It is important to know if a change to a data item appears only in main memory
or if it has been written to the database on disk. In a real database system, the write operation does not necessarily result in the immediate update of the data on
the disk; the write operation may be temporarily stored elsewhere and executed
on the disk later. For now, however, we shall assume that the write operation
updates the database immediately.We shall return to this subject in Chapter 16.