Is synchronization necessary when 500 coroutines simultaneously modify a variable on Main and IO dispatchers?

Yes, synchronization is necessary. If 500 coroutines simultaneously modify the same variable, running on different dispatchers (Main and IO), race conditions and incorrect results can occur without synchronization.

In Kotlin, to protect shared resources, you can use:

• Mutex — for locking critical sections
• Atomic variables from the kotlinx.atomicfu package or java.util.concurrent.atomic
• Special constructs, such as withContext(Dispatchers.Default) with sequential access

Example with Mutex:

val mutex = Mutex()
var counter = 0

suspend fun increment() {
    mutex.withLock {
        counter++
    }
}

Without such protection, the variable can be corrupted when accessed simultaneously.