19 Local Cache

While it is not a clustered service, the Coherence local cache implementation is often used in combination with various Coherence clustered cache services. The Coherence local cache is just that: a cache that is local to (completely contained within) a particular cluster node. There are several attributes of the local cache that are particularly interesting:

The local cache is important to the clustered cache services for several reasons, including as part of Coherence's near cache technology, and with the modular backing map architecture.

Configuring the Local Cache

The key element for configuring the Local Cache is <local-scheme>. Local caches are generally nested within other cache schemes, for instance as the front-tier of a near-scheme. Thus, this element can appear as a sub-element of any of these elements in the coherence-cache-config file: <caching-schemes>, <distributed-scheme>, <replicated-scheme>, <optimistic-scheme>, <near-scheme>, <versioned-near-scheme>, <overflow-scheme>, <read-write-backing-map-scheme>, and <versioned-backing-map-scheme>.

The <local-scheme> provides several optional sub-elements that let you define the characteristics of the cache. For example, the <low-units> and <high-units> sub-elements allow you to limit the cache in terms of size. When the cache reaches its maximum allowable size it prunes itself back to a specified smaller size, choosing which entries to evict according to a specified eviction-policy (<eviction-policy>). The entries and size limitations are measured in terms of units as calculated by the scheme's unit-calculator (<unit-calculator>).

You can also limit the cache in terms of time. The <expiry-delay> sub-element specifies the amount of time from last update that entries will be kept by the cache before being marked as expired. Any attempt to read an expired entry will result in a reloading of the entry from the configured cache store (<cachestore-scheme>). Expired values are periodically discarded from the cache based on the flush-delay.

If a <cache-store-scheme> is not specified, then the cached data will only reside in memory, and only reflect operations performed on the cache itself. See <local-scheme> for a complete description of all of the available sub-elements.

The XML code Example 19-1 illustrates the configuration of a Local Cache. See Sample Cache Configurations for additional examples.

Example 19-1 Local Cache Configuration

<?xml version="1.0"?>

<cache-config>
  <caching-scheme-mapping>
    <cache-mapping>
      <cache-name>example-local-cache</cache-name>
      <scheme-name>example-local</scheme-name>
    </cache-mapping>
  </caching-scheme-mapping>
  <caching-schemes>
    <local-scheme>
      <scheme-name>example-local</scheme-name>
      <eviction-policy>LRU</eviction-policy>
      <high-units>32000</high-units>
      <low-units>10</low-units>
      <unit-calculator>FIXED</unit-calculator>
      <expiry-delay>10ms</expiry-delay>
      <flush-delay>1000ms</flush-delay>
      <cachestore-scheme>
        <class-scheme>
          <class-name>ExampleCacheStore</class-name>
        </class-scheme>
      </cachestore-scheme>
      <pre-load>true</pre-load>
    </local-scheme>
  </caching-schemes>
</cache-config>

For more information, see "Local Cache" in the C++ User Guide or .NET User Guide .