external-scheme

`external-scheme`

Used in: caching-schemes, distributed-scheme, replicated-scheme, optimistic-scheme, near-scheme, versioned-near-scheme, overflow-scheme, read-write-backing-map-scheme, versioned-backing-map-scheme

Description

External schemes define caches which are not JVM heap based, allowing for greater storage capacity.

Implementaion

This scheme is implemented by:

com.tangosol.net.cache.SerializationMap - for unlimited size caches
com.tangosol.net.cache.SerializationCache - for size limited caches

The implementation type is chosen based on the following rule:

if the high-units element is specified and not zero then SerializationCache is used;
otherwise SerializationMap is used.

Pluggable Storage Manager

External schemes use a pluggable store manager to store and retrieve binary key value pairs. Supported store managers include:

async-store-manager - a wrapper providing asynchronous write capabilities for of other store manager implementations
custom-store-manager - allows definition of custom implementations of store managers
bdb-store-manager - uses Berkeley Database JE to implement an on-disk cache
lh-file-manager - uses a Tangosol LH on-disk database cache
nio-file-manager - uses NIO to implement memory-mapped file based cache
nio-memory-manager - uses NIO to implement an off JVM heap, in-memory cache

Size Limited Cache

The cache may be configured as size-limited, which means that once it reaches its maximum allowable size it prunes itself.

Eviction against disk based caches can be expensive, consider using a paged-external-scheme for such cases.

Persistence (long-term storage)

External caches are generally used for temporary storage of large data sets, for example as the back-tier of an overflow-scheme. Certain implementations do however support persistence for non-clustered caches, see the bdb-store-manager and lh-file-manager for details. Clustered persistence should be configured via a read-write-backing-map-scheme on a distributed-scheme.

Elements

The following table describes the elements you can define within the external-scheme element.

Element	Required/Optional	Description
`<scheme-name>`	Optional	Specifies the scheme's name. The name must be unique within a configuration file.
`<scheme-ref>`	Optional	Specifies the name of another scheme to inherit from.
`<class-name>`	Optional	Specifies a custom implementation of the external cache. Any custom implementation must extend either: `com.tangosol.net.cache.SerializationMap` - for unlimited size caches `com.tangosol.net.cache.SerializationCache` - for size limited caches and declare the exact same set of public constructors as the superclass.
`<init-params>`	Optional	Specifies initialization parameters, for use in custom external cache implementations which implement the `com.tangosol.run.xml.XmlConfigurable` interface.
`<listener>`	Optional	Specifies an implementation of a `com.tangosol.util.MapListener` which will be notified of events occurring on the cache.
`<high-units>`	Optional	Used to limit the size of the cache. Contains the maximum number of units that can be placed in the cache before pruning occurs. An entry is the unit of measurement. Once this limit is exceeded, the cache will begin the pruning process, evicting the least recently used entries until the number of units is brought below this limit. The scheme's `class-name` element may be used to provide custom extensions to `SerializationCache`, which implement alternative eviction policies. Legal values are positive integers or `zero`. Zero implies no limit. Default value is `zero`.
`<async-store-manager>`	Optional	Configures the external cache to use an asynchronous storage manager wrapper for any other storage manager.
`<custom-store-manager>`	Optional	Configures the external cache to use a custom storage manager implementation.
`<bdb-store-manager>`	Optional	Configures the external cache to use Berkeley Database JE on-disk databases for cache storage.
`<lh-file-manager>`	Optional	Configures the external cache to use a Tangosol LH on-disk database for cache storage.
`<nio-file-manager>`	Optional	Configures the external cache to use a memory-mapped file for cache storage.
`<nio-memory-manager>`	Optional	Configures the external cache to use an off JVM heap, memory region for cache storage.