Page 191 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Database Management Systems/Managing Database

Notes or more blocks. This assumption is realistic for most data-processing applications, such as our
banking example.
Transactions input information from the disk to main memory, and then output the information
back onto the disk. The input and output operations are done in block units. The blocks residing
on the disk are referred to as physical blocks; the blocks residing temporarily in main memory
are referred to as buffer blocks. The area of memory where blocks reside temporarily is called
the disk buffer.

Block movements between disk and main memory are initiated through the following two
operations:
1. input(B) transfers the physical block B to main memory.

2. output(B) transfers the buffer block B to the disk, and replaces the appropriate physical
block there.
Each transaction T has a private work area in which copies of all the data items accessed and
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updated by T are kept. The system creates this work area when the transaction is initiated; the
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system removes it when the transaction either commits or aborts. Each data item X kept in the
work area of transaction T is denoted by X . Transaction T interacts with the database system by
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transferring data to and from its work area to the system buffer. We transfer data by these two
operations:
1. read(X) assigns the value of data item X to the local variable X . It executes this operation
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as follows:
(a) If block B x on which X resides is not in main memory, it issues input(B x).
(b) It assigns to X the value of X from the buffer block.
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2. write(X) assigns the value of local variable X to data item X in the buffer block. It executes
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this operation as follows:
(a) If block B x on which X resides is not in main memory, it issues input(Bx)
(b) It assigns the value of X to X in buffer B x.
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Notes Both operations may require the transfer of a block from disk to main memory.
They do not, however, specifically require the transfer of a block from main memory to
disk.

A buffer block is eventually written out to the disk either because the buffer manager needs the
memory space for other purposes or because the database system wishes to reflect the change to
B on the disk. We shall say that the database system performs a force-output of buffer B if it
issues an output(B).
When a transaction needs to access a data item X for the first time, it must execute read (X). The
system then performs all updates to X on X . After the transaction accesses X for the final time, it
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must execute write(X) to reflect the change to X in the database itself.
The output(B x) operation for the buffer block B x on which X resides ,does not need to take effect
immediately after write(X) is executed, since the block Bx may contain other data items that are
still being accessed. Thus, the actual output may take place later. Notice that, if the system
crashes after the write(X) operation was executed but before output(Bx) was executed, the new
value of X is never written to disk and, thus, is lost.

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