What is Race Condition?

Race conditions happen when multiple threads, processes, or requests access shared resources concurrently and the outcome depends on execution order. Classic examples include two threads incrementing a counter (lost updates), checking-then-acting on a file (TOCTOU), and double-spending in financial systems. Prevention techniques include mutexes/locks (serialize access), atomic operations, database transactions with proper isolation levels, optimistic locking (version numbers), and lock-free data structures. In web applications, race conditions can cause duplicate charges, inventory overselling, and data corruption. They are notoriously difficult to reproduce and debug because they depend on timing.