Shadow paging in DBMS

This is the method where all the transactions are executed in the primary memory or the shadow copy of database. Once all the transactions completely executed, it will be updated to the database. Hence, if there is any failure in the middle of transaction, it will not be reflected in the database. Database will be updated after all the transaction is complete.

A database pointer will be always pointing to the consistent copy of the database, and copy of the database is used by transactions to update. Once all the transactions are complete, the DB pointer is modified to point to new copy of DB, and old copy is deleted. If there is any failure during the transaction, the pointer will be still pointing to old copy of database, and shadow database will be deleted. If the transactions are complete then the pointer is changed to point to shadow DB, and old DB is deleted.

As we can see in above diagram, the DB pointer is always pointing to consistent and stable database. This mechanism assumes that there will not be any disk failure and only one transaction executing at a time so that the shadow DB can hold the data for that transaction. It is useful if the DB is comparatively small because shadow DB consumes same memory space as the actual DB. Hence it is not efficient for huge DBs. In addition, it cannot handle concurrent execution of transactions. It is suitable for one transaction at a time.


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