Propagasi dan Isolasi Transaksi di Spring @Transactional

1. Perkenalan

Dalam tutorial ini, kita akan membahas anotasi @Transactional serta pengaturan isolasi dan propagasinya .

2. Apa itu @Transactional?

Kita dapat menggunakan @Transactional untuk menggabungkan metode dalam transaksi database.

Ini memungkinkan kami untuk menyetel kondisi propagasi, isolasi, batas waktu, hanya baca, dan rollback untuk transaksi kami. Selain itu, kami dapat menentukan manajer transaksi.

2.1. @Rincian Implementasi Transaksional

Spring membuat proxy atau memanipulasi kode byte kelas untuk mengelola pembuatan, komit, dan rollback transaksi. Dalam kasus proxy, Spring mengabaikan @Transactional dalam panggilan metode internal.

Sederhananya, jika kita memiliki metode seperti callMethod dan kita menandainya sebagai @Transactional, Spring akan membungkus beberapa kode manajemen transaksi di sekitar pemanggilan: metode @Transactional yang disebut:

createTransactionIfNecessary(); try { callMethod(); commitTransactionAfterReturning(); } catch (exception) { completeTransactionAfterThrowing(); throw exception; }

2.2. Cara Menggunakan @Transactional

Kita dapat meletakkan anotasi pada definisi antarmuka, kelas, atau langsung pada metode. Mereka saling menggantikan sesuai dengan urutan prioritas; dari yang terendah ke tertinggi kita memiliki: Antarmuka, kelas super, kelas, metode antarmuka, metode superclass, dan metode kelas.

Spring menerapkan anotasi tingkat kelas ke semua metode publik dari kelas ini yang tidak kami beri anotasi dengan @Transactional .

Namun, jika kita meletakkan anotasi pada metode privat atau dilindungi, Spring akan mengabaikannya tanpa kesalahan.

Mari kita mulai dengan contoh antarmuka:

@Transactional public interface TransferService { void transfer(String user1, String user2, double val); } 

Biasanya, tidak disarankan untuk menyetel @Transactional pada antarmuka. Namun, ini dapat diterima untuk kasus seperti @Repository dengan Spring Data.

Kita dapat meletakkan anotasi pada definisi kelas untuk mengganti pengaturan transaksi dari antarmuka / superclass:

@Service @Transactional public class TransferServiceImpl implements TransferService { @Override public void transfer(String user1, String user2, double val) { // ... } }

Sekarang mari kita menggantinya dengan mengatur anotasi secara langsung pada metode:

@Transactional public void transfer(String user1, String user2, double val) { // ... }

3. Propagasi Transaksi

Propagasi mendefinisikan batas transaksi logika bisnis kita. Spring berhasil memulai dan menjeda transaksi sesuai dengan pengaturan propagasi kami .

Spring memanggil TransactionManager :: getTransaction untuk mendapatkan atau membuat transaksi sesuai dengan propagasi. Ini mendukung beberapa penyebaran untuk semua jenis TransactionManager , tetapi ada beberapa di antaranya yang hanya didukung oleh implementasi tertentu dari TransactionManager .

Sekarang mari kita bahas berbagai propagasi dan cara kerjanya.

3.1. DIBUTUHKAN Perbanyakan

REQUIRED adalah propagasi default. Spring memeriksa apakah ada transaksi aktif, lalu membuat transaksi baru jika tidak ada. Jika tidak, logika bisnis ditambahkan ke transaksi yang sedang aktif:

@Transactional(propagation = Propagation.REQUIRED) public void requiredExample(String user) { // ... }

Juga karena REQUIRED adalah propagasi default, kita dapat menyederhanakan kode dengan menjatuhkannya:

@Transactional public void requiredExample(String user) { // ... }

Mari kita lihat pseudo-code tentang cara kerja pembuatan transaksi untuk propagasi REQUIRED :

if (isExistingTransaction()) { if (isValidateExistingTransaction()) { validateExisitingAndThrowExceptionIfNotValid(); } return existing; } return createNewTransaction();

3.2. DUKUNGAN Perbanyakan

Untuk DUKUNGAN , Spring pertama-tama memeriksa apakah ada transaksi aktif. Jika ada transaksi, maka transaksi yang ada akan digunakan. Jika tidak ada transaksi, itu dijalankan non-transaksional:

@Transactional(propagation = Propagation.SUPPORTS) public void supportsExample(String user) { // ... }

Mari kita lihat pseudo-code pembuatan transaksi untuk SUPPORTS :

if (isExistingTransaction()) { if (isValidateExistingTransaction()) { validateExisitingAndThrowExceptionIfNotValid(); } return existing; } return emptyTransaction;

3.3. Perbanyakan WAJIB

Ketika propagasi WAJIB , jika ada transaksi yang aktif maka akan digunakan. Jika tidak ada transaksi yang aktif, maka Spring memberikan pengecualian:

@Transactional(propagation = Propagation.MANDATORY) public void mandatoryExample(String user) { // ... }

Dan mari kita lihat lagi pseudo-code:

if (isExistingTransaction()) { if (isValidateExistingTransaction()) { validateExisitingAndThrowExceptionIfNotValid(); } return existing; } throw IllegalTransactionStateException;

3.4. JANGAN PERNAH Propagasi

Untuk logika transaksional dengan propagasi NEVER , Spring menampilkan pengecualian jika ada transaksi aktif:

@Transactional(propagation = Propagation.NEVER) public void neverExample(String user) { // ... }

Mari kita lihat pseudo-code tentang cara kerja pembuatan transaksi untuk PERNAH propagasi:

if (isExistingTransaction()) { throw IllegalTransactionStateException; } return emptyTransaction;

3.5. NOT_SUPPORTED Propagation

Spring at first suspends the current transaction if it exists, then the business logic is executed without a transaction.

@Transactional(propagation = Propagation.NOT_SUPPORTED) public void notSupportedExample(String user) { // ... }

The JTATransactionManager supports real transaction suspension out-of-the-box. Others simulate the suspension by holding a reference to the existing one and then clearing it from the thread context

3.6. REQUIRES_NEW Propagation

When the propagation is REQUIRES_NEW, Spring suspends the current transaction if it exists and then creates a new one:

@Transactional(propagation = Propagation.REQUIRES_NEW) public void requiresNewExample(String user) { // ... }

Similar to NOT_SUPPORTED, we need the JTATransactionManager for actual transaction suspension.

And the pseudo-code looks like so:

if (isExistingTransaction()) { suspend(existing); try { return createNewTransaction(); } catch (exception) { resumeAfterBeginException(); throw exception; } } return createNewTransaction();

3.7. NESTED Propagation

For NESTED propagation, Spring checks if a transaction exists, then if yes, it marks a savepoint. This means if our business logic execution throws an exception, then transaction rollbacks to this savepoint. If there's no active transaction, it works like REQUIRED .

DataSourceTransactionManager supports this propagation out-of-the-box. Also, some implementations of JTATransactionManager may support this.

JpaTransactionManager supports NESTED only for JDBC connections. However, if we set nestedTransactionAllowed flag to true, it also works for JDBC access code in JPA transactions if our JDBC driver supports savepoints.

Finally, let's set the propagation to NESTED:

@Transactional(propagation = Propagation.NESTED) public void nestedExample(String user) { // ... }

4. Transaction Isolation

Isolation is one of the common ACID properties: Atomicity, Consistency, Isolation, and Durability. Isolation describes how changes applied by concurrent transactions are visible to each other.

Each isolation level prevents zero or more concurrency side effects on a transaction:

  • Dirty read: read the uncommitted change of a concurrent transaction
  • Nonrepeatable read: get different value on re-read of a row if a concurrent transaction updates the same row and commits
  • Phantom read: get different rows after re-execution of a range query if another transaction adds or removes some rows in the range and commits

We can set the isolation level of a transaction by @Transactional::isolation. It has these five enumerations in Spring: DEFAULT, READ_UNCOMMITTED, READ_COMMITTED, REPEATABLE_READ, SERIALIZABLE.

4.1. Isolation Management in Spring

The default isolation level is DEFAULT. So when Spring creates a new transaction, the isolation level will be the default isolation of our RDBMS. Therefore, we should be careful if we change the database.

We should also consider cases when we call a chain of methods with different isolation. In the normal flow, the isolation only applies when a new transaction created. Thus if for any reason we don't want to allow a method to execute in different isolation, we have to set TransactionManager::setValidateExistingTransaction to true. Then the pseudo-code of transaction validation will be:

if (isolationLevel != ISOLATION_DEFAULT) { if (currentTransactionIsolationLevel() != isolationLevel) { throw IllegalTransactionStateException } }

Now let's get deep in different isolation levels and their effects.

4.2. READ_UNCOMMITTED Isolation

READ_UNCOMMITTED is the lowest isolation level and allows for most concurrent access.

As a result, it suffers from all three mentioned concurrency side effects. So a transaction with this isolation reads uncommitted data of other concurrent transactions. Also, both non-repeatable and phantom reads can happen. Thus we can get a different result on re-read of a row or re-execution of a range query.

We can set the isolation level for a method or class:

@Transactional(isolation = Isolation.READ_UNCOMMITTED) public void log(String message) { // ... }

Postgres does not support READ_UNCOMMITTED isolation and falls back to READ_COMMITED instead. Also, Oracle does not support and allow READ_UNCOMMITTED.

4.3. READ_COMMITTED Isolation

The second level of isolation, READ_COMMITTED, prevents dirty reads.

The rest of the concurrency side effects still could happen. So uncommitted changes in concurrent transactions have no impact on us, but if a transaction commits its changes, our result could change by re-querying.

Here, we set the isolation level:

@Transactional(isolation = Isolation.READ_COMMITTED) public void log(String message){ // ... }

READ_COMMITTED is the default level with Postgres, SQL Server, and Oracle.

4.4. REPEATABLE_READ Isolation

The third level of isolation, REPEATABLE_READ, prevents dirty, and non-repeatable reads. So we are not affected by uncommitted changes in concurrent transactions.

Also, when we re-query for a row, we don't get a different result. But in the re-execution of range-queries, we may get newly added or removed rows.

Moreover, it is the lowest required level to prevent the lost update. The lost update occurs when two or more concurrent transactions read and update the same row. REPEATABLE_READ does not allow simultaneous access to a row at all. Hence the lost update can't happen.

Here is how to set the isolation level for a method:

@Transactional(isolation = Isolation.REPEATABLE_READ) public void log(String message){ // ... }

REPEATABLE_READ is the default level in Mysql. Oracle does not support REPEATABLE_READ.

4.5. SERIALIZABLE Isolation

SERIALIZABLE is the highest level of isolation. It prevents all mentioned concurrency side effects but can lead to the lowest concurrent access rate because it executes concurrent calls sequentially.

In other words, concurrent execution of a group of serializable transactions has the same result as executing them in serial.

Now let's see how to set SERIALIZABLE as the isolation level:

@Transactional(isolation = Isolation.SERIALIZABLE) public void log(String message){ // ... }

5. Conclusion

In this tutorial, we explored the propagation property of @Transaction in detail. Afterward, we learned about concurrency side effects and isolation levels.

As always, you can find the complete code over on GitHub.