Assaf Arkin arkin@exoffice.com

# Transaction And Locking Modes

## The JDO Model

In order to understand how the JDO transaction model affects applications performance and transactional integrity, you must first understand the semantics of the Java Data Objects. Java Data Objects are objects loaded from and stored to the database, but are disassociated from the database itself. That is, once an object has been loaded into memory, changes to the object in memory are not reflect in the database until the transaction commits.

The following table shows a sample code and the state of the JDO object and the relevant database field:

JDO objects

Code | Object value | Record value
--- | --- | ---
`results = oql.execute();` | N/A | X
`obj = results.next();` | X | X
`obj.setValue( Y );` | Y | X
`db.commit();` | Y | Y

Concurrency conflicts do not occur when an object is changed in memory, but rather when the transaction commits and changes are saved back to the database. No changes are saved if the transaction rolls back.

Conflicts could occur if two threads attempted to modify the same object, or the same thread receives two objects associated with the same database record and performs different changes to each object. Castor solves these issues through a transaction-object-identity association.

When the same transaction attempts to query the same database record twice, (e.g. as the result of two different queries) the same object is returned, assuring that different changes will be synchronized through the same object.

When two transactions attempt to query the same database record, each transaction receives a different object mapping to the same record, assuring that changes done in one transaction are not visible to the other transactions. Changes should only become visible when the transaction commits and all its changes are made durable.

This approach places the responsibility of synchronization and deadlock detection on the Castor persistence engine, easing the life of the developer.

## Locking Modes

Concurrent access requires use of locking to synchronize two transactions attempting to work with the same object. The locking mechanism has to take into account several possible use of objects, as well as help minimize database access through caching and is-modified checking.

Locking modes are declared in the class element of the XML mapping on a per class basis.

### Access mode: Shared

The shared mode is the default for all objects, unless otherwise specified in the mapping file. Shared mode allows two transactions to read the same record at the same time. Each transaction will get it's own view of the record as a separate object, to prevent in-memory changes from conflicting with each other. However, the values loaded from the database are the same for both transactions.

When transactions query different objects or query the same objects but for read-only purposes, shared access provides the most efficient means of access in terms of performance, utilizing record caching and eliminating lock contention. For example, when each transaction retrieves a different Customer record and all transactions retrieved the same set of Department records but hardly ever change them, both Customer and Department should be declared as having a shared lock.

However, when two transactions attempt to access the same object, modify it, and commit the changes, a concurrency conflict will occur. Some concurrency conflicts can lead to one of the transactions aborting. For example, if two transactions happen to load the same Account object with a balance of X, one adds 50 and the other adds 60, if both were allowed to commit the changes the new account balance will be either X+50 or X+60, but not X+110.

In the above case either exclusive or db-locked modes should be used to reduce potential conflicts. However exclusive and db-locked modes may cause the application to perform slower due to lock contention and should not be used as the general case. In rare cases conflicts may occur where shared locks are the preferred choice, e.g. when two transactions attempt to modify the same Department object, or somehow get hold of the same Customer records.

Castor detects such conflicts as they occur and uses two mechanisms to deal with them: write locks and deadlock detection. When a transaction commits Castor first tries to determine whether the object has been modified from it's known state (i.e. during query). If the object has not been modified, Castor will not attempt to store the object to the database. If the object has been modified, Castor acquires a write lock on the object preventing other transactions from accessing the object until the current transaction completes storing all changes to the database. A write lock prevents other transactions from acquiring either a write or read lock, preventing them from accidentally loading a dirty image of the object.

If the second transaction has a read lock on the object, the current transaction will block until the second transaction releases the lock, either by rolling back or by not modifying its object. If the other transaction modifies the object and attempts to store it, a deadlock occurs. If both transactions have a read lock, both require a write lock in order to proceed and neither can proceed until the other terminates.

Castor detects such deadlock occurences and causes the second transaction to rollback, allowing the first transaction to commit properly. Such conflicts do not happen often, but when they happen some transactions will fail. The application developer should either be aware of the possibility of failing, or choose to use a more severe lock type.

Conflicts occur not just from other Castor transactions, but also from direct database access. Consider another application modifying the exact same record through direct JDBC access, or a remote server connecting to the same database. To detect such conflicts Castor uses a dirty checking mechanism.

When an object is about to be stored, Castor compares the current values in the database records with those known when the object was originally loaded from the database. Any changes are regarded as dirty fields and cause the transaction to rollback with the proper error message.

Not all fields are necessarily sensitive to dirty checking. The balance in a bank account is a sensitive field, but the date of the last transaction might not be. Since the date does not depend on the original value of the account, but on the last modification to it, dirty checking can be avoided.

A field marked with dirty="ignore" will not take part in dirty checking. Furthermore, modifications to such a field will not require a write lock on the object, further improving throughput. Marking fields as non-dirty should be done with extreme care.

### Access mode: Exclusive

The exclusive mode assures that no two transactions can use the same record at the same time. Exclusive mode works by acquiring a write lock in memory and synchronizing transactions through a lock mechanism with configured timeout.

Using in-memory locks, exclusive access provides some transaction synchronization that is efficient in terms of performance and increases the chance of a commit being successful. It does not, however, guarantee commit. Since the lock is acquired by Castor, it can be used to synchronize Castor access, but other forms of direct database access may still modify the database record.

When a transaction obtains an object that was specified as exclusive access in the mapping file or when performing a query, Castor will always obtain a write lock on that object. The write lock will prevent a second transaction from being able to access the object either for read or write, until the current transaction commits. If the object is already being accessed by another transaction, the current transaction will block until the other transaction commits and release the lock.

It is possible to upgrade from a shared to an exclusive lock by calling the org.exolab.castor.jdo.Database.lock(java.lang.Object) method. This method can be used with shared objects when the application wants to assure that other transactions will be blocked and changes can be made to the current object.

Because direct database access can modify the same record as represented by an exclusive locked object, Castor uses dirty checking when updating the database. Dirty checking does not have a severe affect on performance, and can be disabled by marking all the fields of the object with dirty="ignore".

To reduce the possibility of dirty reads, Castor will always synchronize exclusive locked objects with the database at the beginning of a transaction. That is, when an object is loaded the first time in a transaction with an exclusive lock, Castor will retrieve a fresh copy of the object from the database. Castor will not, however, refresh the object if the lock is upgraded in the middle of a transaction using the lock method.

Exclusive mode does cause lock contention and can have an affect on application performance when multiple transactions attempt to access the same record. However, when used smartly with on a small set of objects it can help reduce the possibility of concurrency conflicts. It can also be used to force an object to be loaded from the database and the cache refreshed.

### Access mode: Database Locked

The locked mode performs optimistic locking using the underlying database engine to assure that only one transaction has access to the record. In addition to acquiring a write lock in memory, Castor performs a query with a special SQL construct (FOR UPDATE in Oracle, HOLDLOCK in Sybase) to guarantee access by one transaction.

In the event that the same database record will be accessed directly through JDBC, stored procedure, or a second machine using Castor, the only way to achieve object locking is through the database layer. However, such write locks should be cooperative, that is, other forms of database access should attempt to use the same locking mechanism.

In some isolation levels, when Castor acquires a write lock on the database it will prevent other applications from accessing the same record until the Castor transaction commits. However, certain isolation levels allow other applications to obtain a dirty image of the record.

Write locks on the database have a severe impact on application performance. They incur overhead in the database manager, and increase lock contention. It is recommended to use database locks with care, pay extra attention to the isolation level being used, and follow good practices recommended by the database vendor with regards to such locks.

In the future long transaction will be supported. Long transactions rely on the dirty checking mechanism and only hold connections open for as long as they are required for queries. Long transactions cannot be used with database locking.

Locked mode must be specified for the object in the mapping file or when conducting the query. It is not possible to upgrade to a locked lock in the middle of a transaction.

Objects loaded in this mode are always synchronized with the database, that is, they will never be obtained from the cache and always re-loaded for each new transaction.

### Read-only Queries

When a query is performed in read-only mode or no mode is specified and the object is marked as read-only in the database, Castor will return a transient object. The returned object will not be locked and will not participate in the transaction commit/rollback.

When the same object is queried twice in a transaction as read-only, Castor will return two separate objects, allowing the caller to modify one object without altering the other. Castor will utilize the cache and only perform one load from the database to the cache.

Read-only access is recommended only when the object is intentionally queried as read-only and changes to the object should not be reflected in the database. If the object will not be modified, or modifications will be stored in the database, it is recommended to use the shared mode. Shared mode allows the same object to be returned twice in the same transaction.

## Visibility of Changes

The visibility of changes occuring in one transaction to other transactions depends upon the transaction isolation level specified for the database connection. Whether or not the changes are visible in the current transaction depends upon the operation being done.

There are four types of changes, the following table summarizes the affect of each change in one transaction on other queries in that transaction as well as other transactions.

Changes

Type of change | Same transaction | Other transaction
--- | --- | ---
Create new object | New object is visible and will be returned from a query | New object might be visible, depending on isolation level, but access is blocked until the first transaction completes
Delete existing object | Object is no longer visible and will not be returned from a query | Object might not be visible, depending on isolation level, but access is blocked until the first transaction completes
setXXX() | Change is visible in object itself | Change is not visible, object is accessible depending on lock
update() | Change is visible in object itself | Change might be visible, depending on isolation level, object might be accessible depending on lock