap

Code that reads correctly line by line can corrupt data when two copies run at once. Before writing any code that reads shared state and then writes based on what it read, ask: what happens if another process changes the state between my read and my write?

The Three Patterns

Almost every race condition is one of these:

1. Check-Then-Act (TOCTOU)

Read state, make a decision, act on it. Another process changes state between check and act.

// RACE: two requests both find no user, both create one
const existing = await db.user.findUnique({ where: { email } });
if (!existing) return await db.user.create({ data: { email } });

2. Read-Modify-Write