Explain how atomicity durability is implemented in DBMS using shadow copy scheme?

Answer 1

The recovery-management component of a database system can support atomicity

and durability by a variety of schemes. We first consider a simple, but extremely inefficient,

scheme called the shadow copy scheme. This scheme, which is based on

making copies of the database, called shadow copies, assumes that only one transaction

is active at a time. The scheme also assumes that the database is simply a file on

disk. A pointer called db-pointer is maintained on disk; it points to the current copy

of the database.

In the shadow-copy scheme, a transaction that wants to update the database first

creates a complete copy of the database. All updates are done on the new database

copy, leaving the original copy, the shadow copy, untouched. If at any point the transaction

has to be aborted, the system merely deletes the new copy. The old copy of the

database has not been affected.

If the transaction completes, it is committed as follows. First, the operating system

is asked to make sure that all pages of the new copy of the database have been written

out to disk. (Unix systems use the flush command for this purpose.) After the operating

system has written all the pages to disk, the database system updates the pointer

db-pointer to point to the new copy of the database; the new copy then becomes

the current copy of the database. The old copy of the database is then deleted.