TransferState: Who guarantees that data is already stored in state?

I'm doing SSR with TransferState and wondering who guarantees that when we do

http.get(...).subscribe(data => {
  transferState.set(DATA_KEY, data)
})

data will be stored in transferState? Because http.get is async operation and content could be generate and provided to client without this data.

from TransferState: Who guarantees that data is already stored in state?