3.14 Concurrency Control
All the examples so far have been single-user applications (only one transaction was accessing the datastore at a time). It is, of course, possible for multiple applications to be running concurrently. The JDO specification does not make any specific statements about concurrency control, because it is a feature of the underlying datastore rather than of JDO itself.
JDO does define the concept of a datastore transaction and says that access to the datastore should be done within the context of a transaction and that these transactions should exhibit ACID properties. This allows the JDO implementation to use the underlying datastore's own concurrency control mechanisms, which may be based on a pessimistic or optimistic locking scheme or some other approach. Because of this, JDO does not expose APIs to explicitly lock persistent objects. If locks are required by the underlying datastore to enforce concurrency control, then it is the responsibility of the JDO implementation to manage them implicitly.
3.14.1 ACID transactions
ACID stands for Atomic, Consistent, Isolated, and Durable, and it defines standard properties for a transaction:
Atomic: Upon commit, either all changes are in the datastore or none of them are in the datastore. This means that, should something fail, arbitrary changes aren't left behind in the datastore.
Consistent: Prior to and after commit, all data is consistent as far as the datastore is concerned. This means that all data accessed during a transaction is initially consistent and that, after commit, all changes leave the data consistent. Consistent means that the data does not violate any datastore constraints or invariants.
Isolated: Prior to commit, any changes made are not visible to other transactions and vice versa. This means that one transaction is not able to see changes made by another transaction; it can see only committed changes. Many datastores support varying levels of isolation that trade off strict transaction isolation against improved concurrency.
Durable: After commit, any changes are guaranteed to persist even in the event of failure. This means that when a transaction ends, any changes are in the datastore.
Different transactional datastores use different mechanisms to enforce ACID transactions. Therefore, when using JDO, when and how concurrency conflicts are detected and resolved is dependent upon which underlying datastore is being used. All that can be guaranteed is that a transaction is ACID in nature.
3.14.2 Optimistic transactions
JDO provides optional support for optimistic transactions. Optimistic transactions are useful for applications that have long running transactions where it is undesirable to hold datastore resources (such as locks) for an extended period of time.
With optimistic transactions, all concurrency control is deferred until the end of the transaction. See Chapter 5 for more details on optimistic transactions.