DBMS Transaction – Learn DBMS in simple and easy steps starting from its overview, Architecture, data models, data schemas, data independence, ED Diagram, Generalization, Aggregation, Codd’s Rules, Relational Data Model, Relational Algebra, Database Design, Normalization, Database Joins, Database Storage, Database File System, Acid properties in dbms pdf, Hashing, Transaction, Concurrency Control, Deadlock, Backup and Recovery. DBMS, Tutorial, Network Architecture, Overview, Architecture, data models, data schemas, data independence, ED Diagram, Generalization, Aggregation, Codd’s Rules, Relational Data Model, Relational Algebra, Database Design, Normalization, Database Joins, Database Storage, Database File System, Indexing, Hashing, Transaction, Concurrency Control, Deadlock, Backup and Recovery. A transaction can be defined as a group of tasks. A single task is the minimum processing unit which cannot be divided further.
Let’s take an example of a simple transaction. Suppose a bank employee transfers Rs 500 from A’s account to B’s account. This very simple and small transaction involves several low-level tasks. A transaction is a very small unit of a program and it may contain several lowlevel tasks. This property states that a transaction must be treated as an atomic unit, that is, either all of its operations are executed or none.
There must be no state in a database where a transaction is left partially completed. The database must remain in a consistent state after any transaction. No transaction should have any adverse effect on the data residing in the database. If the database was in a consistent state before the execution of a transaction, it must remain consistent after the execution of the transaction as well.
The database should be durable enough to hold all its latest updates even if the system fails or restarts. If a transaction updates a chunk of data in a database and commits, then the database will hold the modified data. If a transaction commits but the system fails before the data could be written on to the disk, then that data will be updated once the system springs back into action. In a database system where more than one transaction are being executed simultaneously and in parallel, the property of isolation states that all the transactions will be carried out and executed as if it is the only transaction in the system. No transaction will affect the existence of any other transaction. When multiple transactions are being executed by the operating system in a multiprogramming environment, there are possibilities that instructions of one transactions are interleaved with some other transaction. A chronological execution sequence of a transaction is called a schedule.
It is a schedule in which transactions are aligned in such a way that one transaction is executed first. When the first transaction completes its cycle, then the next transaction is executed. Transactions are ordered one after the other. This type of schedule is called a serial schedule, as transactions are executed in a serial manner. In a multi-transaction environment, serial schedules are considered as a benchmark. The execution sequence of an instruction in a transaction cannot be changed, but two transactions can have their instructions executed in a random fashion. This ever-varying result may bring the database to an inconsistent state.
To resolve this problem, we allow parallel execution of a transaction schedule, if its transactions are either serializable or have some equivalence relation among them. If two schedules produce the same result after execution, they are said to be result equivalent. They may yield the same result for some value and different results for another set of values. That’s why this equivalence is not generally considered significant.
Two schedules would be view equivalence if the transactions in both the schedules perform similar actions in a similar manner. If T reads the initial data in S1, then it also reads the initial data in S2. If T reads the value written by J in S1, then it also reads the value written by J in S2. If T performs the final write on the data value in S1, then it also performs the final write on the data value in S2. Both belong to separate transactions.