For example, nothing about relational database theory requires that the system support transactions, but most commercial relational systems do. Concurrency means that multiple users can operate on the database at the same time. Transactions permit users to make multiple changes appear at once. For example, a transfer of funds between bank accounts needs to be a transaction because the balance in one account is reduced and the balance in the other increases. Obviously, both the customer and the bank are best served if both operations happen at the same instant.
Transactions have well-defined properties in database systems.
They are atomic , so that the changes happen all at once or not at all. They are consistent , so that the database is in a legal state when the transaction begins and when it ends. They are typically isolated , which means that any other users in the database cannot interfere with them while they are in progress. And they are durable , so that if the system or application crashes after a transaction finishes, the changes are not lost. Together, the properties of atomicity , consistency , isolation , and durability are known as the ACID properties.