Cache

There are different caching inner modules associated with an index. They include filter, field and others.

Filter Cache

The filter cache is responsible for caching the results of filters (used in the query). The default implementation of a filter cache (and the one recommended to use in almost all cases) is the node filter cache type.

Node Filter Cache

The node filter cache may be configured to use either a percentage of the total memory allocated to the process or an specific amount of memory. All shards present on a node share a single node cache (thats why its called node). The cache implements an LRU eviction policy: when a cache becomes full, the least recently used data is evicted to make way for new data.

The setting that allows one to control the memory size for the filter cache is indices.cache.filter.size, which defaults to 20%. Note, this is not an index level setting but a node level setting (can be configured in the node configuration).

indices.cache.filter.size can accept either a percentage value, like 30%, or an exact value, like 512mb.

Index Filter Cache

A filter cache that exists on the index level (on each node). Generally, not recommended for use since its memory usage depends on which shards are allocated on each node and its hard to predict it. The types are: resident, soft and weak.

All types support the following settings:

Setting	Description
index.cache.filter.max_size	The max size (count, not byte size) of the cache (per search segment in a shard). Defaults to not set (-1), which is usually fine with soft cache and proper cacheable filters.
index.cache.filter.expire	A time based setting that expires filters after a certain time of inactivity. Defaults to -1. For example, can be set to 5m for a 5 minute expiry.

Field Data Cache

The field data cache is used mainly when sorting on or faceting on a field. It loads all the field values to memory in order to provide fast document based access to those values. The field data cache can be expensive to build for a field, so its recommended to have enough memory to allocate it, and keep it loaded.

The default type for the field data cache is resident (because of the cost of rebuilding it). Other types include soft.

Setting	Description
index.cache.field.max_size	The max size (count, not byte size) of the cache (per search segment in a shard). Defaults to not set (-1).
index.cache.field.expire	A time based setting that expires filters after a certain time of inactivity. Defaults to -1. For example, can be set to 5m for a 5 minute expiry.