What is ACID and Why is it Important for Database Transactions?
Database transactions are operations that modify the data
stored in a database i.e. CRUD. Transactions are required for the reliability,
consistency and accuracy of the data in the database. But, not all transactions
are created equal. Some transactions may have different requirements and
expectations than others, depending on the nature and purpose of the data and
the application.
One way to classify and evaluate transactions is by using
the ACID model. ACID stands for Atomicity, Consistency, Isolation and
Durability. These are four properties that guarantee that a transaction is
executed in a safe and correct manner, regardless of any errors, failures or
concurrency issues that may occur. We will try to explain what each of these
properties means, why they are important, and how they can be achieved in a
database system.
Atomicity
Atomicity means that a transaction is either executed
completely or not at all. There is no state in between. You don't see a partial
completion of a transaction. For example, if you want to transfer money from
one bank account to another, expectation is, either both accounts are updated
with the correct amounts, or none of them are changed at all. You don't want to
end up with a situation where the money is deducted from one account but not
added to the other, or vice versa.
Atomicity prevents partial updates or data loss in case of
failures or errors. If something goes wrong during the execution of a
transaction, such as a power outage, a network failure, or an application bug,
the database system should be able to detect it and roll back the transaction
to its original state before it started. This way, the database remains
consistent and no data is corrupted or lost.
To achieve atomicity, database systems use various
techniques, such as logging, locking, checkpoints and rollback segments. These
techniques allow the database system to keep track of the changes made by a
transaction, lock the resources involved in the transaction, save the state of
the database before the transaction begins, and undo the changes if the
transaction fails.
Consistency
Consistency means that a transaction preserves the validity
and integrity of the database state. A database has certain rules and
constraints that define what constitutes a valid state. For example, a database
may have primary keys that uniquely identify each record, foreign keys that
link records from different tables, check constraints that limit the range or
format of values in a column, or business rules that enforce some logic or calculation
on the data.
Consistency prevents violations of constraints, rules or
business logic in the database. If a transaction tries to insert, update or
delete data that would break any of these rules or constraints, the transaction
should fail and abort. The database should not allow any invalid or
inconsistent data to be stored or retrieved.
To achieve consistency, database systems use various techniques, such as validation checks, triggers and stored procedures. These techniques allow the database system to verify and enforce the rules and constraints on the data before and after a transaction is executed.
Isolation
Isolation means that concurrent transactions do not
interfere with each other. Transactions are concurrent when they are executed
at or near the same time by different users or applications. For example, if
two customers try to book the same flight seat or hotel room at the same time,
they are executing concurrent transactions.
Isolation prevents anomalies such as dirty reads,
non-repeatable reads or phantom reads in the database. A dirty read occurs when
a transaction reads data that has been modified but not committed by another
transaction. A non-repeatable read occurs when a transaction reads the same
data twice but gets different results because another transaction has modified
and committed the data in between. A phantom read occurs when a transaction
reads a set of data that matches some criteria but gets different results
because another transaction has inserted or deleted some records that match or
do not match the criteria in between.
To achieve isolation, database systems use various techniques, such as locking, timestamps and multi-version concurrency control (MVCC). These techniques allow the database system to control and coordinate the access and modification of data by concurrent transactions.
Durability
Durability means that the effects of a committed transaction
are permanent and persistent in the database. A committed transaction is one
that has been successfully executed and verified by the database system. Once a
transaction is committed, it should not be reversed or undone by any subsequent
event.
Durability prevents data loss or corruption in case of power
failures, system crashes or restarts. If any of these events happen after a
transaction is committed, the database system should be able to recover and
restore the data to its latest committed state.
To achieve durability, database systems use various
techniques, such as write-ahead logging (WAL), checkpoints and backups. These
techniques allow the database system to record and save the changes made by a
transaction to persistent storage devices (such as disks), periodically
synchronize the data in memory and on disk, and create copies of the data for
recovery purposes.
Conclusion
ACID properties are essential for ensuring the reliability,
consistency and accuracy of database transactions. They provide a framework for
designing and evaluating database systems and applications that deal with
sensitive and critical data. By following the ACID model, database systems and
applications can avoid many common problems and errors that may compromise the
quality and integrity of the data.
Scalable Alternative to ACID
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