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Elastic Docs ›Logstash Reference [8.15] ›Setting Up and Running Logstash

Logging

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Logging

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Logstash emits internal logs during its operation, which are placed in LS_HOME/logs (or /var/log/logstash for DEB/RPM). The default logging level is INFO. Logstash’s logging framework is based on Log4j 2 framework, and much of its functionality is exposed directly to users.

You can configure logging for a particular subsystem, module, or plugin.

When you need to debug problems, particularly problems with plugins, consider increasing the logging level to DEBUG to get more verbose messages. For example, if you are debugging issues with Elasticsearch Output, you can increase log levels just for that component. This approach reduces noise from excessive logging and helps you focus on the problem area.

You can configure logging using the log4j2.properties file or the Logstash API.

  • log4j2.properties file. Changes made through the log4j2.properties file require you to restart Logstash for the changes to take effect. Changes persist through subsequent restarts.
  • Logging API. Changes made through the Logging API are effective immediately without a restart. The changes do not persist after Logstash is restarted.

Log4j2 configuration

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Logstash ships with a log4j2.properties file with out-of-the-box settings, including logging to console. You can modify this file to change the rotation policy, type, and other log4j2 configuration.

You must restart Logstash to apply any changes that you make to this file. Changes to log4j2.properties persist after Logstash is restarted.

Here’s an example using outputs.elasticsearch:

logger.elasticsearchoutput.name = logstash.outputs.elasticsearch
logger.elasticsearchoutput.level = debug

The previous example defines a name and level for the logger logstash.outputs.elasticsearch. The logger is usually identified by a Java class name, such as org.logstash.dissect.Dissector, for example. It can also be a partial package path as in org.logstash.dissect. For Ruby classes, like LogStash::Outputs::Elasticsearch, the logger name is obtained by lowercasing the full class name and replacing double colons with a single dot.

Consider using the default log4j configuration that is shipped with Logstash, as it is configured to work well for most deployments. The next section describes how the rolling strategy works in case you need to make adjustments.

Rollover settings

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The log4j2.properties file has three appenders for writing to log files: one for plain text, one with json format, and one to split log lines on per pipeline basis when you set the pipeline.separate_logs value.

These appenders define:

  • triggering policies that determine if a rollover should be performed, and
  • rollover strategy to defines how the rollover should be done.

By default, two triggering policies are defined—​time and size.

  • The time policy creates one file per day.
  • The size policy forces the creation of a new file after the file size surpasses 100 MB.

The default strategy also performs file rollovers based on a maximum number of files. When the limit of 30 files has been reached, the first (oldest) file is removed to create space for the new file. Subsequent files are renumbered accordingly.

Each file has a date, and files older than 7 days (default) are removed during rollover.

appender.rolling.type = RollingFile 
appender.rolling.name = plain_rolling
appender.rolling.fileName = ${sys:ls.logs}/logstash-plain.log 
appender.rolling.filePattern = ${sys:ls.logs}/logstash-plain-%d{yyyy-MM-dd}-%i.log.gz 
appender.rolling.policies.type = Policies
appender.rolling.policies.time.type = TimeBasedTriggeringPolicy 
appender.rolling.policies.time.interval = 1
appender.rolling.policies.time.modulate = true
appender.rolling.layout.type = PatternLayout
appender.rolling.layout.pattern = [%d{ISO8601}][%-5p][%-25c]%notEmpty{[%X{pipeline.id}]}%notEmpty{[%X{plugin.id}]} %m%n
appender.rolling.policies.size.type = SizeBasedTriggeringPolicy 
appender.rolling.policies.size.size = 100MB
appender.rolling.strategy.type = DefaultRolloverStrategy
appender.rolling.strategy.max = 30 
appender.rolling.strategy.action.type = Delete 
appender.rolling.strategy.action.basepath = ${sys:ls.logs}
appender.rolling.strategy.action.condition.type = IfFileName
appender.rolling.strategy.action.condition.glob = logstash-plain-* 
appender.rolling.strategy.action.condition.nested_condition.type = IfLastModified
appender.rolling.strategy.action.condition.nested_condition.age = 7D

The appender type, which rolls older log files.

Name of the current log file.

Name’s format definition of the rolled files, in this case a date followed by an incremental number, up to 30 (by default).

Time policy to trigger a rollover at the end of the day.

Size policy to trigger a rollover once the plain text file reaches the size of 100 MB.

Rollover strategy defines a maximum of 30 files.

Action to execute during the rollover.

The file set to consider by the action.

Condition to execute the rollover action: older than 7 days.

The rollover action can also enforce a disk usage limit, deleting older files to match the requested condition, as an example:

appender.rolling.type = RollingFile
...
appender.rolling.strategy.action.condition.glob = pipeline_${ctx:pipeline.id}.*.log.gz
appender.rolling.strategy.action.condition.nested_condition.type = IfAccumulatedFileSize
appender.rolling.strategy.action.condition.nested_condition.exceeds = 5MB

Deletes files if total accumulated compressed file size is over 5MB.

Logging APIs

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For temporary logging changes, modifying the log4j2.properties file and restarting Logstash leads to unnecessary downtime. Instead, you can dynamically update logging levels through the logging API. These settings are effective immediately and do not need a restart.

By default, the logging API attempts to bind to tcp:9600. If this port is already in use by another Logstash instance, you need to launch Logstash with the --api.http.port flag specified to bind to a different port. See Command-Line Flags for more information.

Retrieve list of logging configurations

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To retrieve a list of logging subsystems available at runtime, you can do a GET request to _node/logging

curl -XGET 'localhost:9600/_node/logging?pretty'

Example response:

{
...
  "loggers" : {
    "logstash.agent" : "INFO",
    "logstash.api.service" : "INFO",
    "logstash.basepipeline" : "INFO",
    "logstash.codecs.plain" : "INFO",
    "logstash.codecs.rubydebug" : "INFO",
    "logstash.filters.grok" : "INFO",
    "logstash.inputs.beats" : "INFO",
    "logstash.instrument.periodicpoller.jvm" : "INFO",
    "logstash.instrument.periodicpoller.os" : "INFO",
    "logstash.instrument.periodicpoller.persistentqueue" : "INFO",
    "logstash.outputs.stdout" : "INFO",
    "logstash.pipeline" : "INFO",
    "logstash.plugins.registry" : "INFO",
    "logstash.runner" : "INFO",
    "logstash.shutdownwatcher" : "INFO",
    "org.logstash.Event" : "INFO",
    "slowlog.logstash.codecs.plain" : "TRACE",
    "slowlog.logstash.codecs.rubydebug" : "TRACE",
    "slowlog.logstash.filters.grok" : "TRACE",
    "slowlog.logstash.inputs.beats" : "TRACE",
    "slowlog.logstash.outputs.stdout" : "TRACE"
  }
}

Update logging levels

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Prepend the name of the subsystem, module, or plugin with logger..

Here is an example using outputs.elasticsearch:

curl -XPUT 'localhost:9600/_node/logging?pretty' -H 'Content-Type: application/json' -d'
{
    "logger.logstash.outputs.elasticsearch" : "DEBUG"
}
'

While this setting is in effect, Logstash emits DEBUG-level logs for all the Elasticsearch outputs specified in your configuration. Please note this new setting is transient and will not survive a restart.

If you want logging changes to persist after a restart, add them to log4j2.properties instead.

Reset dynamic logging levels

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To reset any logging levels that may have been dynamically changed via the logging API, send a PUT request to _node/logging/reset. All logging levels will revert to the values specified in the log4j2.properties file.

curl -XPUT 'localhost:9600/_node/logging/reset?pretty'

Log file location

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You can specify the log file location using --path.logs setting.

Slowlog

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Slowlog for Logstash adds the ability to log when a specific event takes an abnormal amount of time to make its way through the pipeline. Just like the normal application log, you can find slowlogs in your --path.logs directory. Slowlog is configured in the logstash.yml settings file with the following options:

slowlog.threshold.warn (default: -1)
slowlog.threshold.info (default: -1)
slowlog.threshold.debug (default: -1)
slowlog.threshold.trace (default: -1)

Slowlog is disabled by default. The default threshold values are set to -1nanos to represent an infinite threshold. No slowlog will be invoked.

Enable slowlog

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The slowlog.threshold fields use a time-value format which enables a wide range of trigger intervals. You can specify ranges using the following time units: nanos (nanoseconds), micros (microseconds), ms (milliseconds), s (second), m (minute), h (hour), d (day).

Slowlog becomes more sensitive and logs more events as you raise the log level.

Example:

slowlog.threshold.warn: 2s
slowlog.threshold.info: 1s
slowlog.threshold.debug: 500ms
slowlog.threshold.trace: 100ms

In this example:

  • If the log level is set to warn, the log shows events that took longer than 2s to process.
  • If the log level is set to info, the log shows events that took longer than 1s to process.
  • If the log level is set to debug, the log shows events that took longer than 500ms to process.
  • If the log level is set to trace, the log shows events that took longer than 100ms to process.

The logs include the full event and filter configuration that are responsible for the slowness.

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