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.

When debugging problems, particularly problems with plugins, it can be helpful to increase the logging level to DEBUG to emit more verbose messages. Previously, you could only set a log level that applied to the entire Logstash product. Starting with 5.0, you can configure logging for a particular subsystem in Logstash. For example, if you are debugging issues with Elasticsearch Output, you can increase log levels just for that component. This way you can reduce noise due to excessive logging and focus on the problem area effectively.

Log file locationedit

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

Log4j 2 Configurationedit

Logstash ships with a log4j2.properties file with out-of-the-box settings. You can modify this file directly to change the rotation policy, type, and other log4j2 configuration. You must restart Lostash to apply any changes that you make to this file.

Logging APIsedit

You could modify the log4j2.properties file and restart your Logstash, but that is both tedious and 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. To update logging levels, take the subsystem/module you are interested in and prepend logger. to it. For example:

PUT /_node/logging
    "logger.logstash.outputs.elasticsearch" : "DEBUG"

While this setting is in effect, Logstash will begin to emit 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.

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

GET /_node/logging?pretty

Example response:

"loggers" : {
   "logstash.registry" : "WARN",
   "logstash.instrument.periodicpoller.os" : "WARN",
   "logstash.instrument.collector" : "WARN",
   "logstash.runner" : "WARN",
   "logstash.inputs.stdin" : "WARN",
   "logstash.outputs.stdout" : "WARN",
   "logstash.agent" : "WARN",
   "logstash.api.service" : "WARN",
   "logstash.instrument.periodicpoller.jvm" : "WARN",
   "logstash.pipeline" : "WARN",
   "logstash.codecs.line" : "WARN"