Program guide > Access data with client applications > Programming for transactions > Transaction processing overview
Transactions have many advantages for data storage and manipulation. Use transactions to protect the data grid from concurrent changes, to apply multiple changes as a concurrent unit, to replicate data and to implement a life cycle for locks on changes.
When a transaction starts, WebSphere eXtreme Scale allocates a special difference map to hold the current changes or copies of key and value pairs that the transaction uses. Typically, when a key and value pair is accessed, the value is copied before the application receives the value. The difference map tracks all changes for operations such as insert, update, get, remove, and so on. Keys are not copied because they are assumed to be immutable. If an ObjectTransformer object is specified, then this object is used for copying the value. If the transaction is using optimistic locking, then before images of the values are also tracked for comparison when the transaction commits.
If a transaction is rolled back, then the difference map information is discarded, and locks on entries are released. When a transaction commits, the changes are applied to the maps and locks are released. If optimistic locking is being used, then eXtreme Scale compares the before image versions of the values with the values that are in the map. These values must match for the transaction to commit. This comparison enables a multiple version locking scheme, but at a cost of two copies being made when the transaction accesses the entry. All values are copied again and the new copy is stored in the map. WebSphere eXtreme Scale performs this copy to protect itself against the application changing the application reference to the value after a commit.
You can avoid using several copies of the information. The application can save a copy by using pessimistic locking instead of optimistic locking as the cost of limiting concurrency. The copy of the value at commit time can also be avoided if the application agrees not to change a value after a commit.
Advantages of transactionsUse transactions for the following reasons:
By using transactions, you can:
- Roll back changes if an exception occurs or business logic needs to undo state changes.
- To apply multiple changes as an atomic unit at commit time.
- Hold and release locks on data to apply multiple changes as an atomic unit at commit time.
- Protect a thread from concurrent changes.
- Implement a life cycle for locks on changes.
- Produce an atomic unit of replication.
Transaction sizeLarger transactions are more efficient, especially for replication. However, larger transactions can adversely impact concurrency because the locks on entries are held for a longer period of time. If you use larger transactions, you can increase replication performance. This performance increase is important when you are pre-loading a Map. Experiment with different batch sizes to determine what works best for the scenario.
Larger transactions also help with loaders. If a loader is being used that can perform SQL batching, then significant performance gains are possible depending on the transaction and significant load reductions on the database side. This performance gain depends on the Loader implementation.
Automatic commit modeIf no transaction is actively started, then when an application interacts with an ObjectMap object, an automatic begin and commit operation is done on behalf of the application. This automatic begin and commit operation works, but prevents rollback and locking from working effectively. Synchronous replication speed is impacted because of the very small transaction size. If you are using an entity manager application, then do not use automatic commit mode because objects that are looked up with the EntityManager.find method immediately become unmanaged on the method return and become unusable.
External transaction coordinatorsTypically, transactions begin with the session.begin method and end with the session.commit method. However, when eXtreme Scale is embedded, the transactions might be started and ended by an external transaction coordinator. If you are using an external transaction coordinator, you do not need to call the session.begin method and end with the session.commit method. If you are using WAS, you can use the WebSphereTranscationCallback plug-in.
Parent topic:Transaction processing overview
Map entry locking
Single-partition and cross-data-grid transactions