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<blockquote>
<p>The Indexer is an optional and <a href="../development/experimental.html">experimental</a> feature.
Its memory management system is still under development and will be significantly enhanced in later releases.</p>
</blockquote>
<p>The Apache Druid (incubating) Indexer process is an alternative to the MiddleManager + Peon task execution system. Instead of forking a separate JVM process per-task, the Indexer runs tasks as separate threads within a single JVM process.</p>
<p>The Indexer is designed to be easier to configure and deploy compared to the MiddleManager + Peon system and to better enable resource sharing across tasks.</p>
<h3><a class="anchor" aria-hidden="true" id="configuration"></a><a href="#configuration" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Configuration</h3>
<p>For Apache Druid (incubating) Indexer Process Configuration, see <a href="../configuration/index.html#indexer">Indexer Configuration</a>.</p>
<h3><a class="anchor" aria-hidden="true" id="http-endpoints"></a><a href="#http-endpoints" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>HTTP endpoints</h3>
<p>The Indexer process shares the same HTTP endpoints as the <a href="../operations/api-reference.html#middlemanager">MiddleManager</a>.</p>
<h3><a class="anchor" aria-hidden="true" id="running"></a><a href="#running" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Running</h3>
<pre><code class="hljs">org.apache.druid.cli.Main<span class="hljs-built_in"> server </span>indexer
</code></pre>
<h3><a class="anchor" aria-hidden="true" id="task-resource-sharing"></a><a href="#task-resource-sharing" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Task resource sharing</h3>
<p>The following resources are shared across all tasks running inside an Indexer process.</p>
<h4><a class="anchor" aria-hidden="true" id="query-resources"></a><a href="#query-resources" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Query resources</h4>
<p>The query processing threads and buffers are shared across all tasks. The Indexer will serve queries from a single endpoint shared by all tasks.</p>
<p>If <a href="../configuration/index.html#indexer-caching">query caching</a> is enabled, the query cache is also shared across all tasks.</p>
<h4><a class="anchor" aria-hidden="true" id="server-http-threads"></a><a href="#server-http-threads" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Server HTTP threads</h4>
<p>The Indexer maintains two equally sized pools of HTTP threads.</p>
<p>One pool is exclusively used for task control messages between the Overlord and the Indexer (&quot;chat handler threads&quot;). The other pool is used for handling all other HTTP requests.</p>
<p>The size of the pools are configured by the <code>druid.server.http.numThreads</code> configuration (e.g., if this is set to 10, there will be 10 chat handler threads and 10 non-chat handler threads).</p>
<p>In addition to these two pools, 2 separate threads are allocated for lookup handling. If lookups are not used, these threads will not be used.</p>
<h4><a class="anchor" aria-hidden="true" id="memory-sharing"></a><a href="#memory-sharing" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Memory sharing</h4>
<p>The Indexer uses the <code>druid.worker.globalIngestionHeapLimitBytes</code> configuration to impose a global heap limit across all of the tasks it is running.</p>
<p>This global limit is evenly divided across the number of task slots configured by <code>druid.worker.capacity</code>.</p>
<p>To apply the per-task heap limit, the Indexer will override <code>maxBytesInMemory</code> in task tuning configs (i.e., ignoring the default value or any user configured value). <code>maxRowsInMemory</code> will also be overridden to an essentially unlimited value: the Indexer does not support row limits.</p>
<p>By default, <code>druid.worker.globalIngestionHeapLimitBytes</code> is set to 1/6th of the available JVM heap. This default is chosen to align with the default value of <code>maxBytesInMemory</code> in task tuning configs when using the MiddleManager/Peon system, which is also 1/6th of the JVM heap.</p>
<p>The peak usage for rows held in heap memory relates to the interaction between the <code>maxBytesInMemory</code> and <code>maxPendingPersists</code> properties in the task tuning configs. When the amount of row data held in-heap by a task reaches the limit specified by <code>maxBytesInMemory</code>, a task will persist the in-heap row data. After the persist has been started, the task can again ingest up to <code>maxBytesInMemory</code> bytes worth of row data while the persist is running.</p>
<p>This means that the peak in-heap usage for row data can be up to approximately <code>maxBytesInMemory</code> * (2 + <code>maxPendingPersists</code>). The default value of <code>maxPendingPersists</code> is 0, which allows for 1 persist to run concurrently with ingestion work.</p>
<p>The remaining portion of the heap is reserved for query processing and segment persist/merge operations, and miscellaneous heap usage.</p>
<h4><a class="anchor" aria-hidden="true" id="concurrent-segment-persist-merge-limits"></a><a href="#concurrent-segment-persist-merge-limits" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Concurrent segment persist/merge limits</h4>
<p>To help reduce peak memory usage, the Indexer imposes a limit on the number of concurrent segment persist/merge operations across all running tasks.</p>
<p>By default, the number of concurrent persist/merge operations is limited to (<code>druid.worker.capacity</code> / 2), rounded down. This limit can be configured with the <code>druid.worker.numConcurrentMerges</code> property.</p>
<h3><a class="anchor" aria-hidden="true" id="current-limitations"></a><a href="#current-limitations" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Current limitations</h3>
<p>Separate task logs are not currently supported when using the Indexer; all task log messages will instead be logged in the Indexer process log.</p>
<p>The Indexer currently imposes an identical memory limit on each task. In later releases, the per-task memory limit will be removed and only the global limit will apply. The limit on concurrent merges will also be removed.</p>
<p>In later releases, per-task memory usage will be dynamically managed. Please see <a href="https://github.com/apache/incubator-druid/issues/7900">https://github.com/apache/incubator-druid/issues/7900</a> for details on future enhancements to the Indexer.</p>
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