Database Transaction

What is Database transaction? 

A transaction is a unit in the database that typically contains some commands that read and write databases. 

Enterprise Relational Database management system must have ACID characteristic, because this is what make RDBMS someway different than normal database like MS.access.
ACID Property  
Atomicity = mean all the effects of the transaction must be fully successful or rolled back.For example ATM cash withdrawals, machine have to spend money and then debited from your account .
Consistency = Consistency make the transaction can not violate the rules of integrity, the database must be in state Consistent. for example the value of the currency should not be negative, the data in the field of sex must be male or female.
Isolation = means the transaction is not completed bole not be read by other transactions, locking.  
Durability = When there are transactions that have dicommit transaction log to be more resilient, will maintain a database transaction logs can still be recovered at some point

SQL Server transactions Distinguished by how they are made.
IMPLICIT - This transaction is used automatically by SQL Server to ensure the ACID properties of a command, for example update statement is modifying 10 rows, SQL server will run that 10 rows will be updated or not all of them. 

Explicit - translation that starts with BEGIN TRANSACION tsql command and terminated with a COMMIT or ROLLBACK Transaction Transaction. 

Relational and storage engines, SQL SERVER divided into two parts
Relational query engine or also called prosesor, its main function to query optimization and execution. Storage engine is responsible for managing all data IO, and IO requests for the row handles, index pages and allocations and other Other components exist BUFFER Pool that contains a variety of cache in sql server including plan cache and data cache.