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Elastic Docs ›Elasticsearch Guide [2.1] ›Modules ›Indices

Shard request cache

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Shard request cache

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When a search request is run against an index or against many indices, each involved shard executes the search locally and returns its local results to the coordinating node, which combines these shard-level results into a “global” result set.

The shard-level request cache module caches the local results on each shard. This allows frequently used (and potentially heavy) search requests to return results almost instantly. The requests cache is a very good fit for the logging use case, where only the most recent index is being actively updated — results from older indices will be served directly from the cache.

For now, the requests cache will only cache the results of search requests where size=0, so it will not cache hits, but it will cache hits.total, aggregations, and suggestions.

Queries that use now (see Date Math) cannot be cached.

Cache invalidation

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The cache is smart — it keeps the same near real-time promise as uncached search.

Cached results are invalidated automatically whenever the shard refreshes, but only if the data in the shard has actually changed. In other words, you will always get the same results from the cache as you would for an uncached search request.

The longer the refresh interval, the longer that cached entries will remain valid. If the cache is full, the least recently used cache keys will be evicted.

The cache can be expired manually with the clear-cache API:

curl -XPOST 'localhost:9200/kimchy,elasticsearch/_cache/clear?request_cache=true'

Enabling caching by default

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The cache is not enabled by default, but can be enabled when creating a new index as follows:

curl -XPUT localhost:9200/my_index -d'
{
  "settings": {
    "index.requests.cache.enable": true
  }
}
'

It can also be enabled or disabled dynamically on an existing index with the update-settings API:

curl -XPUT localhost:9200/my_index/_settings -d'
{ "index.requests.cache.enable": true }
'

Enabling caching per request

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The request_cache query-string parameter can be used to enable or disable caching on a per-request basis. If set, it overrides the index-level setting:

curl 'localhost:9200/my_index/_search?request_cache=true' -d'
{
  "size": 0,
  "aggs": {
    "popular_colors": {
      "terms": {
        "field": "colors"
      }
    }
  }
}
'

If your query uses a script whose result is not deterministic (e.g. it uses a random function or references the current time) you should set the request_cache flag to false to disable caching for that request.

Cache key

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The whole JSON body is used as the cache key. This means that if the JSON changes — for instance if keys are output in a different order — then the cache key will not be recognised.

Most JSON libraries support a canonical mode which ensures that JSON keys are always emitted in the same order. This canonical mode can be used in the application to ensure that a request is always serialized in the same way.

Cache settings

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The cache is managed at the node level, and has a default maximum size of 1% of the heap. This can be changed in the config/elasticsearch.yml file with:

indices.requests.cache.size: 2%

Also, you can use the indices.requests.cache.expire setting to specify a TTL for cached results, but there should be no reason to do so. Remember that stale results are automatically invalidated when the index is refreshed. This setting is provided for completeness' sake only.

Monitoring cache usage

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The size of the cache (in bytes) and the number of evictions can be viewed by index, with the indices-stats API:

curl 'localhost:9200/_stats/request_cache?pretty&human'

or by node with the nodes-stats API:

curl 'localhost:9200/_nodes/stats/indices/request_cache?pretty&human'
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