Should synchronization mechanisms be used for coroutines if some run in IO thread and others in the main thread when accessing a shared variable for increment?

Yes, if multiple coroutines (regardless of whether they run in different threads, such as IO thread and main thread) access a shared variable for incrementing, synchronization mechanisms or atomic operations must be used.

The reason is that an increment (for example, counter++) is not an atomic operation — it involves reading the value, increasing it, and writing it back. Without synchronization, race conditions can occur where multiple coroutines read and write the value simultaneously, leading to lost updates.

In Kotlin, for such cases, you can use:

• AtomicInteger from java.util.concurrent.atomic
• Mutex from kotlinx.coroutines.sync

Example with AtomicInteger:

val counter = AtomicInteger(0)

launch(Dispatchers.IO) {
    counter.incrementAndGet()
}

launch(Dispatchers.Main) {
    counter.incrementAndGet()
}

Thus, even when running in different threads, the increment will be correct and thread-safe.