refactor query caching docs (#10848)
* refactor query caching
* Update docs/querying/using-caching.md
Co-authored-by: sthetland <steve.hetland@imply.io>
* Update docs/querying/using-caching.md
Co-authored-by: sthetland <steve.hetland@imply.io>
* Update docs/querying/using-caching.md
Co-authored-by: sthetland <steve.hetland@imply.io>
* Update docs/querying/using-caching.md
Co-authored-by: sthetland <steve.hetland@imply.io>
* Update docs/querying/using-caching.md
Co-authored-by: sthetland <steve.hetland@imply.io>
* Update docs/querying/using-caching.md
Co-authored-by: sthetland <steve.hetland@imply.io>
* Update docs/querying/using-caching.md
Co-authored-by: sthetland <steve.hetland@imply.io>
* Update docs/querying/using-caching.md
Co-authored-by: sthetland <steve.hetland@imply.io>
* add description for context link
* accept suggestions
* reword, rework some awkward language
* incorporate feedback, fix errors
* add back perf considerations
* Apply suggestions from code review
applying @suneet-s 's changes
Co-authored-by: Suneet Saldanha <suneet@apache.org>
* Update caching.md
fix link
Co-authored-by: sthetland <steve.hetland@imply.io>
Co-authored-by: Suneet Saldanha <suneet@apache.org>
diff --git a/docs/querying/caching.md b/docs/querying/caching.md
index b5a0d00..3ee0191 100644
--- a/docs/querying/caching.md
+++ b/docs/querying/caching.md
@@ -1,6 +1,7 @@
---
id: caching
title: "Query caching"
+description: "Describes Apache Druid per-segment and whole-query cache types. Identifies services where you can enable caching and suggestions for caching strategy."
---
<!--
@@ -22,63 +23,86 @@
~ under the License.
-->
+You can enable caching in Apache Druid to improve query times for frequently accessed data. This topic defines the different types of caching for Druid. It describes the default caching behavior and provides guidance and examples to help you hone your caching strategy.
-Apache Druid supports query result caching at both the segment and whole-query result level. Cache data can be stored in the
-local JVM heap or in an external distributed key/value store. In all cases, the Druid cache is a query result cache.
-The only difference is whether the result is a _partial result_ for a particular segment, or the result for an entire
-query. In both cases, the cache is invalidated as soon as any underlying data changes; it will never return a stale
-result.
+If you're unfamiliar with Druid architecture, review the following topics before proceeding with caching:
+- [Druid Design](../design/architecture.md)
+- [Segments](../design/segments.md)
+- [Query execution](./query-execution.md)
-Segment-level caching allows the cache to be leveraged even when some of the underling segments are mutable and
-undergoing real-time ingestion. In this case, Druid will potentially cache query results for immutable historical
-segments, while re-computing results for the real-time segments on each query. Whole-query result level caching is not
-useful in this scenario, since it would be continuously invalidated.
+For instructions to configure query caching see [Using query caching](./using-caching.md).
-Segment-level caching does require Druid to merge the per-segment results on each query, even when they are served
-from the cache. For this reason, whole-query result level caching can be more efficient if invalidation due to real-time
-ingestion is not an issue.
+## Cache types
+
+Druid supports the following types of caches:
+
+- **Per-segment** caching which stores _partial results_ of a query for a specific segment. Per-segment caching is enabled on Historicals by default.
+- **Whole-query** caching which stores all results for a query.
+
+To avoid returning stale results, Druid invalidates the cache the moment any underlying data changes for both types of cache.
+
+Druid can store cache data on the local JVM heap or in an external distributed key/value store. The default is a local cache based upon [Caffeine](https://github.com/ben-manes/caffeine). Maximum cache storage defaults to the minimum value of 1 GiB or the ten percent of the maximum runtime memory for the JVM with no cache expiration. See [Cache configuration](../configuration/index.md#cache-configuration) for information on how to configure cache storage.
+
+### Per-segment caching
+
+The primary form of caching in Druid is the **per-segment cache** which stores query results on a per-segment basis. It is enabled on Historical services by default.
+
+When your queries include data from segments that are mutable and undergoing real-time ingestion, use a segment cache. In this case Druid caches query results for immutable historical segments when possible. It re-computes results for the real-time segments at query time.
+
+For example, you have queries that frequently include incoming data from a Kafka or Kinesis stream alongside unchanging segments. Per-segment caching lets Druid cache results from older immutable segments and merge them with updated data. Whole-query caching would not be helpful in this scenario because the new data from real-time ingestion continually invalidates the cache.
+
+### Whole-query caching
+
+If real-time ingestion invalidating the cache is not an issue for your queries, you can use **whole-query caching** on the Broker to increase query efficiency. The Broker performs whole-query caching operations before sending fan out queries to Historicals. Therefore Druid no longer needs to merge the per-segment results on the Broker.
+
+For instance, whole-query caching is a good option when you have queries that include data from a batch ingestion task that runs every few hours or once a day. Per-segment caching would be less efficient in this case because it requires Druid to merge the per-segment results for each query, even when the results are cached.
+
+## Where to enable caching
+
+**Per-segment cache** is available as follows:
+
+- On Historicals, the default. Enable segment-level cache population on Historicals for larger production clusters to prevent Brokers from having to merge all query results. When you enable cache population on Historicals instead of Brokers, the Historicals merge their own local results and put less strain on the Brokers.
+
+- On ingestion tasks in the Peon or Indexer service. Larger production clusters should enable segment-level cache population on task services only to prevent Brokers from having to merge all query results. When you enable cache population on task execution services instead of Brokers, the the task execution services to merge their own local results and put less strain on the Brokers.
+
+ Task executor services only support caches that store data locally. For example the `caffeine` cache. This restriction exists because the cache stores results at the level of intermediate partial segments generated by the ingestion tasks. These intermediate partial segments may not be identical across task replicas. Therefore task executor services ignore remote cache types such as `memcached`.
+
+- On Brokers for small production clusters with less than five servers.
+
+ Do not use per-segment caches on the Broker for large production clusters. When `druid.broker.cache.populateCache` is `true` and query context parameter `populateCache` _is not_ `false`, Historicals return results on a per-segment basis without merging results locally thus negatively impacting cluster scalability.
+
+**Whole-query cache** is only available on Brokers.
+
+## Performance considerations for caching
+Caching enables increased concurrency on the same system, therefore leading to noticeable performance improvements for queries on Druid clusters handling throughput for concurrent, mixed workloads.
+
+If you are looking to improve response time for a single query or page load, you should ignore caching. In general, response time for a single task should meet performance objectives even when the cache is cold.
+
+During query processing, the per-segment cache intercepts the query and sends the results directly to the Broker. This way the query bypasses the data server processing threads. For queries requiring minimal processing in the Broker, cached queries are very quick. If work done on the Broker causes a query bottleneck, enabling caching results in little noticeable query improvement.
+
+The largest performance gains from segment caching tend to apply to `topN` and time series queries. For `groupBy` queries, if the bottleneck is in the merging phase on the broker, the impact is less. The same applies to queries with or without joins.
+
+### Scenarios where caching does not increase query performance
+
+Caching does not solve all types of query performance issues. For each cache type there are scenarios where caching is likely to be of little benefit.
+
+**Per-segment caching** doesn't work for the following:
+- queries containing a sub-query in them. However the output of sub-queries may be cached. See [Query execution](./query-execution.md) for more details on sub-queries execution.
+- queries with joins do not support any caching on the broker.
+- GroupBy v2 queries do not support any caching on broker.
+- queries with `bySegment` set in the query context are not cached on the broker.
+
+**Whole-query caching** doesn't work for the following:
+- queries that involve an inline datasource or a lookup datasource.
+- GroupBy v2 queries.
+- queries with joins.
+- queries with a union datasource.
-## Using and populating cache
+## Learn more
+See the following topics for more information:
+- [Using query caching](./using-caching.md) to learn how to configure and use caching.
+- [Druid Design](../design/architecture.md) to learn about Druid processes.
+- [Segments](../design/segments.md) to learn how Druid stores data.
+- [Query execution](./query-execution.md) to learn how Druid services process query statements.
-All caches have a pair of parameters that control the behavior of how individual queries interact with the cache, a 'use' cache parameter, and a 'populate' cache parameter. These settings must be enabled at the service level via [runtime properties](../configuration/index.md) to utilize cache, but can be controlled on a per query basis by setting them on the [query context](../querying/query-context.md). The 'use' parameter obviously controls if a query will utilize cached results. The 'populate' parameter controls if a query will update cached results. These are separate parameters to allow queries on uncommon data to utilize cached results without polluting the cache with results that are unlikely to be re-used by other queries, for example large reports or very old data.
-
-## Query caching on Brokers
-
-Brokers support both segment-level and whole-query result level caching. Segment-level caching is controlled by the
-parameters `useCache` and `populateCache`. Whole-query result level caching is controlled by the parameters
-`useResultLevelCache` and `populateResultLevelCache` and [runtime properties](../configuration/index.md)
-`druid.broker.cache.*`.
-
-Enabling segment-level caching on the Broker can yield faster results than if query caches were enabled on Historicals for small
-clusters. This is the recommended setup for smaller production clusters (< 5 servers). Populating segment-level caches on
-the Broker is _not_ recommended for large production clusters, since when the property `druid.broker.cache.populateCache` is
-set to `true` (and query context parameter `populateCache` is _not_ set to `false`), results from Historicals are returned
-on a per segment basis, and Historicals will not be able to do any local result merging. This impairs the ability of the
-Druid cluster to scale well.
-
-## Query caching on Historicals
-
-Historicals only support segment-level caching. Segment-level caching is controlled by the query context
-parameters `useCache` and `populateCache` and [runtime properties](../configuration/index.md)
-`druid.historical.cache.*`.
-
-Larger production clusters should enable segment-level cache population on Historicals only (not on Brokers) to avoid
-having to use Brokers to merge all query results. Enabling cache population on the Historicals instead of the Brokers
-enables the Historicals to do their own local result merging and puts less strain on the Brokers.
-
-## Query caching on Ingestion Tasks
-
-Task executor processes such as the Peon or the experimental Indexer only support segment-level caching. Segment-level
-caching is controlled by the query context parameters `useCache` and `populateCache`
-and [runtime properties](../configuration/index.md) `druid.realtime.cache.*`.
-
-Larger production clusters should enable segment-level cache population on task execution processes only
-(not on Brokers) to avoid having to use Brokers to merge all query results. Enabling cache population on the
-task execution processes instead of the Brokers enables the task execution processes to do their own local
-result merging and puts less strain on the Brokers.
-
-Note that the task executor processes only support caches that keep their data locally, such as the `caffeine` cache.
-This restriction exists because the cache stores results at the level of intermediate partial segments generated by the
-ingestion tasks. These intermediate partial segments will not necessarily be identical across task replicas, so
-remote cache types such as `memcached` will be ignored by task executor processes.
diff --git a/docs/querying/using-caching.md b/docs/querying/using-caching.md
new file mode 100644
index 0000000..6b68afe
--- /dev/null
+++ b/docs/querying/using-caching.md
@@ -0,0 +1,91 @@
+---
+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 [Query caching](./caching.md). For information on how to configure the caching mechanism, see [Cache configuration](../configuration/index.md#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.broker.cache.useCache=true
+ druid.broker.cache.populateCache=true
+ ```
+See [Historical caching](../configuration/index.md#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](configuration/index.md#peon-caching) and [Indexer caching](configuration/index.md#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 `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 [Broker caching](../configuration/index.md#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](./query-context.md). 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](./sql.md#client-apis).
+
+
+
+## Learn more
+See the following topics for more information:
+- [Query caching](./caching.md) for an overview of caching.
+- [Query context](./query-context.md) for more details and usage for the query context.
+- [Cache configuration](../configuration/index.md#cache-configuration) for information about different cache types and additional configuration options.
diff --git a/website/sidebars.json b/website/sidebars.json
index f1e42b9..20c9692 100644
--- a/website/sidebars.json
+++ b/website/sidebars.json
@@ -69,6 +69,7 @@
"querying/multi-value-dimensions",
"querying/multitenancy",
"querying/caching",
+ "querying/using-caching",
"querying/query-context"
]
},
