Logging
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Logging

Logging in Akka is not tied to a specific logging backend. By default log messages are printed to STDOUT, but you can plug-in a SLF4J logger or your own logger. Logging is performed asynchronously to ensure that logging has minimal performance impact. Logging generally means IO and locks, which can slow down the operations of your code if it was performed synchronously.

How to Log

Create a LoggingAdapter and use the error, warning, info, or debug methods, as illustrated in this example:

import akka.actor.ActorRef;
import akka.actor.ActorSystem;
import akka.actor.AllDeadLetters;
import akka.actor.SuppressedDeadLetter;
import akka.event.Logging;
import akka.event.LoggingAdapter;
class MyActor extends UntypedActor {
  LoggingAdapter log = Logging.getLogger(getContext().system(), this);

  @Override
  public void preStart() {
    log.debug("Starting");
  }

  @Override
  public void preRestart(Throwable reason, Option<Object> message) {
    log.error(reason, "Restarting due to [{}] when processing [{}]",
      reason.getMessage(), message.isDefined() ? message.get() : "");
  }

  public void onReceive(Object message) {
    if (message.equals("test")) {
      log.info("Received test");
    } else {
      log.warning("Received unknown message: {}", message);
    }
  }
}

The first parameter to Logging.getLogger could also be any LoggingBus, specifically system.eventStream(); in the demonstrated case, the actor system’s address is included in the akkaSource representation of the log source (see Logging Thread and Akka Source in MDC) while in the second case this is not automatically done. The second parameter to Logging.getLogger is the source of this logging channel. The source object is translated to a String according to the following rules:

  • if it is an Actor or ActorRef, its path is used
  • in case of a String it is used as is
  • in case of a class an approximation of its simpleName
  • and in all other cases the simpleName of its class

The log message may contain argument placeholders {}, which will be substituted if the log level is enabled. Giving more arguments as there are placeholders results in a warning being appended to the log statement (i.e. on the same line with the same severity). You may pass a Java array as the only substitution argument to have its elements be treated individually:

final Object[] args = new Object[] { "The", "brown", "fox", "jumps", 42 };
system.log().debug("five parameters: {}, {}, {}, {}, {}", args);

The Java Class of the log source is also included in the generated LogEvent. In case of a simple string this is replaced with a “marker” class akka.event.DummyClassForStringSources in order to allow special treatment of this case, e.g. in the SLF4J event listener which will then use the string instead of the class’ name for looking up the logger instance to use.

Logging of Dead Letters

By default messages sent to dead letters are logged at info level. Existence of dead letters does not necessarily indicate a problem, but it might be, and therefore they are logged by default. After a few messages this logging is turned off, to avoid flooding the logs. You can disable this logging completely or adjust how many dead letters that are logged. During system shutdown it is likely that you see dead letters, since pending messages in the actor mailboxes are sent to dead letters. You can also disable logging of dead letters during shutdown.

akka {
  log-dead-letters = 10
  log-dead-letters-during-shutdown = on
}

To customize the logging further or take other actions for dead letters you can subscribe to the Event Stream.

Auxiliary logging options

Akka has a couple of configuration options for very low level debugging, that makes most sense in for developers and not for operations.

You almost definitely need to have logging set to DEBUG to use any of the options below:

akka {
  loglevel = "DEBUG"
}

This config option is very good if you want to know what config settings are loaded by Akka:

akka {
  # Log the complete configuration at INFO level when the actor system is started.
  # This is useful when you are uncertain of what configuration is used.
  log-config-on-start = on
}

If you want very detailed logging of all automatically received messages that are processed by Actors:

akka {
  actor {
    debug {
      # enable DEBUG logging of all AutoReceiveMessages (Kill, PoisonPill et.c.)
      autoreceive = on
    }
  }
}

If you want very detailed logging of all lifecycle changes of Actors (restarts, deaths etc):

akka {
  actor {
    debug {
      # enable DEBUG logging of actor lifecycle changes
      lifecycle = on
    }
  }
}

If you want very detailed logging of all events, transitions and timers of FSM Actors that extend LoggingFSM:

akka {
  actor {
    debug {
      # enable DEBUG logging of all LoggingFSMs for events, transitions and timers
      fsm = on
    }
  }
}

If you want to monitor subscriptions (subscribe/unsubscribe) on the ActorSystem.eventStream:

akka {
  actor {
    debug {
      # enable DEBUG logging of subscription changes on the eventStream
      event-stream = on
    }
  }
}

Auxiliary remote logging options

If you want to see all messages that are sent through remoting at DEBUG log level: (This is logged as they are sent by the transport layer, not by the Actor)

akka {
  remote {
    # If this is "on", Akka will log all outbound messages at DEBUG level,
    # if off then they are not logged
    log-sent-messages = on
  }
}

If you want to see all messages that are received through remoting at DEBUG log level: (This is logged as they are received by the transport layer, not by any Actor)

akka {
  remote {
    # If this is "on", Akka will log all inbound messages at DEBUG level,
    # if off then they are not logged
    log-received-messages = on
  }
}

If you want to see message types with payload size in bytes larger than a specified limit at INFO log level:

akka {
  remote {
    # Logging of message types with payload size in bytes larger than
    # this value. Maximum detected size per message type is logged once,
    # with an increase threshold of 10%.
    # By default this feature is turned off. Activate it by setting the property to
    # a value in bytes, such as 1000b. Note that for all messages larger than this
    # limit there will be extra performance and scalability cost.
    log-frame-size-exceeding = 1000b
  }
}

Also see the logging options for TestKit: Tracing Actor Invocations.

Turn Off Logging

To turn off logging you can configure the log levels to be OFF like this.

akka {
  stdout-loglevel = "OFF"
  loglevel = "OFF"
}

The stdout-loglevel is only in effect during system startup and shutdown, and setting it to OFF as well, ensures that nothing gets logged during system startup or shutdown.

Loggers

Logging is performed asynchronously through an event bus. Log events are processed by an event handler actor and it will receive the log events in the same order as they were emitted.

You can configure which event handlers are created at system start-up and listen to logging events. That is done using the loggers element in the Configuration. Here you can also define the log level. More fine grained filtering based on the log source can be implemented in a custom LoggingFilter, which can be defined in the logging-filter configuration property.

akka {
  # Loggers to register at boot time (akka.event.Logging$DefaultLogger logs
  # to STDOUT)
  loggers = ["akka.event.Logging$DefaultLogger"]
  # Options: OFF, ERROR, WARNING, INFO, DEBUG
  loglevel = "DEBUG"
}

The default one logs to STDOUT and is registered by default. It is not intended to be used for production. There is also an SLF4J logger available in the 'akka-slf4j' module.

Example of creating a listener:

import akka.actor.ActorRef;
import akka.actor.ActorSystem;
import akka.actor.AllDeadLetters;
import akka.actor.SuppressedDeadLetter;
import akka.event.Logging;
import akka.event.LoggingAdapter;

import akka.event.Logging.InitializeLogger;
import akka.event.Logging.Error;
import akka.event.Logging.Warning;
import akka.event.Logging.Info;
import akka.event.Logging.Debug;
class MyEventListener extends UntypedActor {
  public void onReceive(Object message) {
    if (message instanceof InitializeLogger) {
      getSender().tell(Logging.loggerInitialized(), getSelf());
    } else if (message instanceof Error) {
      // ...
    } else if (message instanceof Warning) {
      // ...
    } else if (message instanceof Info) {
      // ...
    } else if (message instanceof Debug) {
      // ...
    }
  }
}

Logging to stdout during startup and shutdown

While the actor system is starting up and shutting down the configured loggers are not used. Instead log messages are printed to stdout (System.out). The default log level for this stdout logger is WARNING and it can be silenced completely by setting akka.stdout-loglevel=OFF.

SLF4J

Akka provides a logger for SL4FJ. This module is available in the 'akka-slf4j.jar'. It has one single dependency; the slf4j-api jar. In runtime you also need a SLF4J backend, we recommend Logback:

<dependency>
  <groupId>ch.qos.logback</groupId>
  <artifactId>logback-classic</artifactId>
  <version>1.0.13</version>
</dependency>

You need to enable the Slf4jLogger in the loggers element in the Configuration. Here you can also define the log level of the event bus. More fine grained log levels can be defined in the configuration of the SLF4J backend (e.g. logback.xml). You should also define akka.event.slf4j.Slf4jLoggingFilter in the logging-filter configuration property. It will filter the log events using the backend configuration (e.g. logback.xml) before they are published to the event bus.

akka {
  loggers = ["akka.event.slf4j.Slf4jLogger"]
  loglevel = "DEBUG"
  logging-filter = "akka.event.slf4j.Slf4jLoggingFilter"
}

One gotcha is that the timestamp is attributed in the event handler, not when actually doing the logging.

The SLF4J logger selected for each log event is chosen based on the Class of the log source specified when creating the LoggingAdapter, unless that was given directly as a string in which case that string is used (i.e. LoggerFactory.getLogger(Class c) is used in the first case and LoggerFactory.getLogger(String s) in the second).

Note

Beware that the actor system’s name is appended to a String log source if the LoggingAdapter was created giving an ActorSystem to the factory. If this is not intended, give a LoggingBus instead as shown below:

final LoggingAdapter log = Logging.getLogger(system.eventStream(), "my.string");

Logging Thread and Akka Source in MDC

Since the logging is done asynchronously the thread in which the logging was performed is captured in Mapped Diagnostic Context (MDC) with attribute name sourceThread. With Logback the thread name is available with %X{sourceThread} specifier within the pattern layout configuration:

<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
  <encoder>
    <pattern>%date{ISO8601} %-5level %logger{36} %X{sourceThread} - %msg%n</pattern>
  </encoder>
</appender>

Note

It will probably be a good idea to use the sourceThread MDC value also in non-Akka parts of the application in order to have this property consistently available in the logs.

Another helpful facility is that Akka captures the actor’s address when instantiating a logger within it, meaning that the full instance identification is available for associating log messages e.g. with members of a router. This information is available in the MDC with attribute name akkaSource:

<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
  <encoder>
    <pattern>%date{ISO8601} %-5level %logger{36} %X{akkaSource} - %msg%n</pattern>
  </encoder>
</appender>

For more details on what this attribute contains—also for non-actors—please see How to Log.

More accurate timestamps for log output in MDC

Akka's logging is asynchronous which means that the timestamp of a log entry is taken from when the underlying logger implementation is called, which can be surprising at first. If you want to more accurately output the timestamp, use the MDC attribute akkaTimestamp:

<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
  <encoder>
    <pattern>%X{akkaTimestamp} %-5level %logger{36} %X{akkaSource} - %msg%n</pattern>
  </encoder>
</appender>

MDC values defined by the application

One useful feature available in Slf4j is MDC, Akka has a way for let the application specify custom values, you just need to get a specialized LoggingAdapter, the DiagnosticLoggingAdapter. In order to get it you will use the factory receiving an UntypedActor as logSource:

// Within your UntypedActor
final DiagnosticLoggingAdapter log = Logging.getLogger(this);

Once you have the logger, you just need to add the custom values before you log something. This way, the values will be put in the SLF4J MDC right before appending the log and removed after.

Note

The cleanup (removal) should be done in the actor at the end, otherwise, next message will log with same mdc values, if it is not set to a new map. Use log.clearMDC().

import akka.event.Logging;
import akka.event.DiagnosticLoggingAdapter;
import java.util.HashMap;
import java.util.Map;
class MdcActor extends UntypedActor {

    final DiagnosticLoggingAdapter log = Logging.getLogger(this);

    public void onReceive(Object message) {

        Map<String, Object> mdc;
        mdc = new HashMap<String, Object>();
        mdc.put("requestId", 1234);
        mdc.put("visitorId", 5678);
        log.setMDC(mdc);

        log.info("Starting new request");

        log.clearMDC();
    }
}

Now, the values will be available in the MDC, so you can use them in the layout pattern:

<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
  <encoder>
    <pattern>
      %-5level %logger{36} [req: %X{requestId}, visitor: %X{visitorId}] - %msg%n
    </pattern>
  </encoder>
</appender>

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