mirror of https://github.com/apache/druid.git
69 lines
4.4 KiB
Markdown
69 lines
4.4 KiB
Markdown
---
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id: troubleshooting
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title: "Troubleshooting query execution in Druid"
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sidebar_label: "Troubleshooting"
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---
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~ Licensed to the Apache Software Foundation (ASF) under one
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This topic describes issues that may affect query execution in Druid, how to identify those issues, and strategies to resolve them.
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## Query fails due to internal communication timeout
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In Druid's query processing, when the Broker sends a query to the data servers, the data servers process the query and push their intermediate results back to the Broker.
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Because calls from the Broker to the data servers are synchronous, the Jetty server can time out in data servers in certain cases:
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1. The data servers don't push any results to the Broker before the maximum idle time.
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2. The data servers started to push data but paused for longer than the maximum idle time such as due to [Broker backpressure](../operations/basic-cluster-tuning.md#broker-backpressure).
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When such timeout occurs, the server interrupts the connection between the Broker and data servers which causes the query to fail with a channel disconnection error. For example,
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```json
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{
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"error": {
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"error": "Unknown exception",
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"errorMessage": "Query[6eee73a6-a95f-4bdc-821d-981e99e39242] url[https://localhost:8283/druid/v2/] failed with exception msg [Channel disconnected] (through reference chain: org.apache.druid.query.scan.ScanResultValue[\"segmentId\"])",
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"errorClass": "com.fasterxml.jackson.databind.JsonMappingException",
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"host": "localhost:8283"
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}
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}
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```
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Channel disconnection occurs for various reasons.
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To verify that the error is due to web server timeout, search for the query ID in the Historical logs.
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The query ID in the example above is `6eee73a6-a95f-4bdc-821d-981e99e39242`.
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The `"host"` field in the error message above indicates the IP address of the Historical in question.
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In the Historical logs, you will see a raised exception indicating `Idle timeout expired`:
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```text
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2021-09-14T19:52:27,685 ERROR [qtp475526834-85[scan_[test_large_table]_6eee73a6-a95f-4bdc-821d-981e99e39242]] org.apache.druid.server.QueryResource - Unable to send query response. (java.io.IOException: java.util.concurrent.TimeoutException: Idle timeout expired: 300000/300000 ms)
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2021-09-14T19:52:27,685 ERROR [qtp475526834-85] org.apache.druid.server.QueryLifecycle - Exception while processing queryId [6eee73a6-a95f-4bdc-821d-981e99e39242] (java.io.IOException: java.util.concurrent.TimeoutException: Idle timeout expired: 300000/300000 ms)
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2021-09-14T19:52:27,686 WARN [qtp475526834-85] org.eclipse.jetty.server.HttpChannel - handleException /druid/v2/ java.io.IOException: java.util.concurrent.TimeoutException: Idle timeout expired: 300000/300000 ms
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```
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To mitigate query failure due to web server timeout:
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* Increase the max idle time for the web server.
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Set the max idle time in the `druid.server.http.maxIdleTime` property in the `historical/runtime.properties` file.
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You must restart the Druid cluster for this change to take effect.
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See [Configuration reference](../configuration/index.md) for more information on configuring the server.
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* If the timeout occurs because the data servers have not pushed any results to the Broker, consider optimizing data server performance. Significant slowdown in the data servers may be a result of spilling too much data to disk in [groupBy queries](groupbyquery.md#performance-tuning-for-groupby), large [`IN` filters](filters.md#in-filter) in the query, or an under scaled cluster. Analyze your [Druid query metrics](../operations/metrics.md#query-metrics) to determine the bottleneck.
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* If the timeout is caused by Broker backpressure, consider optimizing Broker performance. Check whether the connection is fast enough between the Broker and deep storage.
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