Database lock mechanism 2

Source: Internet
Author: User


A database is a shared resource used by multiple users. When multiple users access data concurrently, in the database, multiple transactions are generated concurrently to access the same data. If the concurrency operation is not controlled, it is possible to read and store incorrect data and compromise the consistency of the database.

Locking is a very important technology to realize concurrency control of database. When a transaction makes a request to the system before it operates on a data object, it locks it. After locking, the transaction has some control over the data object, and other transactions cannot update the data object until the transaction releases the lock.

"Basic Lock Type"

Locks include row-level and table-level locks

A row-level lock is an exclusive lock that prevents other transactions from modifying this row, and Oracle automatically applies row-level locks when the following statement is used:
SELECT ... The FOR UPDATE statement allows users to lock multiple records at once for updates
Release a lock using a commit or ROLLBACK statement

table-level locks are also divided into 5 categories:

Row SHARE – Prohibit exclusive locking of tables
Row exclusive (Row EXCLUSIVE) – Exclusive and shared locks are not allowed
Shared Lock (SHARE)-lock table, read-only and write-only, multiple users can apply this lock on the same table at the same time
Shared row Exclusive (SHARE row EXCLUSIVE) – more restrictions than shared locks, prohibit use of shared locks and higher locks
Exclusive (EXCLUSIVE) – Restricts the strongest table locks, allowing only other users to query rows of the table. Prohibit modifying and locking tables

"Optimistic lock and pessimistic lock"

The so-called pessimistic lock is based on the implementation of database mechanism. For example, when you add a for update with the SELECT clause, no application can modify the record of a select until the end of the transaction to which the clause was modified.

The so-called optimistic lock is implemented based on the version mechanism of the application. It is common to design a version field V in the table (I usually set this field to timestamp). The general update scenario is this:

1 Select a, v from TB where id=1;

Suppose to get the data is: [' xxx ', 11111]

2 Update TB set a= ' yyyy ', v=systimestamp where v=11111; Note that V is generally not modified at the time of business operations

This requires that each update operation change the version field, or whether the data between processes will be overwritten with each other.

Optimistic locks cannot lock other apps ' operations on the data.

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 and optimistic lock implementation in Hibernate"

One: 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 a database: select * from account where name= ' Erica ' for update this SQL statement locks all records that meet the search criteria (name= "Erica") in the list. 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:

1 String hqlstr = "from TUser as user where ' Erica '";
2 Query query = session.createquery (HQLSTR);
3 Query.setlockmode ("User", Lockmode.upgrade); Locking
4 List userlist = Query.list (); Execute the query,

Gets the 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:

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

Here hibernate implements the pessimistic locking mechanism by using the database's for UPDATE clause. Hibernate's lock modes are:
? 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:
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.

Two: 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 until the entire process of submitting the modification result, even includes the operation Staff to cook coffee in the middle of the time), database records are always locked, you can imagine, if faced with hundreds of thousands of concurrent, this situation will lead to what consequences. The optimistic locking mechanism solves this problem to some extent.optimistic locks are mostly implemented based on the data Versioning (version) record mechanism。 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 $ $.
1: operator A reads it out at this time (version=1) and deducts $ ($100-$50) from its account balance.
2: During operator A's operation, operator B also reads this user information (version=1) and deducts $ $100-$20 from its account balance.
3: operator a completed the modification work, the data version number plus one (version=2), together with account deduction after the balance (BALANCE=$50), submitted to the database update, at this time because the submission version of the data is larger than the current version of database records, the data is updated, database record version updated to 2.
4: operator B completes the operation, and the version number plus one (version=2) attempts to submit data to the database (balance=$80), but at this time compared to the database record version, operator B submits the data version number 2, the database records the current version is 2, does not meet the " The commit version must be larger than the current version of the record to perform the update "optimistic locking policy, so the submission of operator B is dismissed. This avoids the possibility of operator B overwriting operator A's results with the results of old version=1-based data modifications.

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 property to the class descriptor for Tuser:

< hibernate-mapping >
< class
Name = "Org.hibernate.sample.TUser"
Table = "T_user"
Dynamic-update = "true"
Dynamic-insert = "true"
Optimistic-lock = "Version"
</class >

The Optimistic-lock property resembles the following selectable value:
? None No optimistic lock
? Version enables optimistic locking through a versioning mechanism
? Dirty optimistic locking by checking for changed properties
? All by checking all the properties to implement the music

Database lock mechanism 2

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