What is a pessimistic lock?

Pessimistic locking is an approach to managing concurrent access to data, where a resource is locked immediately at the start of an operation to prevent conflicts. That is, when a transaction wants to modify data, it acquires a lock, and other transactions wait until the lock is released.

In Go, this can be implemented using mutexes (sync.Mutex), which lock access to shared data:

var mu sync.Mutex

func updateData() {
    mu.Lock()   // pessimistic lock
    defer mu.Unlock()
    // critical section
}

This approach guarantees that only one goroutine modifies data at a time, but it can lead to reduced parallelism and blocking.