Project: Druid (App Definition and Development / Database)
File: using-caching.md

id: using-caching title: "Using query caching" <!-- ~ Licensed to the Apache Software Foundation (ASF) under one ~ or more contributor license agreements. See the NOTICE file ~ distributed with this work for additional information ~ regarding copyright ownership. The ASF licenses this file ~ to you under the Apache License, Version 2.0 (the ~ "License"); you may not use this file except in compliance ~ with the License. You may obtain a copy of the License at ~ ~ http://www.apache.org/licenses/LICENSE-2.0 ~ ~ Unless required by applicable law or agreed to in writing, ~ software distributed under the License is distributed on an ~ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY ~ KIND, either express or implied. See the License for the ~ specific language governing permissions and limitations ~ under the License. --> This topic covers how to configure services to populate and use the Druid query caches. For a conceptual overview and use cases, see . For information on how to configure the caching mechanism, see . 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. Historicals only support segment-level caching, which is enabled by

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 for a description of all available Historical cache configurations. 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 and for a description of all available task executor service caching options. Brokers support both segment-level and whole-query result level caching. To control segment caching on the Broker, set the `useCache` and `populateCache`runtime 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 for a description of all available Broker cache configurations. As long as the service is set to populate the cache, you can set cache options for individual queries in the query . 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 datasource 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 . See the following topics for more information: for an overview of caching. for more details and usage for the query context. for information about different cache types and additional configuration options.