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§The Logging API

Using logging in your application can be useful for monitoring, debugging, error tracking, and business intelligence. Play provides an API for logging which is accessed through the Logger object and uses Logback as the logging engine.

§Logging architecture

The logging API uses a set of components that help you to implement an effective logging strategy.

§Logger

Your application can define Logger instances to send log message requests. Each Logger has a name which will appear in log messages and is used for configuration.

Loggers follow a hierarchical inheritance structure based on their naming. A logger is said to be an ancestor of another logger if its name followed by a dot is the prefix of descendant logger name. For example, a logger named “com.foo” is the ancestor of a logger named “com.foo.bar.Baz.” All loggers inherit from a root logger. Logger inheritance allows you to configure a set of loggers by configuring a common ancestor.

Play applications are provided a default logger named “application” or you can create your own loggers. The Play libraries use a logger named “play”, and some third party libraries will have loggers with their own names.

§Log levels

Log levels are used to classify the severity of log messages. When you write a log request statement you will specify the severity and this will appear in generated log messages.

This is the set of available log levels, in decreasing order of severity.

OFF - Used to turn off logging, not as a message classification.
ERROR - Runtime errors, or unexpected conditions.
WARN - Use of deprecated APIs, poor use of API, ‘almost’ errors, other runtime situations that are undesirable or unexpected, but not necessarily “wrong”.
INFO - Interesting runtime events such as application startup and shutdown.
DEBUG - Detailed information on the flow through the system.
TRACE - Most detailed information.

In addition to classifying messages, log levels are used to configure severity thresholds on loggers and appenders. For example, a logger set to level INFO will log any request of level INFO or higher (INFO, WARN, ERROR) but will ignore requests of lower severities (DEBUG, TRACE). Using OFF will ignore all log requests.

§Appenders

The logging API allows logging requests to print to one or many output destinations called “appenders.” Appenders are specified in configuration and options exist for the console, files, databases, and other outputs.

Appenders combined with loggers can help you route and filter log messages. For example, you could use one appender for a logger that logs useful data for analytics and another appender for errors that is monitored by an operations team.

Note: For further information on architecture, see the Logback documentation.

§Using Loggers

First import the Logger class and companion object:

import play.api.Logger

§The default Logger

The Logger object is your default logger and uses the name “application.” You can use it to write log request statements:

// Log some debug info
Logger.debug("Attempting risky calculation.")

try {
  val result = riskyCalculation
  
  // Log result if successful
  Logger.debug(s"Result=$result")
} catch {
  case t: Throwable => {
    // Log error with message and Throwable.
    Logger.error("Exception with riskyCalculation", t)
  }
}

Using Play’s default logging configuration, these statements will produce console output similar to this:

[debug] application - Attempting risky calculation.
[error] application - Exception with riskyCalculation
java.lang.ArithmeticException: / by zero
    at controllers.Application$.controllers$Application$$riskyCalculation(Application.scala:32) ~[classes/:na]
    at controllers.Application$$anonfun$test$1.apply(Application.scala:18) [classes/:na]
    at controllers.Application$$anonfun$test$1.apply(Application.scala:12) [classes/:na]
    at play.api.mvc.ActionBuilder$$anonfun$apply$17.apply(Action.scala:390) [play_2.10-2.3-M1.jar:2.3-M1]
    at play.api.mvc.ActionBuilder$$anonfun$apply$17.apply(Action.scala:390) [play_2.10-2.3-M1.jar:2.3-M1]

Note that the messages have the log level, logger name, message, and stack trace if a Throwable was used in the log request.

§Creating your own loggers

Although it may be tempting to use the default logger everywhere, it’s generally a bad design practice. Creating your own loggers with distinct names allows for flexibile configuration, filtering of log output, and pinpointing the source of log messages.

You can create a new logger using the Logger.apply factory method with a name argument:

val accessLogger: Logger = Logger("access")

A common strategy for logging application events is to use a distinct logger per class using the class name. The logging API supports this with a factory method that takes a class argument:

val logger: Logger = Logger(this.getClass())

§Logging patterns

Effective use of loggers can help you achieve many goals with the same tool:

import scala.concurrent.Future
import play.api.Logger
import play.api.mvc._

trait AccessLogging {
  
  val accessLogger = Logger("access")

  object AccessLoggingAction extends ActionBuilder[Request] {

    def invokeBlock[A](request: Request[A], block: (Request[A]) => Future[Result]) = {
      accessLogger.info(s"method=${request.method} uri=${request.uri} remote-address=${request.remoteAddress}")
      block(request)
    }
  }
}

object Application extends Controller with AccessLogging {
  
  val logger = Logger(this.getClass())
  
  def index = AccessLoggingAction {
    try {
      val result = riskyCalculation
      Ok(s"Result=$result")
    } catch {
      case t: Throwable => {
        logger.error("Exception with riskyCalculation", t)
        InternalServerError("Error in calculation: " + t.getMessage())
      }
    }
  }
}

This example uses action composition to define an AccessLoggingAction that will log request data to a logger named “access.” The Application controller uses this action and it also uses its own logger (named after its class) for application events. In configuration you could then route these loggers to different appenders, such as an access log and an application log.

The above design works well if you want to log request data for only specific actions. To log all requests, it’s better to use a filter:

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future
import play.api.Logger
import play.api.mvc._
import play.api._

object AccessLoggingFilter extends Filter {
  
  val accessLogger = Logger("access")
  
  def apply(next: (RequestHeader) => Future[Result])(request: RequestHeader): Future[Result] = {
    val resultFuture = next(request)
    
    resultFuture.foreach(result => {
      val msg = s"method=${request.method} uri=${request.uri} remote-address=${request.remoteAddress}" +
        s" status=${result.header.status}";
      accessLogger.info(msg)
    })
    
    resultFuture
  }
}

object Global extends WithFilters(AccessLoggingFilter) {
  
  override def onStart(app: Application) {
    Logger.info("Application has started")
  }

  override def onStop(app: Application) {
    Logger.info("Application has stopped")
  }
}

In the filter version we’ve added the response status to the log request by logging when the Future[Result] completes. Also note that the Global object is a sensible place to use the default logger for events like application start and stop.

§Configuration

See configuring logging for details on configuration.

Next: Advanced topics