This manual is for an old version of Hazelcast Jet, use the latest stable version.

Configuring Logging

Jet, like Hazelcast IMDG does not depend on a specific logging framework and has built-in adapters for a variety of logging frameworks. You can also write a new adapter to integrate with loggers Jet doesn't natively support. To use one of the built-in adapters, set the hazelcast.logging.type property to one of the following:

  • jdk: java.util.logging (default)
  • log4j: Apache Log4j
  • log4j2: Apache Log4j 2
  • slf4j: SLF4J
  • none: Turn off logging

For example, to configure Jet to use Log4j, you can do one of the following:

System.setProperty("hazelcast.logging.type", "log4j");


JetConfig config = new JetConfig() ;
config.getHazelcastConfig().setProperty( "hazelcast.logging.type", "log4j" );

For more detailed information about how to configure logging, please refer to the IMDG reference manual.

Inspecting Output of Individual Stages

When building pipelines, it's often useful to see what the output of each stage is. This can be achieved by using the peek() stage. For example:

p.drawFrom(Sources.<Long, String>map(...))
   .flatMap(e -> traverseArray(delimiter.split(e.getValue().toLowerCase())))
   .filter(word -> !word.isEmpty())
   .groupBy(wholeItem(), counting())

In this pipeline, the output of the groupBy stage will be logged using the configured logging framework:

16:05:29,650  INFO || - [com.hazelcast.jet.impl.processor.PeekWrappedP.groupByKey.cd6373be.stage2#0] hz._hzInstance_1_jet.jet.cooperative.thread-1 - []:5701 [jet] [0.6-SNAPSHOT] Output to 0: accusers=6
16:05:29,650  INFO || - [com.hazelcast.jet.impl.processor.PeekWrappedP.groupByKey.cd6373be.stage2#0] hz._hzInstance_1_jet.jet.cooperative.thread-1 - []:5701 [jet] [0.6-SNAPSHOT] Output to 0: mutability=2
16:05:29,650  INFO || - [com.hazelcast.jet.impl.processor.PeekWrappedP.groupByKey.cd6373be.stage2#0] hz._hzInstance_1_jet.jet.cooperative.thread-1 - []:5701 [jet] [0.6-SNAPSHOT] Output to 0: lovely=53

The logger name of com.hazelcast.jet.impl.processor.PeekWrappedP.groupByKey.cd6373be.stage2#0 can be decomposed as follows:

  • com.hazelcast.jet.impl.processor.PeekWrappedP: class of the processor writing the log message
  • groupByKey.cd6373be.stage2: Name of the vertex the processor belongs to
  • #0: the unique index (per vertex) of the processor instance

Keep in mind that this can create a large amount of output when dealing with large volumes of data, and should strictly be used in a non-production environment.

For more information about logging when using the Core API, see the Best Practices section.