JBoss.org Community Documentation
For the purpose of this discussion, we can define a transaction as a unit of work containing one or more operations involving one or more shared resources having ACID properties. ACID is an acronym for atomicity, consistency, isolation and durability, the four important properties of transactions. The meanings of these terms is:
Atomicity : A transaction must be atomic. This means that either all the work done in the transaction must be performed, or none of it must be performed. Doing part of a transaction is not allowed.
Consistency : When a transaction is completed, the system must be in a stable and consistent condition.
Isolation : Different transactions must be isolated from each other. This means that the partial work done in one transaction is not visible to other transactions until the transaction is committed, and that each process in a multi-user system can be programmed as if it was the only process accessing the system.
Durability : The changes made during a transaction are made persistent when it is committed. When a transaction is committed, its changes will not be lost, even if the server crashes afterwards.
To illustrate these concepts, consider a simple banking account application. The banking application has a database with a number of accounts. The sum of the amounts of all accounts must always be 0. An amount of money M is moved from account A to account B by subtracting M from account A and adding M to account B. This operation must be done in a transaction, and all four ACID properties are important.
The atomicity property means that both the withdrawal and deposit is performed as an indivisible unit. If, for some reason, both cannot be done nothing will be done.
The consistency property means that after the transaction, the sum of the amounts of all accounts must still be 0.
The isolation property is important when more than one bank clerk uses the system at the same time. A withdrawal or deposit could be implemented as a three-step process: First the amount of the account is read from the database; then something is subtracted from or added to the amount read from the database; and at last the new amount is written to the database. Without transaction isolation several bad things could happen. For example, if two processes read the amount of account A at the same time, and each independently added or subtracted something before writing the new amount to the database, the first change would be incorrectly overwritten by the last.
The durability property is also important. If a money transfer transaction is committed, the bank must trust that some subsequent failure cannot undo the money transfer.