Write short notes on Database Management Systems 10CS54 VTU notes Jan 2013Jan2014June 2016

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Write short notes on Database Management Systems - 10CS54 VTU notes (Jan 2013/Jan2014/June 2016)

i) Write a head log protocol: 
i) Transaction support in SQL: 
iii) Two phase locking protocol:
iv) Time stamp Ordering

Asked by:anonymousguy7799

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i) Write a head log protocol:
When in-place update (immediate or deferred) is used then log is necessary for recovery and it must be available to recovery manager. This is achieved by Write-Ahead Logging (WAL) protocol. WAL states that For Undo: Before a data item’s AFIM is flushed to the database disk (overwriting the BFIM) its BFIM must be written to the log and the log must be saved on a stable store (log disk). For Redo: Before a transaction executes its commit operation, all its AFIMs must be written to the log and the log must be saved on a stable store. 
i) Transaction support in SQL: 
A single SQL statement is always considered to be atomic. Either the statement completes execution without error or it fails and leaves the database unchanged. With SQL, there is no explicit Begin Transaction statement. Transaction initiation is done implicitly when particular SQL statements are encountered. Every transaction must have an explicit end statement, which is either a COMMIT or ROLLBACK. Characteristics specified by a SET TRANSACTION statement in SQL2: Access mode: READ ONLY or READ WRITE. The default is READ WRITE unless the isolation level of READ UNCOMITTED is specified, in which case READ ONLY is assumed. Diagnostic size n, specifies an integer value n, indicating the number of conditions that can be held simultaneously in the diagnostic area. Isolation level , where can be READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ or SERIALIZABLE. The default is SERIALIZABLE. With SERIALIZABLE: the interleaved execution of transactions .
iii) Two phase locking protocol: Two-Phase Locking Techniques: Essential components Two locks modes (a) shared (read) and (b) exclusive (write). Shared mode: shared lock (X). More than one transaction can apply share lock on X for reading its value but no write lock can be applied on X by any other transaction. Exclusive mode: Write lock (X). Only one write lock on X can exist at any time and no shared lock can be applied by any other transaction on X.
iv) Time stamp Ordering: Timestamp T1 VTUlive.com 70 A monotonically increasing variable (integer) indicating the age of an operation or a transaction. A larger timestamp value indicates a more recent event or operation. Timestamp based algorithm uses timestamp to serialize the execution of concurrent transactions 
Basic Timestamp Ordering 1. Transaction T issues a write_item(X) operation: a. If read_TS(X) > TS(T) or if write_TS(X) > TS(T), then an younger transaction has already read the data item so abort and roll-back T and reject the operation. If the condition in part (a) does not exist, then execute write_item(X) of T and set write_TS(X) to TS(T). 2. Transaction T issues a read_item(X) operation: If write_TS(X) > TS(T), then an younger transaction has already written to the data item so abort and roll-back T and reject the operation. If write_TS(X) ≤ TS(T), then execute read_item(X) of T and set read_TS(X) to the larger of TS(T) and the current read_TS(X) 

Answerd By:Nikhil-bharadwaj

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