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This article mainly introduces "introduction of basic characteristics and isolation level of java transaction". In daily operation, I believe that many people have doubts about the basic characteristics and isolation level introduction of java transaction. The editor consulted all kinds of data and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "introduction to basic characteristics and isolation level of java transaction". Next, please follow the editor to study!

What is a transaction?

Transaction is an operation sequence of accessing database, and database application system accesses database through transaction set. The correct execution of the transaction causes the database to transition from one state to another.

Transactions must follow the ACID principles established by ISO/IEC. ACID is an abbreviation for atomicity, consistency, isolation, and durability. These four states mean:

1. Atomicity

That is, it is indivisible, and either all transactions are executed or none are executed. If all subtransactions of the transaction are committed successfully, all database operations are committed and the database state changes; if there are subtransactions that fail, the database operations of other subtransactions are rolled back, that is, the database returns to the state before the transaction execution, and the state transition does not occur.

2. Consistency

The execution of the transaction causes the database to change from one correct state to another.

3. Isolation

Changes to the data made by the transaction are not allowed to be provided to any other transaction until the transaction is committed correctly, that is, its possible results should not be displayed to other transactions until the transaction is committed correctly

4. Persistence

After the transaction is committed correctly, the results will be saved in the database forever, even if there are other failures after the transaction is committed.

The role of the transaction

Transaction management is very important for enterprise applications, it ensures that every operation of the user is reliable, even if there is abnormal access, it will not destroy the integrity of the background data. Just like the bank's ATM ATM, ATM can usually serve customers normally, but it is also inevitable to encounter sudden failures in the course of operation. At this time, the transaction must ensure that the operation on the account does not take effect before the failure, just as the user has not used the ATM machine at all, so as to ensure that the interests of the user and the bank will not be lost.

And send out the problems that will arise from the transaction.

For example, transaction An and transaction B manipulate the same resource, transaction A has several sub-transactions, transaction B also has several sub-transactions, transaction An and transaction B will have a variety of problems in the case of high concurrency. "all kinds of problems", summed up as five main types: the first type of lost updates, the second kind of lost updates, dirty reading, non-repeatable reading, phantom reading. Among the five kinds, the first kind of missing updates and the second kind of missing updates are not important. Let's not talk about dirty reading, unrepeatable reading and phantom reading.

1. Dirty reading

The so-called dirty reading means that transaction A reads the data that transaction B has not yet committed, such as the bank withdraws money, transaction A starts the transaction, then switches to transaction B, transaction B starts the transaction-- > takes 100 yuan, and then switches back to transaction A. transaction A must read the original data in the database, because transaction B took 100 yuan and did not commit, and the account balance in the database must still be the original balance. This is dirty reading.

2. Non-repeatable

The so-called unrepeatable reading means that some data is read twice in a transaction and the data read is inconsistent. Let's take the bank withdrawal as an example. Transaction A starts the transaction-- > finds out that the bank card balance is 1000 yuan. At this time, switch to transaction B to start the transaction-- > transaction B takes away 100 yuan-- > commit, and the balance in the database becomes 900. at this time, switch back to transaction A, and transaction A checks again that the account balance is 900. as far as transaction An is concerned, it is inconsistent to read the account balance data twice in the same transaction. This is unrepeatable.

3. Illusory reading

The so-called illusory reading refers to the discovery of unmanipulated data in the operation of a transaction. For example, student information, transaction An opens transaction-> modify all student check-in status is false on that day, switch to transaction B, transaction B starts transaction-> transaction B inserts a piece of student data, and transaction A switches back to transaction A. when transaction A commits, it finds a piece of data that it has not modified. This is an illusion, as if an illusion had occurred. The premise of phantom reading is that insert and delete operations take place in concurrent transactions.

Transaction isolation level

The transaction isolation level is born to solve the above problems. Why is there a transaction isolation level? because the higher the transaction isolation level, the fewer problems there will be under concurrency, but the greater the performance consumption will be, so there is often a tradeoff between concurrency and performance. Therefore, several transaction isolation levels are set up so that different projects can choose the appropriate transaction isolation level according to the concurrency of their own projects, and compensate for the concurrency problems that arise outside the transaction isolation level in the code.

There are 4 transaction isolation levels, but 5 are provided to users like Spring. Let's take a look at:

1 、 DEFAULT

The default isolation level is different for each database, and if Spring sets isolation to this value when configuring transactions, the default transaction isolation level for the underlying database will be used. By the way, if you are using MySQL, you can use "select @ @ tx_isolation" to view the default transaction isolation level

2 、 READ_UNCOMMITTED

Read uncommitted, that is, data that is not committed can be read, so it is obvious that this level of isolation mechanism can not solve any kind of dirty, unrepeatable or phantom reading, so it is rarely used.

3 、 READ_COMMITED

Reading submitted, that is, being able to read those submitted data, can naturally prevent dirty reading, but can not limit non-repeatable reading and phantom reading.

4 、 REPEATABLE_READ

Repeat reading, that is, add a lock after the data is read, similar to "select * from XXX for update". It is clear that the data is read for update, so add a lock to prevent others from modifying it. The meaning of REPEATABLE_READ is similar. After reading a piece of data, other transactions cannot change the record until the transaction is finished. This solves the problem of dirty reading and unrepeatable reading, but the problem of phantom reading still cannot be solved.

5 、 SERLALIZABLE

Serialization, the highest transaction isolation level, no matter how many transactions, after running all the subtransactions of one transaction one by one, all the subtransactions of another transaction can be executed, thus solving the problems of dirty reading, non-repeatable reading and phantom reading.

There is a graph on the Internet that lists the concurrency problems solved by the transaction isolation level in the form of a table:

It must be emphasized again that it is not that the higher the transaction isolation level is set, the better, and the higher the transaction isolation level is set, which means that it is necessary to use means to lock to ensure the correctness of the transaction, so the efficiency will be reduced. Therefore, in actual development, we often have to make a trade-off between efficiency and concurrent correctness, which is generally set to READ_COMMITED, which avoids dirty reading and concurrency is also good. Then use some other means to solve the problem of unrepeatable reading and phantom reading.

At this point, the study on "introduction to the basic features and isolation levels of java transactions" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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