druid/docs/querying/caching.md

id	title	description
caching	Query caching	Describes Apache Druid per-segment and whole-query cache types. Identifies services where you can enable caching and suggestions for caching strategy.

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.

If you're unfamiliar with Druid architecture, review the following topics before proceeding with caching:

• Druid Design
• Segments
• Query execution

For instructions to configure query caching see Using query caching.

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. 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 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 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.

Learn more

See the following topics for more information:

• Using query caching to learn how to configure and use caching.
• Druid Design to learn about Druid processes.
• Segments to learn how Druid stores data.
• Query execution to learn how Druid services process query statements.