Optimistic lock and pessimistic lock

The comparison between optimistic lock and pessimistic lock:

Category	Realize	Characteristics	Risk
Pessimistic lock	Rely on the lock mechanism of database layer	Exclusive Nature	The large overhead of database performance is often unsustainable, especially for long transactions.
Optimistic lock	Implementation based on the data versioning mechanism	The optimistic locking mechanism avoids the database lock overhead in long transaction, and greatly improves the overall performance of the system under large concurrency.	External System update operations will not be controlled by the system, dirty data may be updated into the database

The following is the text:

Lock (Locking)

In the process of implementing business logic, it is often necessary to guarantee the exclusivity of data access. As in the final settlement of the financial system, we want to process the data for a cut-off point in time, and we do not want the data to change again during the settlement process (which may be a few seconds or maybe a few hours). At this point, we need some mechanism to ensure that the data will not be modified in the process of the outside, such a mechanism, in this case, the so-called "lock", that is, to our selected target data locked, so that it can not be modified by other programs.

Hibernate supports two locking mechanisms: "Pessimistic lock (pessimistic Locking)" and "optimistic lock (optimistic Locking)", which is commonly referred to.

Pessimistic lock (Pessimistic Locking)

Pessimistic locking, as its name implies, is a conservative attitude to the data being modified by the outside world (including other transactions currently in the system, as well as transactions from external systems), so that the data is locked during the entire data processing process. Pessimistic lock implementation, often rely on the database provided by the lock mechanism (also only the database layer provides a lock mechanism to truly guarantee the exclusivity of data access, otherwise, even in this system to implement the locking mechanism, there is no guarantee that the external system will not modify the data).

A typical pessimistic lock call that relies on the database:

SELECT * from account where name= ' Erica ' for update

This SQL statement locks all records in the Account table that match the search criteria (name= "Erica"). These records cannot be modified by the outside world before the transaction commits (the locks in the transaction are freed during transaction commits).

Hibernate's pessimistic lock is also a database-based lock mechanism implementation. The following code implements the locking of the query record:

String hqlstr = "from TUser as user where user.name= ' Erica '"; Query query = session.createquery (HQLSTR); Query.setlockmode ("User", Lockmode.upgrade); Locking list userlist = Query.list ();//execute query, get data

Query.setlockmode locks the record for a particular alias in the query statement (we specify an alias "user" for the TUser Class), which is to lock all the returned user records.

Observe the SQL statements generated by Hibernate during the run time:

Select Tuser0_.id as ID, tuser0_.name as name, tuser0_.group_id as group_id, Tuser0_.user_type as User_type, tuser0_.sex a s sex from T_user tuser0_ where (tuser0_.name= ' Erica ') for update

Here Hibernate implements the pessimistic locking mechanism by using the database's for UPDATE clause.

Hibernate's Lock mode is: Lockmode.none: no lock mechanism. LockMode.WRITE:Hibernate is automatically acquired when the Insert and Update records are recorded. The LockMode.READ:Hibernate is automatically retrieved when the record is read.

These three types of locking mechanisms are typically used internally by hibernate, such as hibernate to automatically add WRITE locks to the target object in the Save method implementation, in order to ensure that the object is not modified by the outside world during the Update process.

Lockmode.upgrade: Use the database's for UPDATE clause to lock. Lockmode. A specific implementation of the Upgrade_nowait:oracle, using Oracle's for UPDATE NOWAIT clause to implement the lock.

The above two kinds of locking mechanism is more commonly used in the application layer, lock is generally implemented by the following methods:

Criteria.setLockModeQuery.setLockModeSession.lock

Note that locking is really done through the lock mechanism of the database only if the lock is set before the query starts (that is, before the hiberate generates SQL), otherwise the data has been loaded through a Select SQL that does not contain a FOR UPDATE clause, so the so-called database lock is not possible.

Optimistic lock (optimistic Locking)

Relative pessimistic lock, the optimistic locking mechanism adopts a more relaxed locking mechanism. Pessimistic locking relies on the lock mechanism of the database in most cases, to ensure the maximum degree of exclusivity of the operation. But it comes with a lot of overhead for database performance, especially for long transactions, which are often unsustainable.

As a financial system, when an operator reads a user's data and modifies it on the basis of the user's data being read (such as changing the user account balance), the pessimistic locking mechanism means that the entire operation (from the operator reads the data, starts the modification, and commits the modified result. Even when the operator takes the time to cook the coffee, the database record is always locked, and you can see what happens if you face hundreds of thousand concurrent cases.

The optimistic locking mechanism solves this problem to some extent. Optimistic locking, mostly based on the data version (versions) recording mechanism implementation. What is a data version? is to add a version identity to the data, which is typically done by adding a "version" field to the database table in the version solution based on the database table.

When the data is read, the version number is read together, and then the version number is added one after the update. At this point, the version data of the submitted data is compared to the current version information of the database table corresponding to the record, and if the submitted version number is greater than the current version number of the database table, it is updated, otherwise it is considered to be outdated data.

For the example above to modify user account information, assume that there is a version field in the Account information table in the database, the current value is 1, and the Current Account balance field (balance) is $ $.

As can be seen from the above example, the optimistic locking mechanism avoids the database lock-up overhead in a long transaction (both operator A and operator B do not locking the database data), which greatly improves the overall performance of the system under large concurrent volume. It should be noted that the optimistic locking mechanism is often based on the data storage logic in the system, so there are some limitations, as in the above example, because the optimistic locking mechanism is implemented in our system, the user balance update from the external system is not controlled by our system, it may cause dirty data to be updated into the database. In the system design phase, we should take full account of the possibility of these situations, and make appropriate adjustments (such as the optimistic locking policy implemented in the database stored procedures, external only open the data update path based on this stored procedure, rather than the database table directly to the public).

Hibernate has built-in optimistic locking implementations in its data access engine. If you do not take into account the external system to update the database, using Hibernate to provide a transparent optimistic lock implementation, will greatly enhance our productivity.

Hibernate can be specified by using the Optimistic-lock property of the class descriptor in conjunction with the version descriptor.

Now, let's add an optimistic locking mechanism to the TUser in the previous example.

1. First add the Optimistic-lock attribute to the class descriptor for TUser:

The Optimistic-lock property resembles the following selectable value:
None: No optimistic lock version: optimistic locking through version mechanism dirty: optimistic locking by checking for changed properties all: Optimistic locking by checking all properties
The optimistic locking mechanism which is implemented by version is the optimistic lock implemented by hibernate, and it is also the only locking mechanism that is still effective when the data object is changed from the Session. Therefore, in general, we all choose the version mode as the Hibernate optimistic lock implementation mechanism.
2. Add a Version Property descriptor
Note The version node must appear after the ID node.
Here we declare a version property, which holds the user's release information, and is saved in the TUser table's versions field.
At this point, if we try to write a piece of code, update the record data in the TUser table, such as:
Criteria = Session.createcriteria (Tuser.class), Criteria.add (Expression.eq ("name", "Erica")); List userlist = Criteria.list (); TUser user = (TUser) userlist.get (0); Transaction tx = Session.begintransaction (); User.setusertype (1); Update Usertype field tx.commit ();
Each time we update the TUser, we can see that the version in the database is incremented.
And if we try to start another Session before Tx.commit, manipulate the user named Erica to emulate the concurrency update scenario:
Session session= getsession (); Criteria = Session.createcriteria (Tuser.class), Criteria.add (Expression.eq ("name", "Erica")); Session Session2 = getsession (); Criteria criteria2 = Session2.createcriteria (Tuser.class); Criteria2.add (Expression.eq ("name", "Erica")); List userlist = Criteria.list (); List userList2 = Criteria2.list (); TUser user = (TUser) userlist.get (0); TUser user2 = (TUser) userlist2.get (0); Transaction tx = Session.begintransaction (); Transaction tx2 = Session2.begintransaction (); User2.setusertype (in); Tx2.commit (); User.setusertype (1); Tx.commit ();
Executing the above code, the code throws an Staleobjectstateexception exception at Tx.commit () and indicates that the version check failed and that the current transaction is attempting to submit an expired data. By catching this exception, we can handle the optimistic lock check when it fails.
Optimistic lock and pessimistic lock