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Druid Caffeine Cache
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A highly performant local cache implementation for Druid based on [Caffeine](https://github.com/ben-manes/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`|

## `druid.cache.cacheExecutorFactory`

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

# Enabling

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

# Metrics
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|