Coherence 3.1 User Guide : overflow-scheme

overflow-scheme

Description

The overflow-scheme defines a two tier cache consisting of a fast, size limited front-tier, and slower but much higher capacity back-tier cache. When the size limited front fills up, evicted entries are transparently moved to the back. In the event of a cache miss, entries may move from the back to the front. A typical deployment might use a local-scheme for the front-tier, and a external-scheme for the back-tier, allowing for fast local caches with capacities larger the the JVM heap would allow.

Implementation

Elements

The following table describes the elements you can define within the overflow-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 overflow cache. Any custom implementation must extend either the `com.tangosol.net.cache.OverflowMap` or `com.tangosol.net.cache.SimpleOverflowMap` class, and declare the exact same set of public constructors.
`<init-params>`	Optional	Specifies initialization parameters, for use in custom overflow 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.
`<front-scheme>`	Required	Specifies the `cache-scheme` to use in creating the front-tier cache. Legal values are: `local-scheme` `external-scheme` `paged-external-scheme` `class-scheme` The eviction policy of the `front-scheme` defines which entries which items are in the front vs back tiers. For Example: <front-scheme> <local-scheme> <eviction-policy>HYBRID</eviction-policy> <high-units>1000</high-units> </local-scheme> </front-scheme>
`<back-scheme>`	Required	Specifies the `cache-scheme` to use in creating the back-tier cache. Legal values are: `local-scheme` `external-scheme` `paged-external-scheme` `class-scheme` For Example: <back-scheme> <external-scheme> <lh-file-manager/> </external-scheme> </back-scheme>
`<miss-cache-scheme>`	Optional	Specifies a `cache-scheme` for maintaining information on cache misses. For caches which are not `expiry-enabled`, the miss-cache is used track keys which resulted in both a front and back tier cache miss. The knowledge that a key is not in either tier allows some operations to perform faster, as they can avoid querying the potentially slow back-tier. A size limited scheme may be used to control how many misses are tracked. If unspecified no cache-miss data will be maintained. Legal values are: `local-scheme`
`<expiry-enabled>`	Optional	Turns on support for automatically-expiring data, as provided by the `com.tangosol.net.cache.CacheMap` API. When enabled the `overflow-scheme` will be implemented using `com.tangosol.net.cache.OverflowMap`, rather then `com.tangosol.net.cache.SimpleOverflowMap`. Legal values are `true` or `false`. Default value is `false`.
`<autostart>`	Optional	The `autostart` element is intended to be used by cache servers (i.e. `com.tangosol.net.DefaultCacheServer`). It specifies whether or not the cache services associated with this cache scheme should be automatically started at a cluster node. Legal values are `true` or `false`. Default value is `false`.

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 overflow cache.

Any custom implementation must extend either the com.tangosol.net.cache.OverflowMap or com.tangosol.net.cache.SimpleOverflowMap class, and declare the exact same set of public constructors.

<init-params>

Optional

Specifies initialization parameters, for use in custom overflow 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.

<front-scheme>

Required

Specifies the cache-scheme to use in creating the front-tier cache.

Legal values are:

The eviction policy of the front-scheme defines which entries which items are in the front vs back tiers.
For Example:

<front-scheme>
  <local-scheme>
    <eviction-policy>HYBRID</eviction-policy>
    <high-units>1000</high-units>
  </local-scheme>
</front-scheme>

<back-scheme>

Required

Specifies the cache-scheme to use in creating the back-tier cache.

Legal values are:

For Example:

<back-scheme>
  <external-scheme>
    <lh-file-manager/>
  </external-scheme>
</back-scheme>

<miss-cache-scheme>

Optional

Specifies a cache-scheme for maintaining information on cache misses. For caches which are not expiry-enabled, the miss-cache is used track keys which resulted in both a front and back tier cache miss. The knowledge that a key is not in either tier allows some operations to perform faster, as they can avoid querying the potentially slow back-tier. A size limited scheme may be used to control how many misses are tracked. If unspecified no cache-miss data will be maintained.

Legal values are:

local-scheme

<expiry-enabled>

Optional

Turns on support for automatically-expiring data, as provided by the com.tangosol.net.cache.CacheMap API.

When enabled the overflow-scheme will be implemented using com.tangosol.net.cache.OverflowMap, rather then com.tangosol.net.cache.SimpleOverflowMap.

Legal values are true or false.

Default value is false.

<autostart>

Optional

The autostart element is intended to be used by cache servers (i.e. com.tangosol.net.DefaultCacheServer). It specifies whether or not the cache services associated with this cache scheme should be automatically started at a cluster node.

Legal values are true or false.

Default value is false.