fun main() {
    runBlocking {
       val foo1 = withContext(IO) {
           delay(100)
           "foo1"
       }

       val fooDeferred = async(IO) {
            delay(100)
            "foo2"
       }

       val foo2 = fooDeferred.await()
       print("$foo1 and $foo2")
    }
}

^ can someone tell me, what the different here between

async(IO)

and

withContext(IO)

? only that async is deferred?

Pretty much. You can think of

withContext

as an optimization, for when you immediately await an async.

withContext is running blocking... look at example https://goo.gl/R8E1Dq

No I would say that analogy withContext as optimization for await/async is wrong way to think about this, because with context not create any coroutines, it's just a way to update context

I agree. That's why I clarified.