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Druid Caffeine Cache

A highly performant local cache implementation for Druid based on Caffeine. Requires a JRE8u60 or higher

Configuration

Below are the configuration options known to this module:

`runtime.properties`	Description	Default
`druid.cache.sizeInBytes`	The maximum size of the cache in bytes on heap.	None (unlimited)
`druid.cache.expireAfter`	The time (in ms) after an access for which a cache entry may be expired	None (no time limit)
`druid.cache.cacheExecutorFactory`	The executor factory to use for Caffeine maintenance. One of `COMMON_FJP`, `SINGLE_THREAD`, or `SAME_THREAD`	ForkJoinPool common pool (`COMMON_FJP`)
`druid.cache.evictOnClose`	If a close of a namespace (ex: removing a segment from a node) should cause an eager eviction of associated cache values	`false`

Here are the possible values for druid.cache.cacheExecutorFactory, which controls how maintenance tasks are run

COMMON_FJP (default) use the common ForkJoinPool. Do NOT use this option unless you are running 8u60 or higher
SINGLE_THREAD Use a single-threaded executor
SAME_THREAD Cache maintenance is done eagerly

To enable the caffeine cache, include this module on the loadList and set druid.cache.type to caffeine in your properties.

In addition to the normal cache metrics, the caffeine cache implementation also reports the following in both total and delta

Metric	Description	Normal value
`query/cache/caffeine/*/requests`	Count of hits or misses	hit + miss
`query/cache/caffeine/*/loadTime`	Length of time caffeine spends loading new values (unused feature)	0
`query/cache/caffeine/*/evictionBytes`	Size in bytes that have been evicted from the cache	Varies, should tune cache `sizeInBytes` so that `sizeInBytes`/`evictionBytes` is approximately the rate of cache churn you desire