1. Introducción
En este tutorial, cubriremos la anotación @Transactional y su configuración de aislamiento y propagación .
2. ¿Qué es @Transactional?
Podemos usar @Transactional para envolver un método en una transacción de base de datos.
Nos permite establecer condiciones de propagación, aislamiento, tiempo de espera, solo lectura y reversión para nuestra transacción. Además, podemos especificar el administrador de transacciones.
2.1. @ Detalles de implementación transaccional
Spring crea un proxy o manipula el código de bytes de la clase para administrar la creación, confirmación y reversión de la transacción. En el caso de un proxy, Spring ignora @Transactional en las llamadas a métodos internos.
En pocas palabras, si tenemos un método como callMethod y lo marcamos como @Transactional, Spring envolvería un código de gestión de transacciones alrededor de la invocación: método @Transactional llamado:
createTransactionIfNecessary(); try { callMethod(); commitTransactionAfterReturning(); } catch (exception) { completeTransactionAfterThrowing(); throw exception; }2.2. Cómo utilizar @Transactional
Podemos poner la anotación en definiciones de interfaces, clases o directamente en métodos. Se anulan entre sí según el orden de prioridad; de menor a mayor tenemos: Interfaz, superclase, clase, método de interfaz, método de superclase y método de clase.
Spring aplica la anotación de nivel de clase a todos los métodos públicos de esta clase que no anotamos con @Transactional .
Sin embargo, si colocamos la anotación en un método privado o protegido, Spring la ignorará sin error.
Comencemos con una muestra de interfaz:
@Transactional public interface TransferService { void transfer(String user1, String user2, double val); } Por lo general, no se recomienda configurar @Transactional en la interfaz. Sin embargo, es aceptable para casos como @Repository con Spring Data.
Podemos poner la anotación en una definición de clase para anular la configuración de transacción de la interfaz / superclase:
@Service @Transactional public class TransferServiceImpl implements TransferService { @Override public void transfer(String user1, String user2, double val) { // ... } }Ahora anulémoslo estableciendo la anotación directamente en el método:
@Transactional public void transfer(String user1, String user2, double val) { // ... }3. Propagación de transacciones
La propagación define los límites de las transacciones de nuestra lógica empresarial. Spring logra iniciar y pausar una transacción de acuerdo con nuestra configuración de propagación .
Spring llama a TransactionManager :: getTransaction para obtener o crear una transacción de acuerdo con la propagación. Admite algunas de las propagaciones para todos los tipos de TransactionManager , pero hay algunas que solo son compatibles con implementaciones específicas de TransactionManager .
Ahora repasemos las diferentes propagaciones y cómo funcionan.
3.1. Propagación REQUERIDA
REQUERIDO es la propagación predeterminada. Spring verifica si hay una transacción activa, luego crea una nueva si no existía nada. De lo contrario, la lógica empresarial se agrega a la transacción actualmente activa:
@Transactional(propagation = Propagation.REQUIRED) public void requiredExample(String user) { // ... }Además, como REQUERIDO es la propagación predeterminada, podemos simplificar el código soltándolo:
@Transactional public void requiredExample(String user) { // ... }Veamos el pseudocódigo de cómo funciona la creación de transacciones para la propagación REQUERIDA :
if (isExistingTransaction()) { if (isValidateExistingTransaction()) { validateExisitingAndThrowExceptionIfNotValid(); } return existing; } return createNewTransaction();3.2. Apoya la propagación
Para SOPORTES , Spring primero verifica si existe una transacción activa. Si existe una transacción, se utilizará la transacción existente. Si no hay una transacción, se ejecuta no transaccional:
@Transactional(propagation = Propagation.SUPPORTS) public void supportsExample(String user) { // ... }Veamos el pseudocódigo de creación de la transacción para SUPPORTS :
if (isExistingTransaction()) { if (isValidateExistingTransaction()) { validateExisitingAndThrowExceptionIfNotValid(); } return existing; } return emptyTransaction;3.3. Propagación OBLIGATORIA
Cuando la propagación sea OBLIGATORIA , si hay una transacción activa, se utilizará. Si no hay una transacción activa, Spring lanza una excepción:
@Transactional(propagation = Propagation.MANDATORY) public void mandatoryExample(String user) { // ... }Y veamos nuevamente el pseudocódigo:
if (isExistingTransaction()) { if (isValidateExistingTransaction()) { validateExisitingAndThrowExceptionIfNotValid(); } return existing; } throw IllegalTransactionStateException;3.4. NUNCA Propagación
Para la lógica transaccional con NUNCA propagación, Spring lanza una excepción si hay una transacción activa:
@Transactional(propagation = Propagation.NEVER) public void neverExample(String user) { // ... }Veamos el pseudocódigo de cómo funciona la creación de transacciones para NUNCA propagación:
if (isExistingTransaction()) { throw IllegalTransactionStateException; } return emptyTransaction;3.5. Propagación NO_SUPPORTADA
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.