Using query caching

This topic covers how to configure services to populate and use the Druid query caches. For a conceptual overview and use cases, see Query caching. For information on how to configure the caching mechanism, see Cache configuration.

All query caches have a pair of parameters that control the way individual queries interact with the cache:

• useCache to instruct queries to use the cache for results.
• populateCache to instruct a query to cache its results.

The separation of concerns, usage and population, lets you include cached results for queries on uncommon data without polluting the cache with results that are unlikely to be reused by other queries, for example, large reports or queries on very old data.

To use caching, it must be enabled in the settings for the service to perform caching in the service’s runtime properties. By default, per-segment cache is enabled on Historicals. For individual queries, you can control cache usage and population within the query context.

Enabling query caching on Historicals

Historicals only support segment-level caching, which is enabled by default. To control caching on the Historical, set the useCache and populateCache runtime properties. For example, to set the Historical to both use and populate the segment cache for queries:

druid.historical.cache.useCache=true
druid.historical.cache.populateCache=true

See Historical caching for a description of all available Historical cache configurations.

Enabling query caching on task executor services

Task executor services, the Peon or the Indexer, only support segment-level caching. To control caching on a task executor service, set the useCache and populateCache runtime properties. For example, to set the Peon to both use and populate the segment cache for queries:

druid.realtime.cache.useCache=true
druid.realtime.cache.populateCache=true

See Peon caching and Indexer caching for a description of all available task executor service caching options.

Enabling query caching on Brokers

Brokers support both segment-level and whole-query result level caching.

To control segment caching on the Broker, set the useCache and populateCacheruntime properties. For example, to set the Broker to use and populate the segment cache for queries:

druid.broker.cache.useCache=true
druid.broker.cache.populateCache=true

To control whole-query caching on the Broker, set the useResultLevelCache and populateResultLevelCache runtime properties. For example, to set the Broker to use and populate the whole-query cache for queries:

druid.broker.cache.useResultLevelCache=true
druid.broker.cache.populateResultLevelCache=true

See Broker caching for a description of all available Broker cache configurations.

Enabling caching in the query context

As long as the service is set to populate the cache, you can set cache options for individual queries in the query context. For example, you can POST a Druid SQL request to the HTTP POST API and include the context as a JSON object:

{
  "query" : "SELECT COUNT(*) FROM data_source WHERE foo = 'bar' AND __time > TIMESTAMP '2020-01-01 00:00:00'",
  "context" : {
    "useCache" : "true",
    "populateCache" : "false"
  }
}

In this example the user has set populateCache to false to avoid filling the result cache with results for segments that are over a year old. For more information, see Druid SQL client APIs.

Learn more

See the following topics for more information:

• Query caching for an overview of caching.
• Query context for more details and usage for the query context.
• Cache configuration for information about different cache types and additional configuration options.