[[transactions]]
== Transactions and concurrency control
:sourcedir: ../../../../../test/java/org/hibernate/userguide/transactions

It is important to understand that the term transaction has many different yet related meanings in regards to persistence and Object/Relational Mapping.
In most use-cases these definitions align, but that is not always the case.

* It might refer to the physical transaction with the database.
* It might refer to the logical notion of a transaction as related to a persistence context.
* It might refer to the application notion of a Unit-of-Work, as defined by the archetypal pattern.

[NOTE]
====
This documentation largely treats the physical and logic notions of a transaction as one-in-the-same.
====

[[transactions-physical]]
=== Physical Transactions

Hibernate uses the JDBC API for persistence. In the world of Java, there are two well-defined mechanisms for dealing with transactions in JDBC: JDBC itself and JTA.
Hibernate supports both mechanisms for integrating with transactions and allowing applications to manage physical transactions.

The transaction handling per `Session` is handled by the `org.hibernate.resource.transaction.spi.TransactionCoordinator` contract,
which are built by the `org.hibernate.resource.transaction.spi.TransactionCoordinatorBuilder` service.
`TransactionCoordinatorBuilder` represents a strategy for dealing with transactions whereas TransactionCoordinator represents one instance of that strategy related to a Session.
Which `TransactionCoordinatorBuilder` implementation to use is defined by the `hibernate.transaction.coordinator_class` setting.

`jdbc` (the default for non-JPA applications):: Manages transactions via calls to `java.sql.Connection`
`jta`:: Manages transactions via JTA. See <<chapters/bootstrap/Bootstrap.adoc#bootstrap-jpa-compliant,Java EE bootstrapping>>

If a JPA application does not provide a setting for `hibernate.transaction.coordinator_class`, Hibernate will 
automatically build the proper transaction coordinator based on the transaction type for the persistence unit.

If a non-JPA application does not provide a setting for `hibernate.transaction.coordinator_class`, Hibernate 
will use `jdbc` as the default. This default will cause problems if the application actually uses JTA-based transactions.
A non-JPA application that uses JTA-based transactions should explicitly set `hibernate.transaction.coordinator_class=jta` 
or provide a custom `org.hibernate.resource.transaction.TransactionCoordinatorBuilder` that builds a 
`org.hibernate.resource.transaction.TransactionCoordinator` that properly coordinates with JTA-based transactions. 

[NOTE]
====
For details on implementing a custom `TransactionCoordinatorBuilder`, or simply better understanding how it works, see the
http://docs.jboss.org/hibernate/orm/{majorMinorVersion}/integrationguide/html_single/Hibernate_Integration_Guide.html[Integration Guide] .
====

Hibernate uses JDBC connections and JTA resources directly, without adding any additional locking behavior.
Hibernate does not lock objects in memory.
The behavior defined by the isolation level of your database transactions does not change when you use Hibernate.
The Hibernate `Session` acts as a transaction-scoped cache providing repeatable reads for lookup by identifier and queries that result in loading entities.

[IMPORTANT]
====
To reduce lock contention in the database, the physical database transaction needs to be as short as possible.

Long-running database transactions prevent your application from scaling to a highly-concurrent load.
Do not hold a database transaction open during end-user-level work, but open it after the end-user-level work is finished.

This concept is referred to as `transactional write-behind`.
====

[[transactions-physical-jtaplatform]]
=== JTA configuration

Interaction with a JTA system is consolidated behind a single contract named `org.hibernate.engine.transaction.jta.platform.spi.JtaPlatform` which exposes access to the `javax.transaction.TransactionManager`
and `javax.transaction.UserTransaction` for that system as well as exposing the ability to register `javax.transaction.Synchronization` instances, check transaction status, etc.

[NOTE]
====
Generally, `JtaPlatform` will need access to JNDI to resolve the JTA `TransactionManager`, `UserTransaction`, etc.
See <<chapters/jndi/JNDI.adoc#jndi,JNDI chapter>> for details on configuring access to JNDI.
====

Hibernate tries to discover the `JtaPlatform` it should use through the use of another service named `org.hibernate.engine.transaction.jta.platform.spi.JtaPlatformResolver`.
If that resolution does not work, or if you wish to provide a custom implementation you will need to specify the `hibernate.transaction.jta.platform` setting.
Hibernate provides many implementations of the `JtaPlatform` contract, all with short names:

`Atomikos`:: `JtaPlatform` for Atomikos.
`Borland`:: `JtaPlatform` for the Borland Enterprise Server.
`Bitronix`:: `JtaPlatform` for Bitronix.
`JBossAS`:: `JtaPlatform` for Arjuna/JBossTransactions/Narayana when used within the JBoss/WildFly Application Server.
`JBossTS`:: `JtaPlatform` for Arjuna/JBossTransactions/Narayana when used standalone.
`JOnAS`:: `JtaPlatform` for JOTM when used within JOnAS.
`JOTM`:: `JtaPlatform` for JOTM when used standalone.
`JRun4`:: `JtaPlatform` for the JRun 4 Application Server.
`OC4J`:: `JtaPlatform` for Oracle's OC4J container.
`Orion`:: `JtaPlatform` for the Orion Application Server.
`Resin`:: `JtaPlatform` for the Resin Application Server.
`SapNetWeaver`:: `JtaPlatform` for the SAP NetWeaver Application Server.
`SunOne`:: `JtaPlatform` for the SunOne Application Server.
`Weblogic`:: `JtaPlatform` for the Weblogic Application Server.
`WebSphere`:: `JtaPlatform` for older versions of the WebSphere Application Server.
`WebSphereExtended`:: `JtaPlatform` for newer versions of the WebSphere Application Server.

[[transactions-api]]
=== Hibernate Transaction API

Hibernate provides an API for helping to isolate applications from the differences in the underlying physical transaction system in use.
Based on the configured `TransactionCoordinatorBuilder`, Hibernate will simply do the right thing when this transaction API is used by the application.
This allows your applications and components to be more portable to move around into different environments.

To use this API, you would obtain the `org.hibernate.Transaction` from the Session. `Transaction` allows for all the normal operations you'd expect: `begin`, `commit` and `rollback`, and it even exposes some cool methods like:

`markRollbackOnly`:: that works in both JTA and JDBC.
`getTimeout` and `setTimeout`:: that again work in both JTA and JDBC.
`registerSynchronization`:: that allows you to register JTA Synchronizations even in non-JTA environments.
In fact, in both JTA and JDBC environments, these `Synchronizations` are kept locally by Hibernate.
In JTA environments, Hibernate will only ever register one single `Synchronization` with the `TransactionManager` to avoid ordering problems.

Additionally, it exposes a getStatus method that returns an `org.hibernate.resource.transaction.spi.TransactionStatus` enum.
This method checks with the underlying transaction system if needed, so care should be taken to minimize its use; it can have a big performance impact in certain JTA setups.

Let's take a look at using the Transaction API in the various environments.

[[transactions-api-jdbc-example]]
.Using Transaction API in JDBC
====
[source, JAVA, indent=0]
----
include::{sourcedir}/TransactionsTest.java[tags=transactions-api-jdbc-example]
----
====

[[transactions-api-cmt-example]]
.Using Transaction API in JTA (CMT)
====
[source, JAVA, indent=0]
----
include::{sourcedir}/TransactionsTest.java[tags=transactions-api-cmt-example]
----
====

[[transactions-api-bmt-example]]
.Using Transaction API in JTA (BMT)
====
[source, JAVA, indent=0]
----
include::{sourcedir}/TransactionsTest.java[tags=transactions-api-bmt-example]
----
====

In the CMT case, we really could have omitted all of the Transaction calls.
But the point of the examples was to show that the Transaction API really does insulate your code from the underlying transaction mechanism.
In fact, if you strip away the comments and the single configuration setting supplied at bootstrap, the code is exactly the same in all 3 examples.
In other words, we could develop that code and drop it, as-is, in any of the 3 transaction environments.

The Transaction API tries hard to make the experience consistent across all environments.
To that end, it generally defers to the JTA specification when there are differences (for example automatically trying rollback on a failed commit).

[[architecture-current-session]]
=== Contextual sessions

Most applications using Hibernate need some form of _contextual_ session, where a given session is in effect throughout the scope of a given context.
However, across applications the definition of what constitutes a context is typically different; different contexts define different scopes to the notion of current.
Applications using Hibernate prior to version 3.0 tended to utilize either home-grown `ThreadLocal`-based contextual sessions, helper classes such as `HibernateUtil`, or utilized third-party frameworks, such as Spring or Pico, which provided proxy/interception-based contextual sessions.

Starting with version 3.0.1, Hibernate added the `SessionFactory.getCurrentSession()` method.
Initially, this assumed usage of `JTA` transactions, where the `JTA` transaction defined both the scope and context of a current session.
Given the maturity of the numerous stand-alone `JTA TransactionManager` implementations, most, if not all, applications should be using `JTA` transaction management, whether or not they are deployed into a `J2EE` container.
Based on that, the `JTA`-based contextual sessions are all you need to use.

However, as of version 3.1, the processing behind `SessionFactory.getCurrentSession()` is now pluggable.
To that end, a new extension interface, `org.hibernate.context.spi.CurrentSessionContext`,
and a new configuration parameter, `hibernate.current_session_context_class`, have been added to allow pluggability of the scope and context of defining current sessions.

See the https://docs.jboss.org/hibernate/orm/{majorMinorVersion}/javadocs/org/hibernate/context/spi/CurrentSessionContext.html[Javadocs] for the `org.hibernate.context.spi.CurrentSessionContext` interface for a detailed discussion of its contract.
It defines a single method, `currentSession()`, by which the implementation is responsible for tracking the current contextual session.
Out-of-the-box, Hibernate comes with three implementations of this interface:

`org.hibernate.context.internal.JTASessionContext`::
current sessions are tracked and scoped by a `JTA` transaction.
The processing here is exactly the same as in the older JTA-only approach.
`org.hibernate.context.internal.ThreadLocalSessionContext`::
current sessions are tracked by thread of execution. See the https://docs.jboss.org/hibernate/orm/{majorMinorVersion}/javadocs/org/hibernate/context/internal/ThreadLocalSessionContext.html[Javadocs] for more details.
`org.hibernate.context.internal.ManagedSessionContext`::
current sessions are tracked by thread of execution.
However, you are responsible to bind and unbind a `Session` instance with static methods on this class; it does not open, flush, or close a `Session`.

Typically, the value of this parameter would just name the implementation class to use.
For the three out-of-the-box implementations, however, there are three corresponding short names: _jta_, _thread_, and _managed_.

The first two implementations provide a _one session - one database transaction_ programming model.
This is also known and used as __session-per-request__.
The beginning and end of a Hibernate session is defined by the duration of a database transaction.
If you use programmatic transaction demarcation in plain Java SE without JTA, you are advised to use the Hibernate `Transaction` API to hide the underlying transaction system from your code.
If you use JTA, you can utilize the JTA interfaces to demarcate transactions.
If you execute in an EJB container that supports CMT, transaction boundaries are defined declaratively and you do not need any transaction or session demarcation operations in your code. 

The `hibernate.current_session_context_class` configuration parameter defines which `org.hibernate.context.spi.CurrentSessionContext` implementation should be used.
For backward compatibility, if this configuration parameter is not set but a `org.hibernate.engine.transaction.jta.platform.spi.JtaPlatform` is configured, Hibernate will use the `org.hibernate.context.internal.JTASessionContext`.

=== Transactional patterns (and anti-patterns)

[[session-per-operation]]
==== Session-per-operation anti-pattern

This is an anti-pattern of opening and closing a `Session` for each database call in a single thread.
It is also an anti-pattern in terms of database transactions.
Group your database calls into a planned sequence.
In the same way, do not auto-commit after every SQL statement in your application.
Hibernate disables or expects the application server to disable, auto-commit mode immediately.
Database transactions are never optional.
All communication with a database must be encapsulated by a transaction.
Avoid auto-commit behavior for reading data because many small transactions are unlikely to perform better than one clearly-defined unit of work, and are more difficult to maintain and extend.

[NOTE]
====
Using auto-commit does not circumvent database transactions.

Instead, when in auto-commit mode, JDBC drivers simply perform each call in an implicit transaction call.
It is as if your application called commit after each and every JDBC call.
====

[[session-per-request]]
==== Session-per-request pattern

This is the most common transaction pattern.
The term request here relates to the concept of a system that reacts to a series of requests from a client/user.
Web applications are a prime example of this type of system, though certainly not the only one.
At the beginning of handling such a request, the application opens a Hibernate Session, starts a transaction, performs all data related work, ends the transaction and closes the Session.
The crux of the pattern is the one-to-one relationship between the transaction and the Session.

Within this pattern, there is a common technique of defining a current session to simplify the need of passing this `Session` around to all the application components that may need access to it.
Hibernate provides support for this technique through the `getCurrentSession` method of the `SessionFactory`.
The concept of a _current_ session has to have a scope that defines the bounds in which the notion of _current_ is valid.
This is the purpose of the `org.hibernate.context.spi.CurrentSessionContext` contract.

There are 2 reliable defining scopes:

* First is a JTA transaction because it allows a callback hook to know when it is ending, which gives Hibernate a chance to close the `Session` and clean up.
This is represented by the `org.hibernate.context.internal.JTASessionContext` implementation of the `org.hibernate.context.spi.CurrentSessionContext` contract.
Using this implementation, a `Session` will be opened the first time `getCurrentSession` is called within that transaction.
* Secondly is this application request cycle itself.
This is best represented with the `org.hibernate.context.internal.ManagedSessionContext` implementation of the `org.hibernate.context.spi.CurrentSessionContext` contract.
Here an external component is responsible for managing the lifecycle and scoping of a _current_ session.
At the start of such a scope, `ManagedSessionContext#bind()` method is called passing in the `Session`.
In the end, its `unbind()` method is called.
Some common examples of such _external components_ include:
** `javax.servlet.Filter` implementation
** AOP interceptor with a pointcut on the service methods
** A proxy/interception container

[IMPORTANT]
====
The `getCurrentSession()` method has one downside in a JTA environment.
If you use it, `after_statement` connection release mode is also used by default.
Due to a limitation of the JTA specification, Hibernate cannot automatically clean up any unclosed `ScrollableResults` or `Iterator` instances returned by `scroll()` or `iterate()`.
Release the underlying database cursor by calling `ScrollableResults#close()` or `Hibernate.close(Iterator)` explicitly from a finally block.
====

[[long-conversations]]
==== Conversations (application-level transactions)

The session-per-request pattern is not the only valid way of designing units of work.
Many business processes require a whole series of interactions with the user that are interleaved with database accesses.
In web and enterprise applications, it is not acceptable for a database transaction to span a user interaction. Consider the following example:

The first screen of a dialog opens.
The data seen by the user is loaded in a particular `Session` and database transaction.
The user is free to modify the objects.

The user uses a UI element to save their work after five minutes of editing.
The modifications are made persistent.
The user also expects to have exclusive access to the data during the edit session.

Even though we have multiple databases access here, from the point of view of the user, this series of steps represents a single unit of work.
There are many ways to implement this in your application.

A first naive implementation might keep the `Session` and database transaction open while the user is editing, using database-level locks to prevent other users from modifying the same data and to guarantee isolation and atomicity.
This is an anti-pattern because lock contention is a bottleneck which will prevent scalability in the future.

Several database transactions are used to implement the conversation.
In this case, maintaining isolation of business processes becomes the partial responsibility of the application tier.
A single conversation usually spans several database transactions.
These multiple database accesses can only be atomic as a whole if only one of these database transactions (typically the last one) stores the updated data.
All others only read data.
A common way to receive this data is through a wizard-style dialog spanning several request/response cycles.
Hibernate includes some features which make this easy to implement.

[cols=",",]
|=======================================================================
|Automatic Versioning |Hibernate can perform automatic optimistic concurrency control for you.
It can automatically detect (at the end of the conversation) if a concurrent modification occurred during user think time.

|Detached Objects |If you decide to use the session-per-request pattern, all loaded instances will be in the detached state during user think time.
Hibernate allows you to reattach the objects and persist the modifications.
The pattern is called session-per-request-with-detached-objects.
Automatic versioning is used to isolate concurrent modifications.

|Extended `Session` |The Hibernate `Session` can be disconnected from the underlying JDBC connection after the database transaction has been committed and reconnected when a new client request occurs.
This pattern is known as session-per-conversation and makes even reattachment unnecessary.
Automatic versioning is used to isolate concurrent modifications, and the `Session` will not be allowed to flush automatically, only explicitly.
|=======================================================================

Session-per-request-with-detached-objects and session-per-conversation each have advantages and disadvantages.

[[session-per-application]]
==== Session-per-application anti-pattern

The _session-per-application_ is also considered an anti-pattern.
The Hibernate `Session`, like the JPA `EntityManager`, is not a thread-safe object and it is intended to be confined to a single thread at once.
If the `Session` is shared among multiple threads, there will be race conditions as well as visibility issues, so beware of this.

An exception thrown by Hibernate means you have to rollback your database transaction and close the `Session` immediately. 
If your `Session` is bound to the application, you have to stop the application. 
Rolling back the database transaction does not put your business objects back into the state they were at the start of the transaction. 
This means that the database state and the business objects will be out of sync. 
Usually, this is not a problem because exceptions are not recoverable and you will have to start over after rollback anyway.

For more details, check out the <<chapters/pc/PersistenceContext.adoc#pc-exception-handling, _exception handling_>> section in <<chapters/pc/PersistenceContext.adoc#batch,Persistence Context chapter>>.

The `Session` caches every object that is in a persistent state (watched and checked for dirty state by Hibernate).
If you keep it open for a long time or simply load too much data, it will grow endlessly until you get an `OutOfMemoryException`.
One solution is to call `clear()` and `evict()` to manage the `Session` cache, but you should consider a Stored Procedure if you need mass data operations.
Some solutions are shown in the <<chapters/batch/Batching.adoc#batch,Batching chapter>>.
Keeping a `Session` open for the duration of a user session also means a higher probability of stale data.
