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Difference between revisions of "EclipseLink/UserGuide/JPA/Basic JPA Development/Caching/Type and Size"

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{{EclipseLink_Toc}}
 
=Cache Type and Size=
 
=Cache Type and Size=
 
EclipseLink preserves object identity through its cache using the primary key attributes of a persistent entity. These attributes may or may not be assigned through sequencing (see [[Introduction%20to%20Projects (ELUG)#Projects and Sequencing|Projects and Sequencing]]). In a Java application, object identity is preserved if each object in memory is represented by one, and only one, object instance. Multiple retrievals of the same object return references to the same object instance–not multiple copies of the same object.
 
EclipseLink preserves object identity through its cache using the primary key attributes of a persistent entity. These attributes may or may not be assigned through sequencing (see [[Introduction%20to%20Projects (ELUG)#Projects and Sequencing|Projects and Sequencing]]). In a Java application, object identity is preserved if each object in memory is represented by one, and only one, object instance. Multiple retrievals of the same object return references to the same object instance–not multiple copies of the same object.
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====Full Identity Map====
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==Full Identity Map==
 
This option provides full caching and guaranteed identity: objects are never flushed from memory unless they are deleted.
 
This option provides full caching and guaranteed identity: objects are never flushed from memory unless they are deleted.
  
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====Weak Identity Map====
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==Weak Identity Map==
  
 
This option is similar to the full identity map, except that the map holds the objects by using weak references. This method allows full garbage collection and provides full caching and guaranteed identity.
 
This option is similar to the full identity map, except that the map holds the objects by using weak references. This method allows full garbage collection and provides full caching and guaranteed identity.
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====Soft Identity Map====
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==Soft Identity Map==
  
 
This option is similar to the weak identity map, except that the map uses soft references instead of weak references. This method allows full garbage collection and provides full caching and guaranteed identity.
 
This option is similar to the weak identity map, except that the map uses soft references instead of weak references. This method allows full garbage collection and provides full caching and guaranteed identity.
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====Soft Cache Weak Identity Map and Hard Cache Weak Identity Map====
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==Soft Cache Weak Identity Map and Hard Cache Weak Identity Map==
  
 
These options are similar to the weak identity map except that they maintain a most frequently used subcache. The subcache uses soft or hard references to ensure that these objects are garbage-collected only if the system is low on memory.
 
These options are similar to the weak identity map except that they maintain a most frequently used subcache. The subcache uses soft or hard references to ensure that these objects are garbage-collected only if the system is low on memory.
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For more information, see [[#What you may need to Know About the Internals of Weak, Soft, and Hard Identity Maps|What you may need to Know About the Internals of Weak, Soft, and Hard Identity Maps]].
 
For more information, see [[#What you may need to Know About the Internals of Weak, Soft, and Hard Identity Maps|What you may need to Know About the Internals of Weak, Soft, and Hard Identity Maps]].
  
====No Identity Map====
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==No Identity Map==
  
 
This option does not preserve object identity and does not cache objects.
 
This option does not preserve object identity and does not cache objects.
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====Guidelines for Configuring the Cache and Identity Maps====
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=Guidelines for Configuring the Cache and Identity Maps=
  
 
You can configure the cache at the project ([[Configuring%20a%20Project%20(ELUG)#Configuring Cache Type and Size at the Project Level|Configuring Cache Type and Size at the Project Level]]) or descriptor ([[Configuring%20a%20Descriptor%20(ELUG)#Configuring Cache Type and Size at the Descriptor Level|Configuring Cache Type and Size at the Descriptor Level]]) level.
 
You can configure the cache at the project ([[Configuring%20a%20Project%20(ELUG)#Configuring Cache Type and Size at the Project Level|Configuring Cache Type and Size at the Project Level]]) or descriptor ([[Configuring%20a%20Descriptor%20(ELUG)#Configuring Cache Type and Size at the Descriptor Level|Configuring Cache Type and Size at the Descriptor Level]]) level.
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}}
 
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====What You May Need to Know About the Internals of Weak, Soft, and Hard Identity Maps====
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==What You May Need to Know About the Internals of Weak, Soft, and Hard Identity Maps==
  
 
The <tt>WeakIdentiyMap</tt> and <tt>SoftIdentityMap</tt> use JVM weak and soft references to ensure that any object referenced by the application is held in the cache. Once the application releases its' reference to the object, the JVM is free to garbage collection the objects. When a weak and soft reference is garbage collected - is determined by the JVM. In general one could expect a weak reference to be garbage collected on each JVM garbage collector, and a soft reference to be garbage collected when the JVM determines memory is low.
 
The <tt>WeakIdentiyMap</tt> and <tt>SoftIdentityMap</tt> use JVM weak and soft references to ensure that any object referenced by the application is held in the cache. Once the application releases its' reference to the object, the JVM is free to garbage collection the objects. When a weak and soft reference is garbage collected - is determined by the JVM. In general one could expect a weak reference to be garbage collected on each JVM garbage collector, and a soft reference to be garbage collected when the JVM determines memory is low.
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When an object in a <tt>HardCacheWeakIdentityMap</tt> or <tt>SoftCacheWeakIdentityMap</tt> is pushed out of the reference cache, it gets put in the weak cache. Although it is still cached, EclipseLink cannot guarantee that it will be there for any length of time because the JVM can decide to garbage-collect weak references at anytime.
 
When an object in a <tt>HardCacheWeakIdentityMap</tt> or <tt>SoftCacheWeakIdentityMap</tt> is pushed out of the reference cache, it gets put in the weak cache. Although it is still cached, EclipseLink cannot guarantee that it will be there for any length of time because the JVM can decide to garbage-collect weak references at anytime.
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=@Cache Annotation=
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=<cache> XML=
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<source lang="xml">
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cache type="SOFT_WEAK"
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  size="1000"
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  shared="true"
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  always-refresh="false"
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  refresh-only-if-newer="true"
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  disable-hits="false"
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  coordination-type="INVALIDATE_CHANGED_OBJECTS">
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<expiry-time-of-day hour="0" minute="0" />
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/cache>
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</source>
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Revision as of 22:47, 17 June 2010

Cache Type and Size

EclipseLink preserves object identity through its cache using the primary key attributes of a persistent entity. These attributes may or may not be assigned through sequencing (see Projects and Sequencing). In a Java application, object identity is preserved if each object in memory is represented by one, and only one, object instance. Multiple retrievals of the same object return references to the same object instance–not multiple copies of the same object.

Maintaining object identity is extremely important when the application's object model contains circular references between objects. You must ensure that the two objects are referencing each other directly, rather than copies of each other. Object identity is important when multiple parts of the application may be modifying the same object simultaneously.

We recommend that you always maintain object identity. Disable object identity only if absolutely necessary, for example, for read-only objects (see Configuring Read-Only Descriptors).

You can configure how object identity is managed on a class-by-class basis. The ClassDescriptor object provides the cache and identity map options described in this table.


Cache and Identity Map Options

Option (Identity Map) Caching Guaranteed Identity Memory Use

Full Identity Map

Yes

Yes

Very High

Weak Identity Map

Yes

Yes

Low

Soft Identity Map

Yes

Yes

High

Soft Cache Weak Identity Map and Hard Cache Weak Identity Map

Yes

Yes

Medium-high

No Identity Map

No

No

None


For more information, see Guidelines for Configuring the Cache and Identity Maps.


Full Identity Map

This option provides full caching and guaranteed identity: objects are never flushed from memory unless they are deleted.

It caches all objects and does not remove them. Cache size doubles whenever the maximum size is reached. This method may be memory-intensive when many objects are read. Do not use this option on batch operations.

We recommend using this identity map when the data set size is small and memory is in large supply.


Weak Identity Map

This option is similar to the full identity map, except that the map holds the objects by using weak references. This method allows full garbage collection and provides full caching and guaranteed identity.

The weak identity map uses less memory than full identity map but also does not provide a durable caching strategy across client/server transactions. Objects are available for garbage collection when the application no longer references them on the server side (that is, from within the server JVM).


Soft Identity Map

This option is similar to the weak identity map, except that the map uses soft references instead of weak references. This method allows full garbage collection and provides full caching and guaranteed identity.

The soft identity map allows for optimal caching of the objects, while still allowing the JVM to garbage collect the objects if memory is low.


Soft Cache Weak Identity Map and Hard Cache Weak Identity Map

These options are similar to the weak identity map except that they maintain a most frequently used subcache. The subcache uses soft or hard references to ensure that these objects are garbage-collected only if the system is low on memory.

The soft cache weak identity map and hard cache weak identity map provide more efficient memory use. They release objects as they are garbage-collected, except for a fixed number of most recently used objects. Note that weakly cached objects might be flushed if the transaction spans multiple client/server invocations. The size of the subcache is proportional to the size of the identity map as specified by the ClassDescriptor method setIdentityMapSize. You should set this cache size to be as large as the maximum number of objects (of the same type) referenced within a transaction (see Configuring Cache Type and Size at the Descriptor Level).

We recommend using this identity map in most circumstances as a means to control memory used by the cache.

For more information, see What you may need to Know About the Internals of Weak, Soft, and Hard Identity Maps.

No Identity Map

This option does not preserve object identity and does not cache objects.

We do not recommend using the no identity map option. Instead, review the alternatives of cache invalidation and isolated caching.


Guidelines for Configuring the Cache and Identity Maps

You can configure the cache at the project (Configuring Cache Type and Size at the Project Level) or descriptor (Configuring Cache Type and Size at the Descriptor Level) level.

Use the following guidelines when configuring your cache and identity map:

  • If objects with a long life span and object identity are important, use a SoftIdentityMap, SoftCacheWeakIdentityMap or HardCacheWeakIdentityMap policy. For more information on when to choose one or the other, see What you may need to Know About the Internals of Weak, Soft, and Hard Identity Maps.
  • If object identity is important, but caching is not, use a WeakIdentityMap policy.
  • If an object has a long life span or requires frequent access, or object identity is important, use a FullIdentityMap policy.

Elug warning icon.png

Warning: Use the FullIdentityMap only if the class has a small number of finite instances. Otherwise, a memory leak will occur.

  • If an object has a short life span or requires frequent access, and identity is not important, use a CacheIdentityMap policy.
  • If objects are discarded immediately after being read from the database, such as in a batch operation, use a NoIdentityMap policy. The NoIdentityMap does not preserve object identity.

Elug note icon.png

Note: We do not recommend the use of CacheIdentityMap and NoIdentityMap policies.

What You May Need to Know About the Internals of Weak, Soft, and Hard Identity Maps

The WeakIdentiyMap and SoftIdentityMap use JVM weak and soft references to ensure that any object referenced by the application is held in the cache. Once the application releases its' reference to the object, the JVM is free to garbage collection the objects. When a weak and soft reference is garbage collected - is determined by the JVM. In general one could expect a weak reference to be garbage collected on each JVM garbage collector, and a soft reference to be garbage collected when the JVM determines memory is low.

The SoftCacheWeakIdentityMap and HardCacheWeakIdentityMap types of identity map contain the following two caches:

  • Reference cache: implemented as a LinkedList that contains soft or hard references, respectively.
  • Weak cache: implemented as a HashMap that contains weak references.

When you create a SoftCacheWeakIdentityMap or HardCacheWeakIdentityMap with a specified size, the reference cache LinkedList is exactly this size. The weak cache HashMap is initialized to 100 percent of the specified size: the weak cache will grow when more objects than the specified size are read in. Because EclipseLink does not control garbage collection, the JVM can reap the weakly held objects whenever it sees fit.

Because the reference cache is implemented as a LinkedList, new objects are added to the end of the list. Because of this, it is by nature a least recently used (LRU) cache: fixed size, object at the top of the list is deleted, provided the maximum size has been reached.

The SoftCacheWeakIdentityMap and HardCacheWeakIdentityMap are essentially the same type of identity map. The HardCacheWeakIdentityMap was constructed to work around an issue with some JVMs.

If your application reaches a low system memory condition frequently enough, or if your platform's JVM treats weak and soft references the same, the objects in the reference cache may be garbage-collected so often that you will not benefit from the performance improvement provided by it. If this is the case, we recommend that you use the HardCacheWeakIdentityMap. It is identical to the SoftCacheWeakIdentityMap except that it uses hard references in the reference cache. This guarantees that your application will benefit from the performance improvement provided by it.

When an object in a HardCacheWeakIdentityMap or SoftCacheWeakIdentityMap is pushed out of the reference cache, it gets put in the weak cache. Although it is still cached, EclipseLink cannot guarantee that it will be there for any length of time because the JVM can decide to garbage-collect weak references at anytime.


@Cache Annotation

<cache> XML

 cache type="SOFT_WEAK" 
	   size="1000" 
	   shared="true"
	   always-refresh="false" 
	   refresh-only-if-newer="true" 
	   disable-hits="false" 
	   coordination-type="INVALIDATE_CHANGED_OBJECTS">
	<expiry-time-of-day hour="0" minute="0" />
 /cache>



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