This documentation is for an out-of-date version of Apache Flink. We recommend you use the latest stable version.

How to use logging

The logging in Flink is implemented using the slf4j logging interface. As underlying logging framework, log4j2 is used. We also provide logback configuration files and pass them to the JVM’s as properties. Users willing to use logback instead of log4j2 can just exclude log4j2 (or delete it from the lib/ folder).

Configuring Log4j2

Log4j2 is controlled using property files. In Flink’s case, the file is usually called We pass the filename and location of this file using the -Dlog4j.configurationFile= parameter to the JVM.

Flink ships with the following default properties files:

  • Used by the Flink command line client (e.g. flink run) (not code executed on the cluster)
  • Used by the Flink command line client when starting a YARN or Kubernetes session (,
  • JobManager/Taskmanager logs (both standalone and YARN)

Compatibility with Log4j1

Flink ships with the Log4j API bridge, allowing existing applications that work against Log4j1 classes to continue working.

If you have custom Log4j1 properties files or code that relies on Log4j1, please check out the official Log4j compatibility and migration guides.

Configuring Log4j1

To use Flink with Log4j1 you must ensure that:

  • org.apache.logging.log4j:log4j-core, org.apache.logging.log4j:log4j-slf4j-impl and org.apache.logging.log4j:log4j-1.2-api are not on the classpath,
  • log4j:log4j, org.slf4j:slf4j-log4j12, org.apache.logging.log4j:log4j-to-slf4j and org.apache.logging.log4j:log4j-api are on the classpath.

In the IDE this means you have to replace such dependencies defined in your pom, and possibly add exclusions on dependencies that transitively depend on them.

For Flink distributions this means you have to

  • remove the log4j-core, log4j-slf4j-impl and log4j-1.2-api jars from the lib directory,
  • add the log4j, slf4j-log4j12 and log4j-to-slf4j jars to the lib directory,
  • replace all log4j properties files in the conf directory with Log4j1-compliant versions.

Configuring logback

For users and developers alike it is important to control the logging framework. The configuration of the logging framework is exclusively done by configuration files. The configuration file either has to be specified by setting the environment property -Dlogback.configurationFile=<file> or by putting logback.xml in the classpath. The conf directory contains a logback.xml file which can be modified and is used if Flink is started outside of an IDE and with the provided starting scripts. The provided logback.xml has the following form:

    <appender name="file" class="ch.qos.logback.core.FileAppender">
            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{60} %X{sourceThread} - %msg%n</pattern>

    <root level="INFO">
        <appender-ref ref="file"/>

In order to control the logging level of org.apache.flink.runtime.jobgraph.JobGraph, for example, one would have to add the following line to the configuration file.

<logger name="org.apache.flink.runtime.jobgraph.JobGraph" level="DEBUG"/>

For further information on configuring logback see LOGback’s manual.

Best practices for developers

The loggers using slf4j are created by calling

import org.slf4j.LoggerFactory
import org.slf4j.Logger

Logger LOG = LoggerFactory.getLogger(Foobar.class)

In order to benefit most from slf4j, it is recommended to use its placeholder mechanism. Using placeholders allows to avoid unnecessary string constructions in case that the logging level is set so high that the message would not be logged. The syntax of placeholders is the following:"This message contains {} placeholders. {}", 2, "Yippie");

Placeholders can also be used in conjunction with exceptions which shall be logged.

catch(Exception exception){
	LOG.error("An {} occurred.", "error", exception);

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