Logging is an indisputably important feature for applications. Logging monitors what the application is doing, but equally importantly, it logs what the application is not doing. Looking into application logs can help you take an application out of a faulty state, or even help prevent it from happening.

Setting logging levels dynamically

There are many frameworks used for logging in Java applications, for example Logback, tinylog, or log4j2. Kafka uses log4j, but we decided to use the newer log4j2 for Strimzi operators. The main reason is that log4j2 uses the similar principles and format of a configuration file. Strimzi components are split into the two groups regarding which implementation of log4j they use.

log4j

  • Kafka brokers
  • Kafka Connect
  • Kafka MirrorMaker 2.0

log4j2

  • Strimzi Kafka Bridge
  • Strimzi Topic Operator
  • Strimzi User Operator
  • Strimzi Cluster Operator

A Log4j2 implementation supports dynamic reloading of logging configuration from a properties file. Log4j doesn’t support reloading the whole log configuration from a properties file, but it can be changed programmatically using the Log4j APIs. And that is why each implementation requires a different approach, which we will look at now.

Strimzi Cluster Operator

In order to change the Cluster Operator logging config, a simple change is required. You have to configure the logging settings in the ConfigMap used for Cluster Operator configuration. This ConfigMap is by default called strimzi-cluster-operator. To edit the logging configuration use:

$ kubectl edit cm strimzi-cluster-operator

Under the field log4j2.properties you will see the current logging configuration. Now you can edit this configuration to whatever you want with one exception. The entry monitorInterval must stay in place. When it is removed, the configuration will not be reloaded dynamically. To apply changes in such a case, a rolling update needs to be performed on the pod.

The second way to change ConfigMap is to modify the file install/cluster-operator/050-ConfigMap-strimzi-cluster-operator.yaml, and apply the changes by this command:

$ kubectl apply -f install/cluster-operator/050-ConfigMap-strimzi-cluster-operator.yaml

The same restrictions apply. The entry monitorInterval must stay in place.

The change in logging takes some time to be applied. You can find out more in the Implementation details section of this post.

Topic Operator, User Operator and Kafka resources

The approach to changing the logging configuration of User Operator, Topic Operator, Kafka Bridge, Kafka brokers, Kafka Connect and Kafka MirrorMaker 2.0 did not change. The logging is still configurable through the inline or external logging section of a custom resource specification.

Example of inline logging.

...
logging:
  type: inline
  loggers:
    kafka.root.logger.level = INFO
...

Example of external logging.

...
logging:
  type: external
  name: my-external-config-map
...

Only loggers can be changed dynamically. Logging appenders are changed using an external ConfigMap, which triggers a rolling update.

This does not apply for Kafka Bridge, User operator, Topic operator and Cluster operator. Because these use log4j2 framework, even logging appenders are changeable dynmaicaly.

Verifying logging levels are set correctly

In most cases, it takes a few seconds to change the logging level, and then you can observe the change in the logs. If you want to check the loggers and logging levels are changed, you can use these commands.

For Kafka brokers:

$ kubectl  exec <kafka-cluster-name>-kafka-0 -- bin/kafka-configs.sh --bootstrap-server <kafka-cluster-name>-kafka-bootstrap:9092 --entity-type broker-loggers --entity-name 0 --describe

You may wonder what value you should pass to --entity-name? It should be the name of the Kafka broker that can be found in the broker configuration as the broker.id property. In Strimzi, a broker has a name related to the name of pod where is it running. So -kafka-0 should have name '0', -kafka-1 will be '1', etc.

For Kafka Connect, you can get loggers information by accessing Connect API service:

$ kubectl exec -ti <kafka-connect-pod-name> -- curl http://localhost:8083/admin/loggers

Since Kafka MirrorMaker 2.0 is based on the Kafka Connect, getting its loggers is very similar:

$ kubectl exec -ti <kafka-mirror-maker-2-pod-name> -- curl http://localhost:8083/admin/loggers

Strimzi Cluster Operator, Topic Operator, Entity Operator and Strimzi Kafka Bridge use automatic reloading of logging configuration. The only way to check the logging configuration of these components is by looking into the pod for the file on the path /opt/strimzi/custom-config/log4j2.properties. It should contain the configuration and the monitorInterval property. Note that reconciliation time, re-mouting ConfigMap and monitorInterval value affect the time it takes for logging configuration to be reloaded.

Implementation details

To implement dynamically changeable logging levels in Strimzi Cluster Operator, Topic Operator, Entity Operator and Strimzi Kafka Bridge we use thelog4j2 automatic reconfiguration feature.

Automatic reloading works in these steps:

  1. Logging configuration is changed in the custom resource
  2. During reconciliation, the log4j2.properties file containing the configuration from the custom resource is remounted.
  3. log4j2 polls changes in the log4j2.properties file in an interval of monitorInterval seconds

For Kafka broker, Kafka Connect and Kafka MirrorMaker 2.0, we use features already implemented from Kafka. Logging configuration in Kafka Brokers is changed using AdminClient. For more information, see KIP-412

Kafka Connect and Kafka MirrorMaker 2.0 logging configuration is changed using the REST API. For more information, see KIP-495

Further steps

For now, there is no way to set logging configuration for Kafka MirrorMaker. KIP-649 for dynamic client configuration is in progress. However, Kafka MirrorMaker support will eventually be dropped, as it has been replaced by MirrorMaker 2.0.

The inability to set logging configuration dynamically applies to ZooKeeper as well. KIP-500 to replace ZooKeeper with a self-managed metadata quorum is planned, so we do not consider it as priority to implement this feature for the ZooKeeper cluster.

KIP-653 to upgrade log4j to log4j2 has been proposed. It should make the changes in logging even simpler on the Kafka side.

Conclusion

Whether Strimzi is running in development or production environments, it is useful to be able to set an appropriate logging level. Sometimes a Kafka cluster or operator is driven into an edge case. With a less informative logging level it can be difficult to investigate what happened and how this situation can be fixed. Also, changing the logging level causes a rolling update of the pod, which can put the cluster in a regular state and complicate the debugging of such an event. Furthermore, any reason to roll Kafka pods lowers the availability of a Kafka cluster. Applying logging configuration dynamically helps to resolve these issues. And that is the main reason why we decided to implement dynamic logging levels in Strimzi 0.20.0 by using methods described in this post.