Concurrency Security

Patterns for preventing race conditions, double-execution, and state corruption in concurrent systems.

TOCTOU Prevention

Time-of-Check to Time-of-Use: the gap between reading state and acting on it.

// WRONG: check then act - another process can change state between lines
const balance = await db.accounts.findUnique({ where: { id } })
if (balance.amount >= amount) {
  await db.accounts.update({ where: { id }, data: { amount: balance.amount - amount } })
}