[[index-modules-slowlog]]
== Slow log

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-queue-backlog,task queues>>. 

Events that meet the specified threshold are emitted into <<logging,{es} logging>> under the `fileset.name` of `slowlog`. These logs can be viewed in the following locations:

* If <<monitoring-overview,{es} monitoring>> is enabled, from
{kibana-ref}/xpack-monitoring.html[Stack Monitoring]. Slow log events have a `logger` value of `index.search.slowlog` or `index.indexing.slowlog`.

* From local {es} service logs directory. Slow log files have a suffix of `_index_search_slowlog.json` or `_index_indexing_slowlog.json`.

[discrete]
[[slow-log-format]]
=== Slow log format

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

TIP: 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,X-Opaque-Id HTTP header>> for details and best practices.

[source,js]
---------------------------
{
  "@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"
}

---------------------------
// NOTCONSOLE


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

[source,js]
---------------------------
{
  "@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"
}

---------------------------
// NOTCONSOLE

[discrete]
[[enable-slow-log]]
=== Enable slow logging

You can enable slow logging at two levels: 

* For all indices under the <<settings,{es} `log4j2.properties` configuration file>>. This method requires a node restart.
* At the index level, using the <<indices-update-settings,update indices settings API>>

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 <<indices-get-settings,get index settings API>>:

[source,console]
--------------------------------------------------
GET _all/_settings?expand_wildcards=all&filter_path=*.settings.index.*.slowlog
--------------------------------------------------

[discrete]
[[search-slow-log]]
==== Enable slow logging for search events

Search slow logs emit per shard. They must be enabled separately for the shard's link:https://www.elastic.co/blog/understanding-query-then-fetch-vs-dfs-query-then-fetch[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 
<<settings,`log4j2.properties` configuration file>>:

[source,yaml]
--------------------------------------------------
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 <<indices-update-settings,update indices settings API>>:

[source,console]
--------------------------------------------------
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
}
--------------------------------------------------
// TEST[setup:my_index]


[discrete]
[[index-slow-log]]
==== Enable slow logging for indexing events

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 
<<settings,`log4j2.properties` configuration file>>:

[source,yaml]
--------------------------------------------------
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 <<indices-update-settings,update indices settings API>>:

[source,console]
--------------------------------------------------
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
}
--------------------------------------------------
// TEST[setup:my_index]

[discrete]
===== Logging the `_source` field

By default, {es} 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.

[discrete]
[[slow-log-fields]]

[discrete]
[[troubleshoot-slow-log]]
=== Best practices for slow logging

Logging slow requests can be resource intensive to your {es} cluster depending on the qualifying traffic's volume. For example, emitted logs might increase the index disk usage of your <<monitoring-overview,{es} 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 <<indices-update-settings,update indices settings API>>: 

* Enable for search requests: 
+
[source,console]
--------------------------------------------------
PUT /*/_settings
{
  "index.search.slowlog.include.user": true,
  "index.search.slowlog.threshold.fetch.warn": "30s",
  "index.search.slowlog.threshold.query.warn": "30s"
}
--------------------------------------------------
// TEST[setup:my_index]

* Enable for indexing requests:
+
[source,console]
--------------------------------------------------
PUT /*/_settings
{
  "index.indexing.slowlog.include.user": true,
  "index.indexing.slowlog.threshold.index.warn": "30s"
}
--------------------------------------------------
// TEST[setup:my_index]

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 <<task-queue-backlog,live expensive tasks>>.

If you're experiencing search performance issues, then you might also consider investigating searches flagged for their query durations using the <<search-profile,profile API>>. You can then use the profiled query to investigate optimization options using the link:{kibana-ref}/xpack-profiler.html[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,circuit breaker errors>>. If suspect circuit breaker errors, then you should also consider enabling <<enable-audit-logging,audit logging>>, which logs events before they are executed.

[discrete]
=== Learn more

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