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Elastic Docs ›Elasticsearch Guide [8.x] ›Index modules

Slow log

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Slow log

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The slow log records database searching and indexing events that have execution durations above specified thresholds. You can use these logs to investigate analyze or troubleshoot your cluster’s historical search and indexing performance.

Slow logs report task duration at the shard level for searches, and at the index level for indexing, but might not encompass the full task execution time observed on the client. For example, slow logs don’t surface HTTP network delays or the impact of task queues.

Events that meet the specified threshold are emitted into Elasticsearch logging under the fileset.name of slowlog. These logs can be viewed in the following locations:

  • If Elasticsearch monitoring is enabled, from Stack Monitoring. Slow log events have a logger value of index.search.slowlog or index.indexing.slowlog.
  • From local Elasticsearch service logs directory. Slow log files have a suffix of _index_search_slowlog.json or _index_indexing_slowlog.json.

Slow log format

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The following is an example of a search event in the slow log:

If a call was initiated with an X-Opaque-ID header, then the ID is automatically included in Search slow logs in the elasticsearch.slowlog.id field. See X-Opaque-Id HTTP header for details and best practices.

{
  "@timestamp": "2024-12-21T12:42:37.255Z",
  "auth.type": "REALM",
  "ecs.version": "1.2.0",
  "elasticsearch.cluster.name": "distribution_run",
  "elasticsearch.cluster.uuid": "Ui23kfF1SHKJwu_hI1iPPQ",
  "elasticsearch.node.id": "JK-jn-XpQ3OsDUsq5ZtfGg",
  "elasticsearch.node.name": "node-0",
  "elasticsearch.slowlog.id": "tomcat-123",
  "elasticsearch.slowlog.message": "[index6][0]",
  "elasticsearch.slowlog.search_type": "QUERY_THEN_FETCH",
  "elasticsearch.slowlog.source": "{\"query\":{\"match_all\":{\"boost\":1.0}}}",
  "elasticsearch.slowlog.stats": "[]",
  "elasticsearch.slowlog.took": "747.3micros",
  "elasticsearch.slowlog.took_millis": 0,
  "elasticsearch.slowlog.total_hits": "1 hits",
  "elasticsearch.slowlog.total_shards": 1,
  "event.dataset": "elasticsearch.index_search_slowlog",
  "fileset.name" : "slowlog",
  "log.level": "WARN",
  "log.logger": "index.search.slowlog.query",
  "process.thread.name": "elasticsearch[runTask-0][search][T#5]",
  "service.name": "ES_ECS",
  "user.name": "elastic",
  "user.realm": "reserved"
}

The following is an example of an indexing event in the slow log:

{
  "@timestamp" : "2024-12-11T22:34:22.613Z",
  "auth.type": "REALM",
  "ecs.version": "1.2.0",
  "elasticsearch.cluster.name" : "41bd111609d849fc9bf9d25b5df9ce96",
  "elasticsearch.cluster.uuid" : "BZTn4I9URXSK26imlia0QA",
  "elasticsearch.index.id" : "3VfGR7wRRRKmMCEn7Ii58g",
  "elasticsearch.index.name": "my-index-000001",
  "elasticsearch.node.id" : "GGiBgg21S3eqPDHzQiCMvQ",
  "elasticsearch.node.name" : "instance-0000000001",
  "elasticsearch.slowlog.id" : "RCHbt5MBT0oSsCOu54AJ",
  "elasticsearch.slowlog.source": "{\"key\":\"value\"}"
  "elasticsearch.slowlog.took" : "0.01ms",
  "event.dataset": "elasticsearch.index_indexing_slowlog",
  "fileset.name" : "slowlog",
  "log.level" : "TRACE",
  "log.logger" : "index.indexing.slowlog.index",
  "service.name" : "ES_ECS",
  "user.name": "elastic",
  "user.realm": "reserved"
}

Enable slow logging

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You can enable slow logging at two levels:

By default, all thresholds are set to -1, which results in no events being logged.

Slow log thresholds can be enabled for the four logging levels: trace, debug, info, and warn. You can mimic setting log level thresholds by disabling more verbose levels.

To view the current slow log settings, use the get index settings API:

resp = client.indices.get_settings(
    index="_all",
    expand_wildcards="all",
    filter_path="*.settings.index.*.slowlog",
)
print(resp)
const response = await client.indices.getSettings({
  index: "_all",
  expand_wildcards: "all",
  filter_path: "*.settings.index.*.slowlog",
});
console.log(response);
GET _all/_settings?expand_wildcards=all&filter_path=*.settings.index.*.slowlog
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Enable slow logging for search events

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Search slow logs emit per shard. They must be enabled separately for the shard’s query and fetch search phases.

You can use the index.search.slowlog.include.user setting to append user.* and auth.type fields to slow log entries. These fields contain information about the user who triggered the request.

The following snippet adjusts all available search slow log settings across all indices using the log4j2.properties configuration file:

index.search.slowlog.threshold.query.warn: 10s
index.search.slowlog.threshold.query.info: 5s
index.search.slowlog.threshold.query.debug: 2s
index.search.slowlog.threshold.query.trace: 500ms

index.search.slowlog.threshold.fetch.warn: 1s
index.search.slowlog.threshold.fetch.info: 800ms
index.search.slowlog.threshold.fetch.debug: 500ms
index.search.slowlog.threshold.fetch.trace: 200ms

index.search.slowlog.include.user: true

The following snippet adjusts the same settings for a single index using the update indices settings API:

resp = client.indices.put_settings(
    index="my-index-000001",
    settings={
        "index.search.slowlog.threshold.query.warn": "10s",
        "index.search.slowlog.threshold.query.info": "5s",
        "index.search.slowlog.threshold.query.debug": "2s",
        "index.search.slowlog.threshold.query.trace": "500ms",
        "index.search.slowlog.threshold.fetch.warn": "1s",
        "index.search.slowlog.threshold.fetch.info": "800ms",
        "index.search.slowlog.threshold.fetch.debug": "500ms",
        "index.search.slowlog.threshold.fetch.trace": "200ms",
        "index.search.slowlog.include.user": True
    },
)
print(resp)
const response = await client.indices.putSettings({
  index: "my-index-000001",
  settings: {
    "index.search.slowlog.threshold.query.warn": "10s",
    "index.search.slowlog.threshold.query.info": "5s",
    "index.search.slowlog.threshold.query.debug": "2s",
    "index.search.slowlog.threshold.query.trace": "500ms",
    "index.search.slowlog.threshold.fetch.warn": "1s",
    "index.search.slowlog.threshold.fetch.info": "800ms",
    "index.search.slowlog.threshold.fetch.debug": "500ms",
    "index.search.slowlog.threshold.fetch.trace": "200ms",
    "index.search.slowlog.include.user": true,
  },
});
console.log(response);
PUT /my-index-000001/_settings
{
  "index.search.slowlog.threshold.query.warn": "10s",
  "index.search.slowlog.threshold.query.info": "5s",
  "index.search.slowlog.threshold.query.debug": "2s",
  "index.search.slowlog.threshold.query.trace": "500ms",
  "index.search.slowlog.threshold.fetch.warn": "1s",
  "index.search.slowlog.threshold.fetch.info": "800ms",
  "index.search.slowlog.threshold.fetch.debug": "500ms",
  "index.search.slowlog.threshold.fetch.trace": "200ms",
  "index.search.slowlog.include.user": true
}
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Enable slow logging for indexing events

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Indexing slow logs emit per index document.

You can use the index.indexing.slowlog.include.user setting to append user.* and auth.type fields to slow log entries. These fields contain information about the user who triggered the request.

The following snippet adjusts all available indexing slow log settings across all indices using the log4j2.properties configuration file:

index.indexing.slowlog.threshold.index.warn: 10s
index.indexing.slowlog.threshold.index.info: 5s
index.indexing.slowlog.threshold.index.debug: 2s
index.indexing.slowlog.threshold.index.trace: 500ms

index.indexing.slowlog.source: 1000
index.indexing.slowlog.reformat: true

index.indexing.slowlog.include.user: true

The following snippet adjusts the same settings for a single index using the update indices settings API:

resp = client.indices.put_settings(
    index="my-index-000001",
    settings={
        "index.indexing.slowlog.threshold.index.warn": "10s",
        "index.indexing.slowlog.threshold.index.info": "5s",
        "index.indexing.slowlog.threshold.index.debug": "2s",
        "index.indexing.slowlog.threshold.index.trace": "500ms",
        "index.indexing.slowlog.source": "1000",
        "index.indexing.slowlog.reformat": True,
        "index.indexing.slowlog.include.user": True
    },
)
print(resp)
const response = await client.indices.putSettings({
  index: "my-index-000001",
  settings: {
    "index.indexing.slowlog.threshold.index.warn": "10s",
    "index.indexing.slowlog.threshold.index.info": "5s",
    "index.indexing.slowlog.threshold.index.debug": "2s",
    "index.indexing.slowlog.threshold.index.trace": "500ms",
    "index.indexing.slowlog.source": "1000",
    "index.indexing.slowlog.reformat": true,
    "index.indexing.slowlog.include.user": true,
  },
});
console.log(response);
PUT /my-index-000001/_settings
{
  "index.indexing.slowlog.threshold.index.warn": "10s",
  "index.indexing.slowlog.threshold.index.info": "5s",
  "index.indexing.slowlog.threshold.index.debug": "2s",
  "index.indexing.slowlog.threshold.index.trace": "500ms",
  "index.indexing.slowlog.source": "1000",
  "index.indexing.slowlog.reformat": true,
  "index.indexing.slowlog.include.user": true
}
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Logging the _source field

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By default, Elasticsearch logs the first 1000 characters of the _source in the slow log. You can adjust how _source is logged using the index.indexing.slowlog.source setting. Set index.indexing.slowlog.source to false or 0 to skip logging the source entirely. Set index.indexing.slowlog.source to true to log the entire source regardless of size.

The original _source is reformatted by default to make sure that it fits on a single log line. If preserving the original document format is important, then you can turn off reformatting by setting index.indexing.slowlog.reformat to false. This causes source to be logged with the original formatting intact, potentially spanning multiple log lines.

Best practices for slow logging

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Logging slow requests can be resource intensive to your Elasticsearch cluster depending on the qualifying traffic’s volume. For example, emitted logs might increase the index disk usage of your Elasticsearch monitoring cluster. To reduce the impact of slow logs, consider the following:

  • Enable slow logs against specific indices rather than across all indices.
  • Set high thresholds to reduce the number of logged events.
  • Enable slow logs only when troubleshooting.

If you aren’t sure how to start investigating traffic issues, consider enabling the warn threshold with a high 30s threshold at the index level using the update indices settings API:

  • Enable for search requests:

    resp = client.indices.put_settings(
    index="*",
    settings={
        "index.search.slowlog.include.user": True,
        "index.search.slowlog.threshold.fetch.warn": "30s",
        "index.search.slowlog.threshold.query.warn": "30s"
    },
)
print(resp)
    const response = await client.indices.putSettings({
  index: "*",
  settings: {
    "index.search.slowlog.include.user": true,
    "index.search.slowlog.threshold.fetch.warn": "30s",
    "index.search.slowlog.threshold.query.warn": "30s",
  },
});
console.log(response);
    PUT /*/_settings
{
  "index.search.slowlog.include.user": true,
  "index.search.slowlog.threshold.fetch.warn": "30s",
  "index.search.slowlog.threshold.query.warn": "30s"
}
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  • Enable for indexing requests:

    resp = client.indices.put_settings(
    index="*",
    settings={
        "index.indexing.slowlog.include.user": True,
        "index.indexing.slowlog.threshold.index.warn": "30s"
    },
)
print(resp)
    const response = await client.indices.putSettings({
  index: "*",
  settings: {
    "index.indexing.slowlog.include.user": true,
    "index.indexing.slowlog.threshold.index.warn": "30s",
  },
});
console.log(response);
    PUT /*/_settings
{
  "index.indexing.slowlog.include.user": true,
  "index.indexing.slowlog.threshold.index.warn": "30s"
}
    Copy as curlTry in Elastic 

Slow log thresholds being met does not guarantee cluster performance issues. In the event that symptoms are noticed, slow logs can provide helpful data to diagnose upstream traffic patterns or sources to resolve client-side issues. For example, you can use data included in X-Opaque-ID, the _source request body, or user.* fields to identify the source of your issue. This is similar to troubleshooting live expensive tasks.

If you’re experiencing search performance issues, then you might also consider investigating searches flagged for their query durations using the profile API. You can then use the profiled query to investigate optimization options using the query profiler. This type of investigation should usually take place in a non-production environment.

Slow logging checks each event against the reporting threshold when the event is complete. This means that it can’t report if events trigger circuit breaker errors. If suspect circuit breaker errors, then you should also consider enabling audit logging, which logs events before they are executed.

Learn more

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To learn about other ways to optimize your search and indexing requests, refer to tune for search speed and tune for indexing speed.

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