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| <h1 id="query-caching">Query Caching</h1> |
| |
| <p>Apache Druid (incubating) 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 <em>partial result</em> 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.</p> |
| |
| <p>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.</p> |
| |
| <p>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.</p> |
| |
| <h2 id="query-caching-on-brokers">Query caching on Brokers</h2> |
| |
| <p>Brokers support both segment-level and whole-query result level caching. Segment-level caching is controlled by the |
| parameters <code>useCache</code> and <code>populateCache</code>. Whole-query result level caching is controlled by the parameters |
| <code>useResultLevelCache</code> and <code>populateResultLevelCache</code> and <a href="../configuration/index.html">runtime properties</a> |
| <code>druid.broker.cache.*</code>..</p> |
| |
| <p>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 <em>not</em> recommended for large production clusters, since when the property <code>druid.broker.cache.populateCache</code> is |
| set to <code>true</code> (and query context parameter <code>populateCache</code> is <em>not</em> set to <code>false</code>), 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.</p> |
| |
| <h2 id="query-caching-on-historicals">Query caching on Historicals</h2> |
| |
| <p>Historicals only support segment-level caching. Segment-level caching is controlled by the query context |
| parameters <code>useCache</code> and <code>populateCache</code> and <a href="../configuration/index.html">runtime properties</a> |
| <code>druid.historical.cache.*</code>.</p> |
| |
| <p>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.</p> |
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