Optimistic locking what is

And of course, let me tell a couple words about the other locking strategy: pessimistic locking. Most relational databases support pessimistic locking out of the box but you can also implement it yourself in the application code. When it comes to choosing between optimistic and pessimistic locking, I would say go with the former by default.

Pessimistic locking is useful when the cost of merging simultaneous changes is high. Which is not the case in the vast majority of domains. Alright, back to the original question. So, how to combine optimistic locking and automatic retry? The short answer is: nohow. Remember that the locking mechanism is a way to ensure that all changes are taken into consideration when changing a record in the database. In other words, someone should review the new version of the record and make an informed decision as to whether they still want to submit the update.

Can we make the automatic retry in this case? The answer is: also, no. The default "last transaction wins" mode will suffice here. Sometimes, you can automate the merge process. In most cases, however, you do want the client to intervene. If any values have changed since the object was read, the update or delete fails. This comparison is only on a per table basis. If you perform an update on an object mapped to multiple tables including multiple table inheritance , only the changed tables appear in the where clause.

This allows multiple clients to modify different parts of the same row without failure. Using this policy, a delete compares only on the primary key. When you update or delete an object under SelectedFieldsLockingPolicy , the Unit of Work compares a list of selected fields in the update statement. Under most circumstances, the application handles this exception by refreshing the object and reapplying changes.

Each of these policies requires an additional field in the database to operate:. For VersionLockingPolicy , add a numeric field to the database.

For TimestampLockingPolicy , add a timestamp field to the database. OracleAS TopLink records the version as it reads an object from a table. When the client attempts to write the object, OracleAS TopLink compares the object version with the version in the table record.

If the versions match, OracleAS TopLink writes the updated object to the table and updates the version of both the table record and the object. These two version locking policies have different ways of writing the version fields back to the database:.

TimestampLockingPolicy inserts a new timestamp into the row. The timestamp is configurable to get the time from the server or the local system. For either policy, you write the value of the write lock field in either the identity map or in a writable mapping within the object.

If you store the value in the identity map, you do not require an attribute mapping for the version field. However, if the application does map the field, the mappings must be read-only to allow OracleAS TopLink to control writing the fields. If you need absolute certainty for versioning, and especially if your database does not offer fine time granularity, implement the VersionLockingPolicy.

This policy uses integers for field locking and guarantees that you recognize changes. If your database time offers a fine granularity, or if you need to know when an object was last updated, implement the TimestampLockingPolicy. Skip Headers. OracleAS TopLink works with relational databases to provide support for several types of locking policy, including: Optimistic Lock: All users have read access to the object.

No Locking: The application does not verify that data is current. Note: Most OracleAS TopLink applications use either optimistic locking or optimistic read locking, because they are the safest and most efficient of these locking strategies.

Using Optimistic Locking Optimistic locking, also known as write locking, allows unlimited read access to a given object, but allows a client to modify the object only if the object has not changed since the client last read it.

Note: Using optimistic locking by itself does not protect against having different copies of the same object existing in multiple nodes. Advantages and Disadvantages of Optimistic Locking Here are the advantages of optimistic locking: It prevents users and applications from editing stale data. It notifies users of any locking violation immediately, when updating the object. It does not require you to lock up the database resource. It prevents database deadlocks.

Optimistic Read Locking Optimistic read lock is an advanced type of optimistic lock that enables you to force lock checking on objects that are not modified by the current transaction. Notice that the thread forces a version update. When is an Object Considered Changed?

Pessimistic Locking Pessimistic locking locks objects when the transaction accesses them, before commit time, ensuring that only one client is editing the object at any given time. Pessimistic Locking and the Cache When you acquire a pessimistic lock on an object, you refresh the object in the session cache. Although this is the standard way of implementing pessimistic locking in the database, not all databases support row-level locking functionality.

The locks are placed as soon as any piece of the row is accessed, making it impossible for two or more users to update the row at the same time. Depending on the lock mode shared , exclusive , or update , other users might be able to read the data even though a lock has been placed. For more details on the lock modes, see Lock modes: shared, exclusive, and update.

Instead of locking every record every time that it is used, the system merely looks for indications that two users actually did try to update the same record at the same time. For example, if User1 updates a record and User2 only wants to read it, then User2 simply reads whatever data is on the disk and then proceeds, without checking whether the data is locked. User2 might see slightly out-of-date information if User1 has read the data and updated it, but has not yet committed the transaction.

Optimistic locking is available on disk-based tables D-tables only. Thus the system discards the version of the data, aborts the transaction, and returns an error message. Each time a record is updated, the version number is updated as well. This way the data that each user sees is consistent throughout the transaction, and users are able to concurrently access the database.

Note Even though the optimistic concurrency control mechanism is sometimes called optimistic locking, it is not a true locking scheme—the system does not place any locks when optimistic concurrency control is used.