Media failure: It can be happen due to the disk controller failure / disk head crash / disk to be lost.
It is ht most dangerous failure.
Natural physical disasters: It can be happen due to the natural disasters like fires, floods,
earthquakes, power failure, etc.
More about Transactions: The transaction is the unit of execution of database operations. Assuring
that transactions are executed correctly is the job of a transaction manager, a subsystem that performs
several functions, including:
1. Issuing signals to the log manager (described below) so that necessary information in the form of ―log
records‖ can be stored on the log.
2. Assuring that concurrently executing transactions do not interfere with each other in ways that introduce
(a) The transaction manager: It will send messages about actions of transactions to the log manager,
to the buffer manager about when it is possible or necessary to copy the buffer back to disk, and to
the query processor to execute the queries and other database operations that comprise the
(b) The log manager: it maintains the log. It must deal with the buffer manager, since space for the log
initially appears in main-memory buffers, and at certain times these buffers must be copied to disk.
(c) The recovery manager : When there is a crash, the recovery manager is activated. It examines the
log and uses it to repair the data, if necessary. As always, access to the disk is through the buffer
A fundamental assumption about transactions is:
Correctness Principle: If a transaction executes in the absence of any other transactions or system errors,
and it starts with the database in a consistent state, then the database is also in a consistent state when the
Operations of Transactions:
1. INPUT(X): Copy the disk block containing database element X to a memory buffer.
2. READ(X, t): Copy the database element X to the transaction’s local variable t from memory buffer then
3. WRITE (X. t): Copy the value of local variable t to database element X in a memory buffer.
4. OUTPUT (X): Copy the block containing X from its buffer to disk.