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.

Bytecode instrumentation

A more efficient approach would be to mark dirty properties upon value changing. Analogue to the original deep comparison strategy, it’s good practice to decouple the domain model structures from the change detection logic. The automatic entity change detection mechanism is a cross-cutting concern, that can be woven either at build-time or at runtime.

The entity class can be appended with bytecode level instructions implementing the automatic dirty checking mechanism.

Weaving types

The bytecode enhancement can happen at:

  • Build-time

    After the hibernate entities are compiled, the build tool (e.g. ANT, Maven) will insert bytecode level instructions into each compiled entity class. Because the classes are enhanced at build-time, this process exhibits no extra runtime penalty. Testing can be done against enhanced class versions, so that the actual production code is validated before the project gets built.

  • Runtime

    The runtime weaving can be done using:

If you enjoyed this article, I bet you are going to love my book as well.

Hibernate 5 improvements

Hibernate 3 has been offering bytecode instrumentation through an ANT target but it never became mainstream and most Hibernate projects are still currently using the default deep comparison approach.
Hibernate 5 has redesigned the bytecode enhancement mechanism, being more reliable than it used to be.

If you liked this article, you might want to subscribe to my newsletter too.

Advertisements

7 thoughts on “The anatomy of Hibernate dirty checking mechanism

  1. You mentioned that ” 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” Are you sure about that?

    I have a test case http://stackoverflow.com/questions/27595252/hibernate-sets-dirty-flag-and-issues-update-even-though-client-did-not-change
    which shows that the delta is calculated between the persistent value and the client’s values. Or maybe there is more to it, such as hibernate watching for certain PK’s. If an entity with that PK gets and update, you get a check/ timestamp checking, etc… ???

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s