One of the most critical concepts in database design is ACID. ACID stands for Atomicity, Consistency, Isolation, and Durability. These are standard properties of almost all databases that ensure data consistency and reliability.
Developers often mention ACID in the context of business data processing. It's vital because ACID guarantees that data is processed reliably and accurately. ACID also ensures the lowest possible risk of corruption or data loss in case of failures, such as power outages or computer crashes. ACID principles promise that a loss of power won't ruin your data itself.
If databases were not ACID-compliant, businesses wouldn't be able to trust their numbers. A company that relies on data processing cannot afford to have its operations interrupted by a system crash. It has to be able to recover its systems as quickly as possible if they do happen, which is why you will often see large companies offering very high levels of redundancy at each stage of a data processing system. Databases that implement ACID principles ensure that these companies have reliable access to their (often precious) information.
Now, let's look at each of the four principles that ACID-compliant databases implement.
Atomicity refers to the ability of a transaction to be atomic or indivisible. In short, atomicity means that the entire transaction either completes successfully or does not take place at all. This principle prevents partial data corruption in the event of a system failure.
For example, imagine transferring money from one bank account to another. If you move more than is available in the first account, or if the second does not exist, atomicity requires that you rollback (undo) any partial transactions, so both accounts are left with funds as they were before the operation began. You don't want your funds in a weird intermediate state.
If ACID properties do not hold, there will be inconsistency in the database, which is confusing at best and devastating at worst. ACID guarantees that atomicity will prevent data inconsistency.
Consistency refers to a transaction's requirement to leave the database in a consistent state. Consistency means that all constraints on the data must be satisfied after the transaction completes. For example, if you have a table of employees with the condition that the salary must be greater than 0, then any transaction must adhere to this constraint.
Consistency also requires that any changes made to data during a transaction are visible to all other transactions. This principle prevents dirty reads, where one transaction sees data in an inconsistent state caused by another running concurrently.
Isolation ensures that the transaction results are only visible to other transactions after the isolation level has been achieved. This principle prevents dirty reads and writes, where one transaction sees data changes made by another transaction before a user commits them.
For example, imagine two users trying to change the same record in a database. If a database does not enforce isolation, one user might see the changes made by the other user before they commit them. This situation could lead to incorrect data reaching the database.
Developers usually implement isolation levels using locks, which prevent any other transactions from accessing the data until the lock has been released. Locks can be held for a short time (short lock) or for a long time (long lock).
Short locks allow more transactions to proceed concurrently but may reduce performance. Longer locks ensure that the data will not change during processing and enhance concurrency.
Durability guarantees that the database will store the results once a transaction completes. ACID guarantees durability by flushing all data changes made during a transaction to disk before it defines them as committed (i.e., makes them permanent).
If transactions do not commit, there will be inconsistency in the database, which is terrible because developers need to know that the data they are working with is correct. ACID guarantees that transactions will commit so that developers can rely on accurate data.
ACID is a standard set of properties for almost all databases. ACID ensures atomicity, consistency, isolation, and durability (no data loss). In addition, ACID provides guidelines on properly structuring transactions, preventing dirty reads and writes. ACID is vital for both developers and users to ensure data reliability. These reasons explain why almost all modern databases conform to ACID principles.