Package com.gemstone.gemfire.cache

Provides an implementation of distributed object caching that can leverage GemFire's distribution capabilities.

See:
Description

Interface Summary

AttributesMutator	Supports modification of certain region attributes after the region has been created.
Cache	Caches are obtained from static methods on the CacheFactory class.
CacheCallback	User-defined objects that can be plugged into caching to receive callback notifications.
CacheEvent	A region- or entry-related event affecting the cache.
CacheListener	A listener that can be implemented to handle region- or entry-related events.
CacheLoader	Allows data from outside of the VM to be placed into a region.
CacheStatistics	Defines common statistics information for both region and entries.
CacheTransactionManager	The CacheTransactionManager interface allows applications to manage transactions on a per Cache basis.
CacheWriter	A user-defined object defined in the RegionAttributes that is called synchronously before a region or entry in the cache is modified.
CustomExpiry	This is the contract that a custom-expiry element must honor.
Declarable	An object that can be described in a declarative caching XML file.
DiskWriteAttributes	Immutable parameter object for describing how region entries should be written to disk.
DynamicRegionListener	DynamicRegionListener is an interface that can be implemented to handle dynamic region-related events.
EntryEvent	Contains information about an event affecting an entry, including its identity and the the circumstances of the event.
EvictionAttributesMutator	The EvictionAttributesMutator allows changes to be made to a EvictionAttributes.
LoaderHelper	Provides a set of APIs to help the implementation of the CacheLoader load method.
PartitionAttributes	Attributes that define the partitioned character of a Partitioned Region.
Region	Manages subregions and cached data.
Region.Entry	A key-value pair containing the cached data in a region.
RegionAttributes	Defines attributes for configuring a region.
RegionEvent	Contains information about an event affecting a region, including its identity and the circumstances of the event.
RegionMembershipListener	A listener that can be implemented to handle region membership events.
RegionRoleListener	A listener that can be implemented to handle region reliability membership events.
RoleEvent	Contains information about an event affecting a region reliability, including its identity and the circumstances of the event.
SerializedCacheValue	Class SerializedCacheValue represents a serialized cache value.
TransactionEvent	An event that describes the culmination of an entire transaction.
TransactionId	The TransactionId interface is a "marker" interface that represents a unique GemFire transaction.
TransactionListener	A listener that can be implemented to handle transaction related events.

Class Summary

AttributesFactory	Creates instances of RegionAttributes.
CacheFactory	A factory class that must be used to obtain instances of Cache.
DataPolicy	Enumerated type for region data policy.
DiskWriteAttributesFactory	Factory for getting DiskWriteAttribute objects
DynamicRegionFactory	DynamicRegionFactory provides a distributed region creation service.
DynamicRegionFactory.Config	Configuration for dynamic region factory.
EvictionAction	The action that an EvictionAlgorithm takes.
EvictionAlgorithm	The algorithm used to determine when to perform an EvictionAction
EvictionAttributes	Attributes that describe how a Region's size is managed through an eviction controller.
ExpirationAction	Enumerated type for expiration actions.
ExpirationAttributes	Immutable parameter object for accessing and setting the attributes associated with timeToLive and idleTimeout.
InterestPolicy	Enumerated type for region subscription interest policy.
InterestResultPolicy	Class InterestResultPolicy is an enumerated type for a register interest result.
LossAction	Specifies how access to the region is affected when one or more required roles are lost.
MembershipAttributes	Configuration attributes for defining reliability requirements and behavior for a Region.
MirrorType	Deprecated. as of GemFire 5.0, use DataPolicy instead.
NotAvailable	Singleton token indicating an unavailable object, used in EntryEvent
Operation	Enumerated type for an event operation.
PartitionAttributesFactory	A factory that creates instances of PartitionAttributes which are used to create a partitioned Region.
RegionFactory	RegionFactory is a factory that encapsulates the following four steps to simplify Region creation.
RequiredRoles	Provides information on presence or absence of a Region's required roles.
ResumptionAction	Specifies how the region is affected by resumption of reliability when one or more missing required roles return to the distributed membership.
Scope	Enumerated type for region distribution scope.
SubscriptionAttributes	Configuration attributes for defining subscriber requirements and behavior for a Region.

Exception Summary

CacheClosedException	Indicates that the caching system has been closed.
CacheException	A generic exception, which indicates a cache error has occurred.
CacheExistsException	Thrown when attempting to create a Cache if one already exists.
CacheLoaderException	Thrown from the CacheLoader.load(com.gemstone.gemfire.cache.LoaderHelper) method indicating that an error occurred when a CacheLoader was attempting to load a value.
CacheRuntimeException	A generic runtime exception that indicates a cache error has occurred.
CacheWriterException	An exception thrown by a CacheWriter method.
CacheXmlException	Thrown when a problem is encountered while parsing a declarative caching XML file.
CommitConflictException	Thrown when a commit fails due to a write conflict.
CommitDistributionException	Indicates that an attempt to notify required participants of a transaction involving one or more regions that are configured with MembershipAttributes may have failed.
CommitIncompleteException	Thrown when a commit fails to complete due to errors
DiskAccessException	Indicates that an IOException during a disk region operation.
EntryDestroyedException	Indicates that a method was invoked on an entry that has been destroyed.
EntryExistsException	Thrown when attempting to create a Region.Entry that already exists in the Region.
EntryNotFoundException	Thrown when an operation is invoked on Region for an entry that doesn't exist in the Region.
EntryNotFoundInRegion	Deprecated. this class is no longer in use
FailedSynchronizationException	Thrown when a cache transaction fails to register with the UserTransaction (aka JTA transaction), most likely the cause of the UserTransaction's javax.transaction.Status#STATUS_MARKED_ROLLBACK status.
GatewayException	An exception thrown by a Gateway.
IncompatibleVersionException	An IncompatibleVersionException that the client version was not compatible with the server version.
NoQueueServersAvailableException	Indicates that this client cannot contact any queue servers and therefore cannot perform operations that require a queue, such as registering interest.
NoSubscriptionServersAvailableException	Indicates that this client cannot contact any servers and therefore cannot perform operations that require subscriptions, such as registering interest.
OperationAbortedException	Indicates that the operation that would have otherwise affected the cache has been aborted.
PartitionedRegionDistributionException	Indicates a failure to perform a distributed operation on a Partitioned Region after multiple attempts.
PartitionedRegionStorageException	Description of the conditions under which this exception is thrown When a PartitionedRegionStorageException message contains the string: There are not enough data store members to create a bucket.
RegionAccessException	Indicates that an attempt to access the region has failed.
RegionDestroyedException	Indicates that the region has been destroyed.
RegionDistributionException	Indicates that an attempt to send a distributed cache event to one or more required roles may have failed.
RegionExistsException	Indicates that the requested region already exists when a region is being created.
RegionReinitializedException	Indicates that the region has been reinitialized.
RegionRoleException	Indicates that a Region reliability failure has occurred.
RoleException	RoleException is the superclass of those exceptions that can be thrown to indicate a reliability failure on one or more regions that have been configured with	StatisticsDisabledException	Thrown if statistics are requested when statistics are disabled on the region.
SynchronizationCommitConflictException	Thrown when a commit operation of a JTA enlisted cache transaction fails
TimeoutException	Thrown if a netSearch times out without getting a response back from a cache, or when attempting to acquire a distributed lock.
UnsupportedVersionException	An UnsupportedVersionException indicates an unsupported version.
VersionException	An VersionException is an exception that indicates a client / server version mismatch exception has occurred.

Package com.gemstone.gemfire.cache Description

Provides an implementation of distributed object caching that can leverage GemFire's distribution capabilities. Refer to the programmer's guide for performance guidelines.

• Distributed Caching
• Cache Regions
• Partitioned Regions
• Region Entries
• Distribution and Consistency Models
• Storage Model
• Cache Loaders
• Cache Writers
• Expiration
• Cache Events
• Eviction Attributes
• Statistics
• Declarative Caching
• Client/Server Caching
• Cache Transactions
• Membership Attributes

Distributed Caching

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GemFire's distributed caching implementation allows application data to be efficiently shared among multiple threads in a VM, multiple VMs running on the same physical machine, and multiple VMs running on multiple machines. Cached data resides in "regions" whose contents are stored in a VM's heap.

The CacheFactory class provides the entry point to the caching API. A CacheFactory is configured to create a "cache instance" that resides in the VM. The cache may configured to be part of a DistributedSystem.

Cache Regions

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Application data is cached in a "region". The RegionFactory class provides the simpliest entry point into the Region API. A Region implements Map, however, it also provides caching behavior such as data loading, eviction control, and distribution. Every region has a name and regions may be nested to provide a cache-naming hierarchy ("parent regions" with "subregions"). The root regions of the naming hierarchy (that is, the regions with no parent) are obtained with the Cache.getRegion(java.lang.String) method.

Region properties such as the region's cache loader, data policy, and storage model are specified by an instance of RegionAttributes. A region RegionAttributes object can be specified when creating a region.

Partitioned Regions

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Region data can be partitioned across many distributed system members to create one large logical heap. PartitionAttributes are used to configure a partitioned region. A partitioned region can be configured to be highly available, surviving the loss of one or more system members, by maintaining copies of data. These extra copies also benefit read operations by allowing load balancing across all the copies.

Partitioned Regions have the added feature of allowing storage sizes larger than a single Java VM can provide and with multiple Java VMs comes multiple garbage collectors, improving the performance of the entire Region in the face of a full garbage collection cycle.

Partitioned Region caveats

As of GemFire 5.0, Partitioned Regions do not support the following GemFire features:

1. Eviction
2. Expiration
3. Cache Transactions
4. Membership Attributes
5. Disk Storage
6. Region scope

Region Entries

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A region contains key/value pairs of objects known as the region's "entries". The Region class provides a number of methods for manipulating the region's entries such as create, put, invalidate, and destroy . The following diagram describes the life cycle of a region entry.

Distribution and Consistency Models

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A region's scope attribute determines how the region's entries will be distributed to other caches. A region with local scope will not distribute any of its changes to any other members of the distributed system, nor will it receive changes when another cache instance is updated.

When a change (such as a put or invalidate) is made to a region with non-local scope, that change is distributed to the other members of the distributed system that have created that region in their cache instance. There are three kinds of distributed scope, each of which guarantees a different level of consistency for distributed data. "Global" scope provides the highest level of data consistency by obtaining a distributed lock on a region entry before propagating a change to other members of the distributed system. With globally-scoped regions, only one thread in the entire distributed system may modify the region entry at a time.

"Distributed ACK" scope provides slightly weaker data consistency than global scope. With distributed ACK scope, the method that modifies the region (such as a call to Region.destroy(java.lang.Object)) will not return until an acknowledgment of the change has been received from every member of the distributed system. Multiple threads may modify the region concurrently, but the modifying thread may proceed knowing that its change to the region has been seen by all other members.

"Distributed NO ACK" scope provides the weakest data consistency of all the scopes, but also provides the best performance. A method invocation that modifies a region with distributed NO ACK scope will return immediately after it has updated the contents of the region in its own cache instance. The updates are distributed asynchronously.

Storage Model

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The contents of a region (that is, the region's key/value pairs) may be stored in either the JVM's heap or on a disk drive.

Data Policy

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A region's "data policy" attribute determines if data is stored in the local cache. The normal policy will store region data in the local cache. The empty policy will never store region data in the local cache. They act as proxy regions that distribute write operations to others and receive events from others. The replication policies may reduce the number of net searches that a caching application has to be perform, and can provide a backup mechanism. The replicated region initializes itself when it is created with the keys and value of the region as found in other caches. The replicate policy simply store the relicate data in memory and the the persistent replicate policy stores the data in memory and disk.

Disk Storage

GemFire supports several modes of region persistence as determined by the RegionAttributes.getEvictionAttributes()'s eviction action and persistBackup configuration attributes. The following table summarizes the different modes and their configuration.

persistBackup	eviction controller	mode	description
false	No eviction controller or a eviction controller with an eviction action other than "overflow to disk"	No Disk	The cache Region persists no data to the disk. This is the default configuration.
false	An eviction controller using an EvictionAlgorithm with an eviction action of overflow to disk.	Disk for overflow	Once the amount of data stored in the region exceeds the eviction controller's threshold, least recently used data is written to disk and removed from the VM until the region's size is below the threshold.
true	No evcition controller or an eviction controller with an eviction action other than "overflow to disk"	Disk for persistence	All data in the region is scheduled to be written to disk as soon as it is placed in the region. Thus, the data on disk contains a complete backup of the region. No information about recently used data is maintained and, therefore, the size of the VM will continue to grow as more data is added to the region. "Disk for persistence" mode is appropriate for situations in which the user wants to back up a region whose data fits completely in the VM.
true	An eviction controller using an EvictionAlgorithm with an eviction action of overflow to disk.	Disk for overflow and persistence	All data in the region is scheduled to be written to disk as soon as it is placed in the region. But unlike "disk for persistence" mode, least recently used data will only be removed from the VM once the eviction controller's threshold is reached.

There are several things to keep in mind when working with regions that store data on disk.

• Operations (such as invalidate and Region.destroy(java.lang.Object)) that remove data from the cache also remove data from disk. In order for data to be removed from the VM and not from disk, EvictionAction.OVERFLOW_TO_DISK overflow} must be used.
• When disk overflow is enabled, only the value of the least recently used entry is removed from the VM. That is, the entry's key and the entry itself remain in the VM.
• When disk overflow is enabled, and backup is not then in each disk directory files whose name start with OVERFLOW_ will be created. These files are deleted automatically when they are no longer being used. If a VM crashes it may not clean up its overflow files. In that case they will be cleaned up the next time the same region is created.
• GemFire will look for the value of a region entry on disk before it will look in other cache instances. That is, it will not perform a net search or a load if the value exists on disk.
• When data is read from or written to disk, no cache events are fired.
• Region operations such as destroy and invalidate effect the data that is stored on disk in addition to the data stored in the VM. That is, if an entry is destroyed in the VM (even by an ExpirationAction), it will also be destroyed on disk.

Region backup and restore

A put on a region that is configured to have a disk "backup" (persistBackup is true) will result in the immediate scheduling of a disk write according to the region's DiskWriteAttributes (synchronous or asynchronous).

The actual backup data is stored in each of the specified disk directories. If any one of these directories runs out of space then any further writes to the backed up region will fail with a DiskAccessException. The actual file names begin with BACKUP_ and KEYS_. If you wish to store a backup in another location or offline, then all of these files need to be saved. All of the files in the same directory must always be kept together in the same directory. It is ok to change the directory name.

When a region with a disk backup is created, it initializes itself with a "bulk load" operation that reads region entry data from its disk files. Note that the bulk load operation does not create cache events and it does not send update messages to other members of the distributed system. Additionally, bulk loading reads only the keys of the region entries and not the values. Entry values will be lazily read into the VM as they are requested.

A common high-availability scenario may involve replicated regions that are configured to have disk backups. When a replicated backup region is created in a distributed system that already contains a replicated backup region, GemFire optimizes the initialization of the backup region by streaming the contents of the backup file to the region being initialized. If there is no other replicated backup region in the distributed system, the backup file for the region being initialized may contain stale data. (That is, the value of region entries may have changed while the backup VM was down.) In this situation, the region being initialized will consult other VMs in the distributed system to obtain an up-to-date version of the cached data.

Cache Loaders

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A cache loader allows data from outside of the VM to be placed into a region. When Region.get(java.lang.Object) is called for a region entry that has a null value, the load method of the region's cache loader is invoked. The load method creates the value for the desired key by performing an operation such as a database query. The load may also perform a net search that will look for the value in a cache instance hosted by another member of the distributed system.

If a region was not created with a user-specified cache loader, the get operation will, by default, perform a special variation of net search: if the value cannot be found in any of the members of the distributed system, but one of those members has defined a cache loader for the region, then that remote cache loader will be invoked (a "net load") and the loaded value returned to the requester. Note that a net load does not store the loaded value in the remote cache's region.

Cache Writers

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The CacheWriter is a type of event handler that is invoked synchronously before the cache is modified, and has the ability to abort the operation. Only one CacheWriter in the distributed system is invoked before the operation that would modify a cache. A CacheWriter is typically used to update an external database.

Expiration

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Sometimes cached data has a limited lifetime. The region attributes regionTimeToLive, regionIdleTimeout, entryTimeToLive, and entryIdleTimeout, specify how data is handled when it becomes too old. There are two conditions under which cache data is considered too old: data has resided unchanged in a region for a given amount of time ("time to live") and data has not been accessed for a given amount of time ("idle time"). GemFire's caching implementation launches an "expiration thread" that periodically monitors region entries and will expire those that have become too old. When a region entry expires, it can either be invalidated, destroyed, locally invalidated, or locally destroyed.

Cache Events

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The CacheListener interface provides callback methods that are invoked synchronously in response to certain operations (such as a put or invalidate) being performed on a region. The event listener for a region is specified with the setCacheListener method. Each callback method on the CacheListener receives a CacheEvent describing the operation that caused the callback to be invoked and possibly containing information relevant to the operation (such as the old and new values of a region entry).

Eviction Attributes

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Before a new entry is created in a region, the region's eviction controller is consulted. The eviction controller may perform some action on the region (usually an action that makes the region smaller) based on certain criteria. For instance, an eviction controller could keep track of the sizes of the entry values. Before a new entry is added, the eviction controller could remove the entry with the largest value to free up space in the cache instance for new data. GemFire provides EvictionAttributes that will create an eviction controller that destroys the "least recently used" Region Entry once the Region exceeds a given number of entries.

Statistics

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The CacheStatistics class provides statistics information about the usage of a region or entry. Statistics are often used by eviction controllers to determine which entries should be invalidated or destroyed when the region reaches its maximum size.

Declarative Caching

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A "caching XML file" declares regions, entries, and attributes. When a Cache is created its contents can be initialized according to a caching XML file.

The Document Type Definition for a declarative cache XML file can be found in "doc-files/cache5_0.dtd" and examples of declarative cache XML files can be found here, here, and here.

Client/Server Caching

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GemFire caches can be configured in a client/server hierarchy. In this configuration, GemFire cache regions are configured as clients to regions in GemFire server caches running in a separate distributed system. The GemFire servers are generally run as cacheserver processes. Client regions are configured with a "pool" that manages connections to the server caches. When a client updates its region, the update is forwarded to the server. When a client get results in a local cache miss, the get request is forwarded to the server. The clients may also subscribe to server-side events.

Cache Transactions

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The GemFire cache supports transactions providing enhanced data consistency across multiple Regions. GemFire Transactions are designed to run in a highly concurrent environment and as such have optimistic conflict behavior. They are optimistic in the sense that they assume little data overlap between transactions. Using that assumption, they do not reserve entries that are being changed by the transaction until the commit operation. For example, when two transactions operate on the same Entry, the last one to commit will detect a conflict and fail to commit. The changes made by the successful transaction are only available to other threads after the commit has finished, or in other words Gemfire Transactions exhibit Read Committed behavior.

To provide application integration with GemFire transactions, a TransactionListener with associated TransactionEvents is provided as a Cache attribute. The listener is notified of commits, both failed and successful as well as explicit rollbacks. When a commit message is received by a distributed member with the same Region, again the TransactionListener is invoked.

GemFire transactions also integrate well with JTA transactions. If a JTA transaction has begun and an existing GemFire transaction is not already present, any transactional region operation will create a GemFire transaction and register it with the JTA transaction, causing the JTA transaction manager to take control of the GemFire commit/rollback operation.

Similar to JTA transactions, GemFire transactions are associated with a thread. Only one transaction is allowed at a time per thread and a transaction is not allowed to be shared amount threads. The changes made changed by a GemFire transaction are distributed to other distributed memebers as per the Region's Scope.

Finally, GemFire transactions allow for the "collapse" of multiple operations on an Entry, for example if an application destroys an Entry and follows with a create operation and then a put operations, all three operations are combined into one action reflecting the sum of all three.

Membership Attributes

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The GemFire region can be configured to require the presence of one or more user-defined membership roles. Each Role is assigned to any number of applications when each application connects to the GemFire distributed system. MembershipAttributes are then used to specify which roles are required to be online and present in that region's membership for access to the cache region being configured.

In addition to specifying which roles are required, MembershipAttributes are used to specify the LossAction. The loss action determines how access to the region is affected when one or more required roles are offline and no longer present in the system membership. The region can be made completely "NO_ACCESS", which means that the application cannot read from or write to that region. "LIMITED_ACCESS" means that the application cannot write to that region. "FULL_ACCESS" means that the application can read from and write to that region as normal. If "FULL_ACCESS" is selected, then the application would only be aware of the missing required role if it registered a RegionRoleListener. This listener provides callbacks to notify the application when required roles are gained or lost, thus providing for custom integration of required roles with the application.

ResumptionAction defined in the MembershipAttributes specifies how the application responds to having a missing required role return to the distributed membership. "None" results in no special action, while "Reinitialize" causes the region to be reinitialized. Reinitializing the region will clear all entries from that region. If it is a replicate, the region will repopulate with entries available from other distributed members.

RequiredRoles provides methods to check for missing required roles for a region and to wait until all required roles are present. In addition, this class can be used to check if a role is currently present in a region's membership.