Release 2 (9.0.2)
Part Number A95879-01
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| Oracle9iAS Containers for J2EE Services Guide Release 2 (9.0.2) Part Number A95879-01 |
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This chapter describes the Oracle9iAS Containers for J2EE (OC4J) Java Object Cache, including its architecture and programming features.
This chapter covers the following topics:
Oracle9iAS offers the Java Object Cache to help e-businesses manage Web-site performance issues for dynamically generated content. The Java Object Cache improves the performance, scalability, and availability of Web sites running on Oracle9iAS.
By storing frequently accessed or expensive-to-create objects in memory or on disk, the Java Object Cache eliminates the need to repeatedly create and load information within a Java program. The Java Object Cache retrieves content faster and greatly reduces the load on application servers.
The Oracle9iAS cache architecture includes the following cache components:
The Web Cache is an HTTP-level cache, maintained outside the application, providing very fast cache operations. It is a pure, content-based cache, capable of caching static data (such as HTML, GIF, or JPEG files) or dynamic data (such as servlet or JSP results). Given that it exists as a flat content-based cache outside the application, it cannot cache objects (such as Java objects or XML DOM objects) in a structured format. In addition, it offers relatively limited post-processing abilities on cached data.
A custom tag library or API allows you to define page fragment boundaries and to capture, store, reuse, process, and manage the intermediate and partial execution results of JSP pages and servlets as cached objects. Each block can produce its own resulting cache object. The cached objects can be HTML or XML text fragments, XML DOM objects, or Java serializable objects. These objects can be cached conveniently in association with HTTP semantics. Alternatively, they can be reused outside HTTP, such as in outputting cached XML objects through Simple Mail Transfer Protocol (SMTP), Java Messaging Service (JMS), Advanced Queueing (AQ), or Simple Object Access Protocol (SOAP).
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Note: This chapter focuses on the Java Object Cache. For a full discussion of all three caches and their differences, see the Oracle9iAS Containers for J2EE JSP Tag Libraries and Utilities Reference. |
For a programmer using the Java Object Cache, information has one of three characteristics:
Hashtable.
Figure 13-1 shows the basic architecture for the Java Object Cache. The cache delivers information to a user process. The process could be a servlet application that generates HTML pages or any other Java application.
For simplicity, availability, and performance, the Java object cache is specific to each process (object creation is not centrally controlled). However, using distributed object management, the Java Object Cache provides coordination of updates and invalidations between processes. If an object is updated or invalidated in one process, it is also updated or invalidated in all other associated processes. This distributed management allows a system of processes to stay synchronized, without the overhead of centralized control.
Figure 13-2 shows the architecture for the Java Object Cache, using distributed object management. The cache delivers information to a user process. The user process could be a servlet application that generates HTML pages or any other Java application. Using the distributed object management message layer, the application uses the Java Object Cache to share the information across processes and between caches.
The Java Object Cache manages Java objects within a process, across processes, or on a local disk. The Java Object Cache provides a powerful, flexible, and easy-to-use service that significantly improves Java performance by managing local copies of Java objects. There are very few restrictions on the types of Java objects that can be cached or on the original source of the objects. Programmers use the Java Object Cache to manage objects that, without cache access, are expensive to retrieve or to create.
The Java Object Cache is easy to integrate into new and existing applications. Objects can be loaded into the object cache, using a user-defined object, the CacheLoader, and can be accessed through a CacheAccess object. The CacheAccess object supports local and distributed object management. Most of the functionality of the Java Object Cache does not require administration or configuration. Advanced features support configuration using administration application programming interfaces (APIs) in the Cache class. Administration includes setting configuration options, such as naming local disk space or defining network ports. The administration features allow applications to fully integrate the Java Object Cache.
Each cached Java object has a set of associated attributes that control how the object is loaded into the cache, where the object is stored, and how the object is invalidated. Cached objects are invalidated based on time or an explicit request (notification can be provided when the object is invalidated). Objects can be invalidated by group or individually.
Figure 13-3 shows the basic Java Object Cache APIs. Figure 13-3 does not show distributed cache management.
The Java Object Cache is organized as follows:
StreamAccess objects.
Table 13-1 provides a summary of the constructs in the cache environment and the cache object types.
The Java Object Cache provides the following features:
This section describes the object types that the Java Object Cache manages, including:
Memory objects are Java objects that the Java Object Cache manages. Memory objects are stored in the Java VM's heap space as Java objects. Memory objects can hold HTML pages, the results of a database query, or any information that can be stored as a Java object.
Memory objects are usually loaded into the Java Object Cache with an application-supplied loader. The source of the memory object may be controlled externally (for example, using data in a table on the Oracle9i Database Server). The application supplied loader accesses the source and either creates or updates the memory object. Without the Java Object Cache, the application would be responsible for accessing the source directly, rather than using the loader.
You can update memory objects by obtaining a private copy of the memory object, applying the changes to the copy, and then placing the updated object back in the cache (using CacheAccess.replace()).
The CacheAccess.defineObject() method associates attributes with an object. If attributes are not defined, the object inherits the default attributes from its associated region, subregion, or group.
An application can request that a memory object be spooled to a local disk (using the SPOOL attribute). Setting this attribute allows the Java Object Cache to handle memory objects that are large, or costly to re-create and seldom updated. When the disk cache is set up to be significantly larger than the memory cache, objects on disk usually stay in the disk cache longer than objects in memory.
Combining memory objects that are spooled to a local disk with the distributed feature from the DISTRIBUTE attribute provides object persistence (when the Java Object Cache is running in distributed mode). Object persistence allows you to re-create objects when the system or the Java VM is restarted after the process fails or shuts down.
There are very few restrictions on Java Object Cache memory objects. Memory objects can contain any Java object.
Disk objects are stored on a local disk and are accessed directly from the disk by the application using the Java Object Cache. Disk objects may be shared by all Java Object Cache processes, or they may be local to a particular process, depending on the setting for the DISTRIBUTE attribute (and whether the Java Object Cache is running in distributed or local mode).
Disk objects can be invalidated explicitly or by setting the TimeToLive or IdleTime attributes. Disk objects can be updated by obtaining a private copy of the disk object (file). When the Java Object Cache requires additional space, disk objects that are not being referenced may be removed from the cache.
There are very few restrictions on disk objects in the Java Object Cache.
StreamAccess objects are objects that are accessed as a stream, and are automatically loaded to the disk cache. The object is loaded as an OutputStream and read as an InputStream. The Java Object Cache determines how to access the StreamAccess object based on the size of the object and the capacity of the cache. Smaller objects are accessed from memory, while larger objects are streamed directly from disk.
The cache user's access to the StreamAccess object is through an InputStream. All the attributes that apply to memory objects and disk objects also apply to StreamAccess objects. A StreamAccess object does not provide a mechanism to manage a stream; for example, StreamAccess objects cannot manage socket endpoints. InputStream and OutputStream objects are available to access fixed sized, potentially very large objects.
The Java Object Cache places some restrictions on StreamAccess objects.
A pool object is a special class of objects that the Java Object Cache manages. A pool object contains a set of identical object instances. The pool object itself is a shared object, while the objects within the pool are private objects. Individual objects within the pool can be checked out to be used and then returned to the pool when they are no longer needed.
Attributes, including TimeToLive or IdleTime may be associated with a pool object. These attributes apply to the pool object as a whole, or they can be applied to the objects within the pool individually.
The Java Object Cache instantiates objects within a pool using an application-defined factory object. The size of a pool decreases or increases based on demand and on the values of the TimeToLive or IdleTime attributes. A minimum size for the pool is specified when the pool is created. The minimum-size value is interpreted as a request rather than a guaranteed minimum value. Objects within a pool object are subject to removal from the cache due to lack of space, so the pool may decrease below the requested minimum value. A maximum pool size value can be set that puts a hard limit on the number of objects available in the pool.
The Java Object Cache environment includes the following:
This section describes these Java Object Cache environment constructs.
Objects that use the Java Object Cache service are managed within a cache region. A cache region defines a name space within the cache. Each object within a cache region must be uniquely named, and the combination of the cache region name and the object name must uniquely identify an object. Thus, cache region names must be unique from other region names, and all objects within a region must be uniquely named relative to the region (multiple objects can have the same name if they are within different regions or subregions).
You can define as many regions as you need to support your application. However, most applications only require one region. The Java Object Cache provides a default region; when a region is not specified, objects are placed in the default region.
Attributes may be defined for a region and are then inherited by the objects, subregions, and groups within the region.
Objects that use the Java Object Cache are managed within a cache region. Specifying a subregion within a cache region defines a child hierarchy. A cache subregion defines a name space within a cache region, or cache subregion. Each object within a cache subregion must be uniquely named, and the combination of the cache region name, the cache subregion name, and the object name must uniquely identify an object.
You can define as many subregions as you need to support your application.
A subregion inherits its attributes from its parent region or subregion unless the attributes are defined when the subregion is defined. A subregion's attributes are inherited by the objects within the subregion. If a subregion's parent region is invalidated or destroyed, the subregion is also invalidated or destroyed.
A cache group creates an association between objects within the Java Object Cache. Cache groups allow related objects to be manipulated together. Objects are typically associated in a cache group because they need to be invalidated together or they use common attributes. Any set of cache objects within the same region or subregion can be associated using a cache group, which may in turn, include other cache groups.
An Java Object Cache object can only belong to one group at any given time. Before an object can be associated with a group, the group must be explicitly created. A group is defined with a name. A group may have its own attributes, or it may inherit its attributes from its parent region, subregion, or group.
Group names are not used to identify individual objects. A group defines a set or collection of objects that have something in common. A group does not define a hierarchical name space. Object type does not distinguish objects for naming purposes; therefore, a region cannot include a group and a memory object with the same name. Use subregions to define a hierarchical name space within a region.
Groups can contain groups, with the groups having a parent and child relationship. The child group inherits attributes from the parent group.
Cache object Attributes (Attributes) affect how the Java Object Cache manages objects. Each object type, region, subregion, and group has a set of associated attributes. An object's applicable attributes contain either the default attribute values; the attribute values inherited from the object's parent region, subregion, or group; or the attribute values that you select for the object.
Attributes fall into two categories:
LOADER attribute. Use the Attributes.setFlags() method to set these attributes.
Some attributes do not apply to certain types of objects. See Object Types sections in the descriptions in Table 13-2 and Table 13-3.
Note:
The attributes shown in Table 13-2 must be defined on an object before the object is loaded. These attributes determine an object's basic management characteristics.
The following list shows the methods you can use to set the attributes shown in Table 13-2 (by setting the values of an Attributes object argument).
CacheAccess.defineRegion()
CacheAccess.defineSubRegion()
CacheAccess.defineGroup()
CacheAccess.defineObject()
CacheAccess.put()
CacheAccess.createPool()
CacheLoader.createDiskObject()
CacheLoader.createStream()
CacheLoader.SetAttributes()
You cannot reset the attributes shown in Table 13-2 by using the
Note:
CacheAccess.resetAttributes()method.
Table 13-2 Java Object Cache Attributes-Set at Object Creation
A set of Java Object Cache attributes can be modified either before or after object loading. Table 13-3 lists these attributes. These attributes can be set using the methods listed in the list shown before Table 13-2, and can be reset using the CacheAccess.resetAttributes() method.
This section describes how to develop applications that use Java Object Cache. This section covers the following topics:
The Oracle installer installs the Java Object Cache jar file cache.jar in the directory $ORACLE_HOME/javacache/lib on UNIX or in %ORACLE_HOME%\javacache\lib on Windows NT.
To use the Java Object Cache, you need to import oracle.ias.cache.
import oracle.ias.cache.*;
All access to the Java Object Cache is through a CacheAccess object. A CacheAccess object provides access to the cache through a cache region. You define a cache region, usually associated with the name of an application, using the CacheAccess.defineRegion()static method. If the cache has not been initialized, defineRegion() initializes the Java Object Cache.
When you define the region, you can also set attributes and create a CacheLoader object. Attributes specify how the Java Object Cache manages objects. The Attributes.setLoader() method sets the name of CacheLoader.
Attributes attr = new Attributes(); MyLoader mloader = new MyLoader; attr.setLoader(mloader); attr.setDefaultTimeToLive(10); final static String APP_NAME_ = "Test Application"; CacheAccess.defineRegion(APP_NAME_, attr);
The first argument for defineRegion uses a String to set the region name. This static method creates a private region name within the Java Object Cache. The second argument defines the attributes for the new region.
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See Also:
"Java Object Cache Environment" and "Implementing a CacheLoader" |
When you want to create an association between two or more objects within the cache, create a cache group. Objects are typically associated in a cache group because they need to be invalidated together or because they have a common set of attributes.
Any set of cache objects within the same region or subregion can be associated using a cache group, including other cache groups. Before an object can be associated with a cache group, the cache group must be defined. A cache group is defined with a name and can use its own attributes, or it can inherit attributes from its parent cache group, subregion, or region. The following code defines a cache group within the region named "Test Application".
final static String APP_NAME_ = "Test Application"; final static String GROUP_NAME_ = "Test Group"; // obtain an instance of CacheAccess object to a named region CacheAccess caccess = CacheAccess.getAccess(APP_NAME_); // Create a group caccess.defineGroup(GROUP_NAME_); // Close the CacheAccess object caccess.close();
Define a subregion when you want to create a private name space within a region or within a previously defined subregion. A subregion's name space is independent of the parent name space. A region can contain two objects with the same name, as long as the objects are within different subregions.
A subregion can contain anything that a region can contain, including cache objects, groups, or additional subregions. Before an object can be associated with a subregion, the subregion must be defined. A cache subregion is defined with a name and can use its own attributes, or it can inherit attributes from its parent cache region or subregion. Use the getParent() method to obtain a subregion's parent.
In the following example, cache subregion is defined within the region named "Test Application".
final static String APP_NAME_ = "Test Application"; final static String SUBREGION_NAME_ = "Test SubRegion"; // obtain an instance of CacheAccess object to a named region CacheAccess caccess = CacheAccess.getAccess(APP_NAME_); // Create a SubRegion caccess.defineSubRegion(SUBREGION_NAME_); // Close the CacheAccess object caccess.close();
You may sometimes want to describe to the Java Object Cache how an individual object should be managed within the cache before the object is loaded. Management options can be specified when the object is loaded, by setting attributes within the CacheLoader.load() method. However, you can also associate attributes with an object by using the CacheAccess.defineObject() method. If attributes are not defined for an object, the Java Object Cache uses the default attributes set for the region, subregion, or group with which the object is associated.
Example 13-1 shows how to set attributes for a cache object.
import oracle.ias.cache.*; final static String APP_NAME_ = "Test Application"; CacheAccess cacc = null; try { cacc = CacheAccess.getAccess(APP_NAME_); // set the default IdleTime for an object using attributes Attributes attr = new Attributes(); // set IdleTime to 2 minutes attr.setIdleTime(120); // define an object and set its attributes cacc.defineObject("Test Object", attr); // object is loaded using the loader previously defined on the region // if not already in the cache. result = (String)cacc.get("Test Object"); } catch (CacheException ex){ // handle exception } finally { if (cacc!= null) cacc.close(); }
Generally, you should use the Java Object Cache to load objects automatically, as needed rather than using the application to directly manage objects in the cache. When an application directly manages objects, it uses the CacheAccess.put() method to insert objects into the cache. To take advantage of automatic loading, you use a CacheLoader object and implement a load() method to insert objects into the cache.
A CacheLoader can be associated with a region, subregion, a group, or an object. Using a CacheLoader allows the Java Object Cache to schedule and manage object loading, and handle the logic for, "if the object is not in cache then load."
When an object is not in the cache, when an application calls CacheAccess.get() or CacheAccess.preLoad(), the CacheLoader executes the load method. When the load method returns, the Java Object Cache inserts the returned object into the cache. Using CacheAccess.get(), if the cache is full the object is returned from the loader and the object is immediately invalidated in the cache (therefore, using CacheAccess.get() with a full cache does not generate a CacheFullException).
When a CacheLoader is defined for a region, subregion, or group, it is taken to be the default loader for all objects associated with the region, subregion, or group. A CacheLoader that is defined for an individual object is used only to load the object.
The Java Object Cache supports several CacheLoader methods that you can use within a load() method implementation. Table 13-4 summarizes the available CacheLoader methods.
Example 13-2 shows a CacheLoader using the cacheLoader.netSearch() method to check if the object being loaded is available in distributed Java Object Cache caches. If the object is not found using netSearch(), the load method uses a more expensive call to retrieve the object (an expensive call might involve an HTTP connection to a remote Web site or a connection to the Oracle9i Database Server). For this example, the Java Object Cache stores the result as a String.
import oracle.ias.cache.*; class YourObjectLoader extends CacheLoader{ public YourObjectLoader () { } public Object load(Object handle, Object args) throws CacheException { String contents; // check if this object is loaded in another cache try { contents = (String)netSearch(handle, 5000);// wait for up to 5 scnds return new String(contents); } catch(ObjectNotFoundException ex){} try { contents = expensiveCall(args); return new String(contents); } catch (Exception ex) {throw exceptionHandler("Loadfailed", ex);} } private String expensiveCall(Object args) { String str = null; // your implementation to retrieve the information. // str = ... return str; } }
An object can be removed from the cache either by setting the TimeToLive attribute for the object, group, subregion, or region; or by explicitly invalidating or destroying the object.
Invalidating an object marks the object for removal from the cache. Invalidating a region, subregion, or a group invalidates all the individual objects from the region, subregion, or group, leaving the environment, including all groups, loaders, and attributes available in the cache. Invalidating an object does not undefine the object. The object loader remains associated with the name. To completely remove an object from the cache, destroy the object using the CacheAccess.destroy() method.
An object may be invalidated automatically based on the TimeToLive or IdleTime attributes. When the TimeToLive or IdleTime expires, objects are by default, invalidated and not destroyed.
If an object, group, subregion, or region is defined as distributed, the invalidate request is propagated to all caches in the distributed environment.
To invalidate an object, group, subregion, or region use CacheAccess.invalidate().
CacheAccess cacc = CacheAccess.getAccess("Test Application"); cacc.invalidate("Test Object"); // invalidate an individual object cacc.invalidate("Test Group"); // invalidate all objects associated with a group cacc.invalidate(); // invalidate all objects associated with the region cacc cacc.close(); // close the CacheAccess access
An object can be removed from the cache either by setting the TimeToLive attribute for the object, group, subregion, or region; or by explicitly invalidating or destroying the object.
Destroying an object marks the object and the associated environment, including any associated loaders, event handlers, and attributes for removal from the cache. Destroying a region, subregion, or a group marks all objects associated with the region, subregion, or group for removal, including the associated environment.
An object may be destroyed automatically based on the TimeToLive or IdleTime attributes. By default, objects are invalidated and are not destroyed. If the objects need to be destroyed, set the attribute GROUP_TTL_DESTROY. Destroying a region also closes the CacheAccess object used to access the region.
To destroy an object, group, subregion, or region use the CacheAccess.destroy() method.
CacheAccess cacc = CacheAccess.getAccess("Test Application"); cacc.destroy("Test Object"); // destroy an individual object cacc.destroy("Test Group"); // destroy all objects associated with // the group "Test Group" cacc.destroy(); // destroy all objects associated with the region // including groups and loaders
During initialization, the Java Object Cache sets values for configuration properties. Table 13-5 lists the configuration properties for Java Object Cache. By default, the first time a region is created, or the default region is accessed, the Java Object Cache initializes the configuration properties. When the Java Object Cache is installed, the installer updates values for certain administrative properties and places the updated values in the javacache.properties configuration file, in the directory $ORACLE_HOME/javacache/admin on UNIX or in %ORACLE_HOME\javacache\admin on Windows NT.
You can modify the javacache.properties file to use values other than the default configuration property values. For configuration property values that are not specified in javacache.properties, the Java Object Cache uses the default values included in Table 13-5.
When the Java Object Cache is initialized, it uses either the default administration property values, or values specified in javacache.properties. No explicit method calls are required to configure the administrative properties using this initialization technique. The Java Object Cache also supports other initialization techniques (see the Cache object methods in the Javadoc for details).
The format for the values in the properties javacache.properties file is:
property=value
A # character in a configuration file starts a comment. When the # is in the first column, the entire line is a comment. When the # is occurs after a property value specification, it applies to the remainder of the line.
Table 13-5 lists the valid property names and lists the valid types for each property.
There are a number of events that can occur in the life cycle of a cached object, including object creation and object invalidation. This sections shows how an application can be notified when cache events occur.
To receive notification of an object's creation, implement event notification as part of the cacheLoader. For notification of invalidation or updates, implement a CacheEventListener and associate the CacheEventListener with an object, group, region, or subregion using Attributes.setCacheEventListener().
CacheEventListener is an interface that extends java.util.EventListener. The cache event listener provides a mechanism to establish a callback method that is registered, and then executes when the event occurs. In the Java Object Cache, the event listener executes when a cached object is invalidated or updated.
An event listener is associated with a cached object, group, region, or subregion. If an event listener is associated with a group, region, or subregion, the listener only runs when the group, region, or subregion itself is invalidated. Invalidating a member does not trigger the event. Attributes.setCacheEventListener() takes a boolean argument, that if true, applies the event listener to each member of the region, subregion, or group, rather than to the region, subregion, or group itself. In this case, the invalidation of an object within the region, subregion, or group triggers the event.
The CacheEventListener interface has one method, handleEvent(). This method takes a single argument, a CacheEvent object that extends java.util.EventObject. This object has two methods getID(),which returns the type of event (OBJECT_INVALIDATION or OBJECT_UPDATED), and getSource(), which returns the object being invalidated. For group objects, the getSource() method returns the name of the group.
The handleEvent() method is executed in the context of a background thread that the Java Object Cache manages. Avoid using JNI code in this method, as the expected thread context may not be available.
Example 13-3 shows how a CacheEventListener is implemented and associated with an object or a group.
import oracle.ias.cache.*; // A CacheEventListener for a cache object class MyEventListener implements CacheEventListener { public void handleEvent(CacheEvent ev) { MyObject obj = (MyObject)ev.getSource(); obj.cleanup(); } // A CacheEventListener for a group object class MyGroupEventListener implements CacheEventListener { public void handleEvent(CacheEvent ev) { String groupName = (String)ev.getSource(); app.notify("group " + groupName + " has been invalidated"); } }
Use the Attributes.listener attribute to specify the CacheEventListener for a region, subregion, group, or object.
Example 13-4 shows how to set a cache event listener on an object. Example 13-5 shows how to set a cache event listener on a group.
import oracle.ias.cache.*; class YourObjectLoader extends CacheLoader { public YourObjectLoader () { } public Object load(Object handle, Object args) { Object obj = null; Attributes attr = new Attributes(); MyEventListener el = new MyEventListener(); attr.setCacheEventListener(CacheEvent.OBJECT_INVALIDATED, el); // your implementation to retrieve or create your object setAttributes(handle, attr); return obj; } }
import oracle.ias.cache.*; try { CacheAccess cacc = CacheAccess.getAccess(myRegion); Attributes attr = new Attributes (); MyGroupEventListener listener = new MyGroupEventListener(); attr.setCacheEventListener(CacheEvent.OBJECT_INVALIDATED, listener); cacc.defineGroup("myGroup", attr); //.... cacc.close(); }catch(CacheException ex) { // handle exception }
This section covers restrictions and programming pointers to keep in mind when using the Java Object Cache.
CacheAccess object should not be shared between threads. This object represents a user to the caching system. The CacheAccess object contains the current state of the user's access to the cache: what object is currently being accessed, what objects are currently owned, and so on. Trying to share the CacheAccess object is unnecessary and can result in nondeterministic behavior.
CacheAccess object only holds a reference to one cached object at a time. If multiple cached objects are being accessed concurrently, multiple CacheAccess objects should be used. For objects stored in memory, the consequences of not doing this are minor since Java prevents the cached object from being garbage collected even if the cache believes it is not being referenced. For disk objects, if the cache reference is not maintained, the underlying file could be removed by another user or by time-based invalidation, causing unexpected exceptions. To optimize resource management, you should keep the cache reference open as long as the cached object is being used.
CacheAccess object should always be closed when it is no longer being used. The CacheAccess objects are pooled. They acquire other cache resources on behalf of the user. If the access object is not closed when it is not being used, these resources are not returned to the pool and are not cleaned up until they are garbage collected by the Java VM. If CacheAccess objects are continually allocated and not closed, available resources and a consequent degradation in performance may occur.
Attributes.DISTRIBUTE attribute) are saved to disk using the CacheAccess.save() method they do not survive the termination of the process. By definition, local objects are only visible to the cache instance where they were loaded. If that cache instance goes away for any reason, the objects it manages, including on disk, are lost. If an object needs to survive process termination, both the object and the cache need to be defined DISTRIBUTE.
CacheAccess.waitForResponse() or CacheAccess.releaseOwnership() method call times out, it must be called again until it returns successfully. Call these methods with a -1 timeout value to free up resources, and eliminate waits.
SYNCHRONIZE attribute set, ownership is required to load or replace the object. However, ownership is not required for general access to the object or to invalidate the object.
CLASSPATH when the Java VM is initialized, rather than by a user defined class loader. Specifically, any objects that are shared between applications or may be saved or spooled to disk need to be defined in the system CLASSPATH. Failure to do so may result in ClassNotFoundExceptions or ClassCastExceptions.
1024 or greater may be an appropriate value for the number of open file descriptors.
The Java Object Cache can manage objects on disk as well as in memory.
This section covers the following topics:
To configure the Java Object Cache to use a disk cache, set the value of the diskPath configuration property in the javacache.properties file.
To configure the Java Object Cache to use a disk cache, the diskPath property in the configuration properties file should be set to the path of the root directory for the disk cache. The default value for diskPath is null, which specifies that the Java Object Cache should not enable the disk cache.
This section covers the following topics:
When operating in local mode, all objects are treated as local objects (even when the DISTRIBUTE attribute is set for an object). In local mode, all objects in the disk cache are only visible to the Java Object Cache cache that loaded them, and they do not survive after process termination. In local mode, objects stored in the disk cache are lost when the process using the cache dies.
When operating in distributed mode, disk cache objects are shared by all caches that have access to the file system hosting the disk cache. This configuration allows for better utilization of disk resources and allows disk objects to persist beyond the life of the Java Object Cache process. Distributed memory objects are not shared by all caches since individual copies of each memory object reside in the individual caches across the system.
Objects stored in the disk cache are identified using the concatenation of the path specified in the diskPath configuration property and an internally generated String representing the remaining path to the file. Thus, caches that share a disk cache can have a different directory structure, as long as the diskPath represents the same directory on the physical disk and is accessible to the Java Object Cache processes.
If a memory object that is saved to disk is also distributed, the memory object can survive the death of the process that spooled it.
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See Also:
"Automatically Adding Objects" for information on using the |
There are several ways to use the disk cache with the Java Object Cache, including:
The Java Object Cache automatically adds certain objects to the disk cache. Such objects may reside either in the memory cache or in the disk cache. If an object in the disk cache is needed, it is copied back to the memory cache. The action of spooling to disk occurs when the Java Object Cache determines that it requires free space in the memory cache. The Java Object Cache automatically moves objects from the memory cache to the disk cache in two cases.
SPOOL attribute set, the memory object is written to disk before it is removed. Spooling saves the memory object to the disk cache, and avoids re-creating the object when or if it is needed again. You should set the SPOOL attribute for objects that are expensive to create, especially if the time required to create the object is greater than the cost of loading the object from disk.
"Cache Object Attributes" and "Working with StreamAccess Objects"
See Also:
In some situations, you may want to force one or more objects to be written to the Java Object Cache disk cache. Using the CacheAccess.save() method, a region, subregion, group, or object is synchronously written to the disk cache (if the object or objects are already in the disk cache, they are not written again).
Calling CacheAccess.save() on a region, subregion, or group saves all the objects within the region, subregion, or group to the disk cache. During a CacheAccess.save() method call, if an object is encountered that cannot be written to disk, either because it is not serializable, or for other reasons, the event is recorded in the Java Object Cache log and the save operation continues with the next object.
Objects that you only access directly from disk cache are loaded into the disk cache by calling CacheLoader.createDiskObject() from the CacheLoader.load() method. The createDiskObject() method returns a File object that the application can use to load the disk object. If the disk object's attributes are not defined for the disk object, set them using the createDiskObject() method. The system manages local and distributed disk objects differently; the determination of local or distributed is made when the system creates the object, based on the specified attributes.
When CacheAccess.get() is called on a disk object, the full path name to the file is returned, and the application can open the file, appropriate to its needs.
Disk objects are stored on a local disk and accessed directly from the disk by the application using the Java Object Cache. Disk objects may be shared by all Java Object Cache processes, or they may be local to a particular process, depending on the setting for the DISTRIBUTE attribute (and the mode the Java Object Cache is running in, either distributed, or local).
Example 13-6 shows a loader object that loads a disk object into the cache.
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See Also:
"Implementing a CacheLoader" and "Java Object Cache Environment" |
import oracle.ias.cache.*; class YourObjectLoader extends CacheLoader { public Object load(Object handle, Object args) { File file; FileOutputStream = out; Attributes attr = new Attributes(); attr.setFlags(Attributes.DISTRIBUTE); try { file = createDiskObject(handle, attr); out = new FileOutputStream(file); out.write((byte[])getInfofromsomewhere()); out.close(); } catch (Exception ex) { // translate exception to CacheException, and log exception throw exceptionHandler("exception in file handling", ex) } return file; } }
Example 13-7 shows application code that uses an Java Object Cache disk object. This example assumes the region named "Stock-Market" is already defined with the "YourObjectLoader" loader set up in Example 13-6 as the default loader for the region.
import oracle.ias.cache.*; try { FileInputStream in; File file; String filePath; CacheAccess cacc = CacheAccess.getAccess("Stock-Market"); filePath = (String)cacc.get("file object"); file = new File(filePath); in = new FileInputStream(filePath); in.read(buf); // do something interesting with the data in.close(); cacc.close(); } catch (Exception ex) { // handle exception }
StreamAccess objects are objects that are accessed as a stream and are automatically loaded to the disk cache. The object is loaded as an OutputStream and read as an InputStream. Smaller StreamAccess objects can be accessed from memory or from the disk cache, while larger StreamAccess objects are streamed directly from disk. The Java Object Cache automatically determines where to access the StreamAccess object based on the size of the object and the capacity of the cache.
The user is always presented with a stream object, an InputStream for reading and an OutputStream for writing, regardless of whether the object is in a file or in memory. The StreamAccess object allows the Java Object Cache user to always access the object in a uniform manner, without regard to object size or resource availability.
To create a StreamAccess object, call the CacheLoader.createStream() method from the CacheLoader.load() method when the object is loaded into the cache. The createStream() method returns an OutputStream object. The OutputStream object can be used to load the object into the cache.
If the attributes have not already been defined for the object, they should be set using the createStream() method. The system manages local and distributed disk objects differently; the determination of local or distributed is made when the system creates the object, based on the attributes.
Example 13-8 shows a loader object that loads a StreamAccess object into the cache.
import oracle.ias.cache.*; class YourObjectLoader extends CacheLoader { public Object load(Object handle, Object args) { OutputStream = out; Attributes attr = new Attributes(); attr.setFlags(Attributes.DISTRIBUTE); try { out = createStream(handle, attr); out.write((byte[])getInfofromsomewhere()); } catch (Exception ex) { // translate exception to CacheException, and log exception throw exceptionHandler("exception in write", ex) } return out; } }
A pool object is a special cache object that the Java Object Cache manages. A pool object contains a set of identical object instances. The pool object itself is a shared object, stored as a static across the entire cache instance, while the objects within the pool object are private objects that the Java Object Cache manages. Users access individual objects within the pool with a check out, using a pool access object, and then return the objects to the pool when they are no longer needed.
This section covers the following topics:
To create a pool object, use CacheAccess.createPool(). The CreatePool() method takes as arguments a PoolInstanceFactory, and an Attributes object, plus two integer arguments. The integer arguments specify the maximum pool size and the minimum pool size. By supplying a group name as an argument to CreatePool(), a pool object is associated with a group.
Attributes, including TimeToLive or IdleTime may be associated with a pool object. These attributes can be applied to the pool object itself, when specified in the attributes set with CacheAccess.createPool(), or they can be applied to the objects within the pool individually.
Using CacheAccess.createPool(), specify minimum and maximum sizes with the integer arguments. The minimum is specified first. It sets the minimum number of objects to create within the pool. The minimum size is interpreted as a request rather than a guaranteed minimum. Objects within a pool object are subject to removal from the cache due to lack of resources, so the pool may decrease the number of objects below the requested minimum value. The maximum pool size puts a hard limit on the number of objects available in the pool.
Example 13-9 shows how to create a pool object.
import oracle.ias.cache.*; try { CacheAccess cacc = CacheAccess.getAccess("Stock-Market"); Attributes attr = new Attributes(); QuoteFactory poolFac = new QuoteFactory(); // set IdleTime for an object in the pool to three minutes attr.setIdleTime(180); // create a pool in the "Stock-Market" region with a minimum of // 5 and a maximum of 10 object instances in the pool cacc.createPool("get Quote", poolFac, attr, 5, 10); cacc.close(); } catch(CacheException ex) { // handle exception } }
To access objects in a pool, use a PoolAccess object. The PoolAccess.getPool() static method returns a handle to a specified pool. The PoolAccess.get() method returns an instance of an object from within the pool (this checks out an object from the pool). When an object is no longer needed, return it to the pool, using the PoolAccess.returnToPool() method, which checks the object back into the pool. Finally, call the PoolAccess.close() method when the pool handle is no longer needed.
Example 13-10 shows the calls required to create a PoolAccess object, check an object out of the pool, and then check the object back in and close the PoolAccess object.
PoolAccess pacc = PoolAccess.getPool("Stock-Market", "get Quote"); //get an object from the pool GetQuote gq = (GetQuote)pacc.get(); // do something useful with the gq object // return the object to the pool pacc.returnToPool(gq); pacc.close();
The Java Object Cache instantiates and removes objects within a pool, using an application-defined factory object, a PoolInstanceFactory. The PoolInstanceFactory is an abstract class with two methods that you must implement, createInstance() and destroyInstance().
The Java Object Cache calls createInstance() to create instances of objects being accumulated within the pool. The Java Object Cache calls destroyInstance() when an instance of an object is being removed from the pool (object instances from within the pool are passed into destroyInstance()).
The size of a pool object, that is the number of objects within the pool, is managed using these PoolInstanceFactory() methods. The system decreases or increases the size and number of objects in the pool, based on demand, and based on the values of the TimeToLive or IdleTime attributes. Example 13-11 shows the calls required when implementing a PoolInstanceFactory.
import oracle.ias.cache.*; public class MyPoolFactory implements PoolInstanceFactory { public Object createInstance() { MyObject obj = new MyObject(); obj.init(); return obj; } public void destroyInstance(Object obj) { ((MyObject)obj).cleanup(); } }
When running in local mode, the Java Object Cache does not share objects or communicate with any other caches running locally on the same machine or remotely across the network. Local mode provides a decentralized architecture that supports a very efficient cache system, with very limited overhead. Object persistence across system shutdowns or program failures is not supported when running in local mode.
By default, the Java Object Cache runs in local mode and all objects in the cache are treated as local objects. When the Java Object Cache is configured in local mode, the cache ignores the DISTRIBUTE attribute for all objects.
In distributed mode, the Java Object Cache can share objects and communicate with other caches running either locally on the same machine or remotely across the network. Object updates and invalidations are propagated between communicating caches. Distributed mode supports object persistence across system shutdowns and program failures. Running in distributed mode has possible disadvantages. Specifically, significant system resources may be required when a large number of distributed objects need to be invalidated, when very large objects are updated, or when updates must be performed rapidly.
This section covers the following topics:
To configure the Java Object Cache to run in distributed mode, set the value of the distribute and discoveryAddress configuration properties in the javacache.properties file.
To start the Java Object Cache in distributed mode, the distribute property should be set to true in the configuration file.
In distributed mode, invalidations, destroys, and replaces are propagated through the cache's messaging system. The messaging system requires a known hostname and port address to allow a cache to join the cache system when it is first initialized. Use the discoveryAddress property in the javacache.properties file to specify a list of hostname and port addresses.
By default, Java Object Cache sets the discoveryAddress to the value :12345 (this is equivalent to localhost:12345). To eliminate conflicts with other software on the site, you should have your system administrator set the discoveryAddress.
If the Java Object Cache spans systems, a comma separated list of hostname and port pairs should be included as the value for discoveryAddress, with one hostname:port pair specified for each node. This avoids any dependency on a particular machine being available or on the order the processes are started.
When the Java Object Cache runs in distributed mode, individual regions, subregions, groups, and objects can be either local, or distributed. By default, objects, regions, subregions, and groups are defined as local. To change the default local value, set the DISTRIBUTE attribute when the object, region, or group is defined.
A distributed cache may contain both local and distributed objects.
Several attributes and methods in the Java Object Cache allow you to work with distributed objects and control the level of consistency of object data across the caches.
When updating, invalidating, or destroying objects across multiple caches, it is useful to know when the action has completed at all the participating sites. Setting the REPLY attribute causes all participating caches to send a reply to the sender when a requested action has completed for the object with the REPLY attribute set. This also enables the wait for response feature for object updates, invalidates, or destroys, and requires the use of the blocking method CacheAcces.waitForResponse().
To wait for a distributed action to complete across multiple caches, use CacheAccess.waitForResponse(). To ignore responses, use the CacheAccess.cancelResponse() method, which frees the cache resources used to collect the responses.
Both CacheAccess.waitForResponse() and CacheAccess.cancelResponse() apply to all objects accessed by the CacheAccess object. This allows the application to update a number of objects, then wait for all the replies.
Example 13-12 illustrates how to set an object as distributed and handle replies when the REPLY attribute is set. In this example, the attributes may also be set for the entire region. Attributes could also be set for a group or individual object, as appropriate for your application.
import oracle.ias.cache.*; CacheAccess cacc; String obj; Attributes attr = new Attributes (); MyLoader loader = new MyLoader(); // mark the object for distribution and have a reply generated // by the remote caches when the change is completed attr.setFlags(Attributes.DISTRIBUTE|Attributes.REPLY); attr.setLoader(loader); CacheAccess.defineRegion("testRegion",attr); cacc = CacheAccess.getAccess("testRegion"); // create region with //distributed attributes obj = (String)cacc.get("testObject"); cacc.replace("testObject", obj + "new version"); // change will be // propagated to other caches cacc.invalidate("invalidObject"); // invalidation is propagated to other caches try { // wait for up to a second,1000 milliseconds, for both the update // and the invalidate to complete cacc.waitForResponse(1000); catch (TimeoutException ex) { // tired of waiting so cancel the response cacc.cancelResponse(); } cacc.close(); }
When updating objects across multiple caches, or when multiple threads access a single object, you may coordinate the update action. Setting the SYNCHRONIZE attribute enables synchronized updates and requires an application to obtain ownership of an object before the object is loaded or updated.
The SYNCHRONIZE attribute also applies to regions, subregions, and groups. When the SYNCHRONIZE attribute is applied to a region, subregion, or group, ownership of the region, subregion, or group must be obtained before an object can be loaded or replaced in the region, subregion, or group.
Setting the SYNCHRONIZE_DEFAULT attribute on a region, subregion, or group applies the SYNCHRONIZE attribute to all of the objects within the region, subregion, or group. Ownership must be obtained for the individual objects within the region, subregion, or group before they can be loaded or replaced.
To obtain ownership of an object, use CacheAccess.getOwnership(). Once ownership is obtained, no other CacheAccess instance is allowed to load or replace the object. Reads and invalidation of objects are not affected by synchronization.
Once ownership has been obtained and the modification to the object is completed, call CacheAccess.releaseOwnership() to release the object. CacheAccess.releaseOwnership() waits up to the specified time for the updates to complete at the remote caches. If the updates complete within the specified time, ownership is released, otherwise a TimeoutException is thrown. If the method times out, call CacheAccess.releaseOwnership() again. CacheAccess.releaseOwnership()must return successfully for ownership to be released. If the time out value is -1, ownership is released immediately without waiting for the responses from the other caches.
import oracle.ias.cache.*; CacheAccess cacc; String obj; Attributes attr = new Attributes (); MyLoader loader = new MyLoader(); // mark the object for distribution and set synchronize attribute attr.setFlags(Attributes.DISTRIBUTE|Attributes.SYNCHRONIZE); attr.setLoader(loader); //create region CacheAccess.defineRegion("testRegion"); cacc = CacheAccess.getAccess("testRegion"); cacc.defineGroup("syncGroup", attr); //define a distributed synchronized group cacc.defineObject("syncObject", attr); // define a distributed synchronized object attr.setFlagsToDefaults() // reset attribute flags // define a group where SYNCHRONIZE is the default for all objects in the group attr.setFlags(Attributes.DISTRIBUTE|Attributes.SYNCHRONIZE_DEFAULT); cacc.defineGroup("syncGroup2", attr); try { // try to get the ownership for the group don't wait more than 5 seconds cacc.getOwnership("syncGroup", 5000); obj = (String)cacc.get("testObject", "syncGroup"); // get latest object // replace the object with a new version cacc.replace("testObject", "syncGroup", obj + "new version"); obj = (String)cacc.get("testObject2", "syncGroup"); // get a second object // replace the object with a new version cacc.replace("testObject2", "syncGroup", obj + "new version"); } catch (TimeoutException ex) { System.out.println("unable to acquire ownership for group"); cacc.close(); return; } try { cacc.releaseOwnership("syncGroup",5000); } catch (TimeoutException ex) { // tired of waiting so just release ownership cacc.releaseOwnership("syncGroup", -1)); } try { cacc.getOwnership("syncObject", 5000); // try to get the ownership for the object // don't wait more than 5 seconds obj = (String)cacc.get("syncObject"); // get latest object cacc.replace("syncObject", obj + "new version"); // replace the object with a new version } catch (TimeoutException ex) { System.out.println("unable to acquire ownership for object"); cacc.close(); return; } try { cacc.releaseOwnership("syncObject", 5000); } catch (TimeoutException ex) { cacc.releaseOwnership("syncObject", -1)); // tired of waiting so just release ownership } try { cacc.getOwnership("Object2", "syncGroup2", 5000); // try to get the ownership for the object // where the ownership is defined as the default for the group don't wait more than 5 seconds obj = (String)cacc.get("Object2", "syncGroup2"); // get latest object // replace the object with new version cacc.replace("Object2", "syncGroup2", obj + "new version"); } catch (TimeoutException ex) { System.out.println("unable to acquire ownership for object"); cacc.close(); return; } try { cacc.releaseOwnership("Object2", 5000); } catch (TimeoutException ex) { cacc.releaseOwnership("Object2", -1)); // tired of waiting so just release ownership } cacc.close(); }
Within the Java Object Cache, each cache manages its own objects locally within its Java VM process. In distributed mode, when using multiple processes or when the system is running on multiple sites, a copy of an object may exist in more than one cache.
The Java Object Cache allows you to specify the consistency level required between copies of objects that are available in multiple caches. The consistency level you specify depends on the application and the objects being cached. The supported levels of consistency vary, from none, to all copies of objects being consistent across all communicating caches.
Setting object attributes specifies the level of consistency. The consistency between objects in different caches is categorized into the following four levels:
If there are no consistency requirements between objects in distributed caches, an object should be defined as a local object (when Attributes.DISTRIBUTE is unset, this specifies a local object). Local is the default setting for objects. For local objects, all updates and invalidation are only visible to the local cache.
To distribute object updates across distributed caches, an object should be defined as distributed by setting the DISTRIBUTE attribute. All modifications to distributed objects are broadcast to other caches in the system. Using this level of consistency does not control or specify when an object is loaded into the cache or updated, and does not provide notification as to when the modification has completed in all caches.
To distribute object updates across distributed caches and wait for the change to complete before continuing, set the object's DISTRIBUTE and REPLY attributes. Using these attributes, notification occurs when a modification has completed in all caches. When Attributes.REPLY is set for an object, replies are sent back to the modifying cache when the modification has been completed at the remote site. These replies are returned asynchronously; that is, the CacheAccess.replace() and CacheAccess.invalidate() methods do not block. Use the CacheAccess.waitForResponse() method to wait for replies and block.
To use Java Object Cache's highest level of consistency set the appropriate attributes on the region, subregion, group, or object to make objects act as synchronized objects.
On a region, subregion, or group, setting Attributes.SYNCHRONIZE_DEFAULT sets the SYNCHRONIZE attribute for all of the objects within the region, subregion, or group.
On an object, setting Attributes.SYNCHRONIZE forces applications to obtain ownership of the object before the object can be loaded or modified. Setting this attribute effectively serializes write access to objects. To obtain ownership of an object, use the CacheAccess.getOwnership() method. Using the Attributes.SYNCHRONIZE attribute, notification is sent to the owner when the update is completed. Use CacheAccess.releaseOwnership() to block until any outstanding updates have completed, and the replies are received. This releases ownership of the object so that other caches can update or load the object.
When using this level of consistency, with Attributes.SYNCHRONIZE, the CacheLoader.load() method should call CacheLoader.netSearch() before loading the object from an external source. Calling CacheLoader.netSearch() in the load method tells the Java Object Cache to search all other caches for a copy of the object. This prevents different versions of the object from being loaded into the cache from an external source.
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