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[[index-modules-shard-query-cache]] | ||
== Shard query cache | ||
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coming[1.4.0] | ||
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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. | ||
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The shard-level query cache module caches the local results on each shard. | ||
This allows frequently used (and potentially heavy) search requests to return | ||
results almost instantly. The query 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. | ||
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[IMPORTANT] | ||
================================== | ||
For now, the query cache will only only cache the results of search requests | ||
where <<count,`?search_type=count`>>, so it will not cache `hits`, | ||
but it will cache `hits.total`, <<search-aggregations,aggregations>>, and | ||
<<search-suggesters,suggestions>>. | ||
Queries that use `now` (see <<date-math>>) cannot be cached. | ||
================================== | ||
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[float] | ||
=== Cache invalidation | ||
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The cache is smart -- it keeps the same _near real-time_ promise as uncached | ||
search. | ||
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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. | ||
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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. | ||
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The cache can be expired manually with the <<indices-clearcache,`clear-cache` API>>: | ||
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[source,json] | ||
------------------------ | ||
curl -XPOST 'localhost:9200/kimchy,elasticsearch/_cache/clear?query_cache=true' | ||
------------------------ | ||
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[float] | ||
=== 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: | ||
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[source,json] | ||
----------------------------- | ||
curl -XPUT localhost:9200/my_index -d' | ||
{ | ||
"settings": { | ||
"index.cache.query.enable": true | ||
} | ||
} | ||
' | ||
----------------------------- | ||
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It can also be enabled or disabled dynamically on an existing index with the | ||
<<indices-update-settings,`update-settings`>> API: | ||
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[source,json] | ||
----------------------------- | ||
curl -XPUT localhost:9200/my_index/_settings -d' | ||
{ "index.cache.query.enable": true } | ||
' | ||
----------------------------- | ||
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[float] | ||
=== Enabling caching per request | ||
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The `query_cache` query-string parameter can be used to enable or disable | ||
caching on a *per-query* basis. If set, it overrides the index-level setting: | ||
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[source,json] | ||
----------------------------- | ||
curl localhost:9200/my_index/_search?search_type=count&query_cache=true -d' | ||
{ | ||
"aggs": { | ||
"popular_colors": { | ||
"terms": { | ||
"field": "colors" | ||
} | ||
} | ||
} | ||
} | ||
' | ||
----------------------------- | ||
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IMPORTANT: 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 | ||
`query_cache` flag to `false` to disable caching for that request. | ||
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[float] | ||
=== 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. | ||
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TIP: 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. | ||
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[float] | ||
=== 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: | ||
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[source,yaml] | ||
-------------------------------- | ||
indices.cache.query.size: 2% | ||
-------------------------------- | ||
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Also, you can use the +indices.cache.query.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. | ||
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[float] | ||
=== 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,`indices-stats`>> API: | ||
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[source,json] | ||
------------------------ | ||
curl -XPOST 'localhost:9200/_stats/query_cache?pretty&human' | ||
------------------------ | ||
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or by node with the <<cluster-nodes-stats,`nodes-stats`>> API: | ||
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[source,json] | ||
------------------------ | ||
curl -XPOST 'localhost:9200/_nodes/stats/indices/query_cache?pretty&human' | ||
------------------------ |
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