How to customize Hibernate dirty checking mechanism

Introduction

In my previous article I described the Hibernate automatic dirty checking mechanism. While you should always prefer it, there might be times when you want to add your own custom dirtiness detection strategy.

Custom dirty checking strategies

Hibernate offers the following customization mechanisms:

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The anatomy of Hibernate dirty checking mechanism

Introduction

The persistence context enqueues entity state transitions that get translated to database statements upon flushing. For managed entities, Hibernate can auto-detect incoming changes and schedule SQL UPDATES on our behalf. This mechanism is called automatic dirty checking.

The default dirty checking strategy

By default Hibernate checks all managed entity properties. Every time an entity is loaded, Hibernate makes an additional copy of all entity property values. At flush time, every managed entity property is matched against the loading-time snapshot value:

DefaultFlushEventFlow

So the number of individual dirty checks is given by the following formula:

N = \sum\limits_{k=1}^n p_{k}

where

n = The number of managed entities
p = The number of properties of a given entity

Even if only one property of a single entity has ever changed, Hibernate will still check all managed entities. For a large number of managed entities, the default dirty checking mechanism may have a significant CPU and memory footprint. Since the initial entity snapshot is held separately, the persistence context requires twice as much memory as all managed entities would normally occupy.

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How does AUTO flush strategy work in JPA and Hibernate

Introduction

Now that I described the the basics of JPA and Hibernate flush strategies, I can continue unraveling the surprising behavior of Hibernate’s AUTO flush mode.

Not all queries trigger a Session flush

Many would assume that Hibernate always flushes the Session before any executing query. While this might have been a more intuitive approach, and probably closer to the JPA’s AUTO FlushModeType, Hibernate tries to optimize that. If the current executed query is not going to hit the pending SQL INSERT/UPDATE/DELETE statements then the flush is not strictly required.

As stated in the reference documentation, the AUTO flush strategy may sometimes synchronize the current persistence context prior to a query execution. It would have been more intuitive if the framework authors had chosen to name it FlushMode.SOMETIMES.

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A beginner’s guide to flush strategies in JPA and Hibernate

Introduction

In my previous post I introduced the entity state transitions Object-relational mapping paradigm.

All managed entity state transitions are translated to associated database statements when the current Persistence Context gets flushed. Hibernate’s flush behavior is not always as obvious as one might think.

Write-behind

Hibernate tries to defer the Persistence Context flushing up until the last possible moment. This strategy has been traditionally known as transactional write-behind.

The write-behind is more related to Hibernate flushing rather than any logical or physical transaction. During a transaction, the flush may occur multiple times.

The flushed changes are visible only for the current database transaction. Until the current transaction is committed, no change is visible by other concurrent transactions.

The persistence context, also known as the first level cache, acts as a buffer between the current entity state transitions and the database.

In caching theory, the write-behind synchronization requires that all changes happen against the cache, whose responsibility is to eventually synchronize with the backing store.

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