Thee ACID properties of transactions in database management systems. These properties ensure that database transactions are processed reliably and maintain the integrity of the data. Here's a detailed explanation of each property:
Atomicity
Atomicity ensures that a transaction is
treated as a single, indivisible unit. This means that all the operations
within a transaction must either be completed successfully or not executed at
all. If any part of the transaction fails, the entire transaction is rolled
back, and the database remains unchanged. This property is crucial for
maintaining data integrity, especially in the event of errors or failures.
Example: Imagine transferring money between
two bank accounts. The transaction involves debiting the amount from one
account and crediting it to another. If the debit operation succeeds but the
credit operation fails, the transaction is rolled back, and no changes are made
to either account.
Consistency
Consistency ensures that a transaction takes
the database from one valid state to another valid state. This means that any
data written to the database must conform to all predefined rules, constraints,
and triggers. A consistent transaction leaves the database in a valid state,
ensuring that all integrity constraints are maintained.
Example: Consider a database with a
constraint that the total amount of money in all accounts must remain constant.
If a transaction violates this constraint by incorrectly updating account
balances, it is considered inconsistent. The transaction must be rolled back to
maintain the integrity of the database.
Isolation
Isolation ensures that the operations of one
transaction do not interfere with the operations of other concurrent
transactions. This property is important for maintaining the integrity of data
in a multi-user environment. Isolation prevents issues such as dirty reads,
non-repeatable reads, and phantom reads.
- Dirty Read: Occurs when a transaction
reads data that has been modified by another transaction but not yet committed.
- Non-Repeatable Read: Occurs when a transaction
reads the same data twice and gets different results because another
transaction has modified the data between the two reads.
- Phantom Read: Occurs when a transaction
reads a set of rows that satisfy a condition and finds additional rows
that satisfy the condition in a subsequent read because another
transaction has inserted new rows.
Example: Imagine two transactions: one
updating a customer's balance and another reading the same balance. Isolation ensures
that the reading transaction does not see the intermediate state of the update,
preventing inconsistencies.
Durability
Durability ensures that once a transaction is
committed, the changes it made to the database are permanent, even in the event
of a system failure. This property is typically achieved through the use of
transaction logs, which record all changes made by a transaction. In the event
of a crash, the recovery manager uses these logs to redo or undo transactions
as needed.
Example: After a successful purchase
transaction, the database updates the inventory and generates an order
confirmation. Even if the system crashes immediately after the transaction, the
changes to the inventory and the order confirmation must be preserved.
Summary of ACID Properties
- Atomicity: All-or-nothing execution of
transactions.
- Consistency: Transitions from one valid
state to another while maintaining integrity constraints. Eg. If stock
available numbers are decremented from tblproduct table, then, there has
to be a related entry in tblproductSales table. The inventory can’t just
disappear.
- Isolation: Concurrent transactions do
not interfere with each other.
- Durability: Committed transactions are
permanent and recoverable. If a system error or power failure occurs before
a set of commands is complete, those commands are undone and the data is
restored to its original state once the system begins running again
These
ACID properties are fundamental to transaction processing in database
management systems, ensuring that the database remains accurate, consistent,
and reliable under all circumstances.
