Logula

Bwah ah ah ah.

Logula is a Scala library which provides a sane log output format and an easy-to-use mixin for adding logging to your code.

It's a thin front-end for log4j 1.2 because java.util.logging was a pain in the neck to deal with.

Requirements

• Java SE 6
• Scala 2.8.1 or 2.9.0-1
• log4j 1.2

How To Use

First, specify Logula as a dependency:

<repositories>
  <repository>
    <id>repo.codahale.com</id>
    <url>http://repo.codahale.com</url>
  </repository>
</repositories>

<dependencies>
  <dependency>
    <groupId>com.codahale</groupId>
    <artifactId>logula_${scala.version}</artifactId>
    <version>2.1.3</version>
  </dependency>
</dependencies>

Second, configure the logging system:

import com.codahale.logula.Logging
import org.apache.log4j.Level

Logging.configure { log =>
  log.registerWithJMX = true

  log.level = Level.INFO
  log.loggers("com.myproject.weebits") = Level.OFF

  log.console.enabled = true
  log.console.threshold = Level.WARN

  log.file.enabled = true
  log.file.filename = "/var/log/myapp/myapp.log"
  log.file.maxSize = 10 * 1024 // KB
  log.file.retainedFiles = 5 // keep five old logs around

  // syslog integration is always via a network socket
  log.syslog.enabled = true
  log.syslog.host = "syslog-001.internal.example.com"
  log.syslog.facility = "local3"
}

Third, add some logging to your classes:

class MyThing extends Logging {
  def complicatedManoeuvre() {
    try {
      log.warn("This is about to get complicated...")
      log.info("Trying to do %d backflips", backflipsToAttempt)
      // complicated bit elided
    } catch {
      case e: Exception => log.error(e, "Horrible things have happened.")
    }
  }
}

Notice that the logging statements use Scala's formatting syntax, and that logged exceptions are passed as the first argument.

Statement Arguments

Unlike a lot of Scala logging libraries, Logula doesn't use pass-by-name semantics (e.g., f: => A) for its logging statements, which means two things:

1. The Scala compiler doesn't have to create one-off closure objects for each logging statement. This should reduce the amount of garbage collection pressure.
2. If your logging arguments are complex to create, that price will be paid regardless of whether or not the statement is logged.

For example:

log.debug("A huge collection: %s", things.mkString(", "))

The mkString call will happen every time. To prevent this, either keep your arguments simple:

log.debug("A huge collection: %s", things)

or only conditionally log them:

if (log.isDebugEnabled) {
  log.debug("A huge collection: %s", things.mkString(", "))
}

In most cases, it's simple enough to just log basic values.

The Log Format

Logula's log format has a few specific goals.

• Be roughly human readable. You shouldn't need another program to make sense of your program.
• Be machine parsable. You shouldn't need another human to make sense of your program. (Shush.)
• Make it easy for sleepy ops folks to figure out why things are pear-shaped at o'dark-thirty using standard UNIXy tools like tail, grep, and fortune.

An example of logging output looks like this:

TRACE [2010-04-06 06:42:35,271] com.codahale.logula.examples.ThingDoer: Contemplating doing a thing.
DEBUG [2010-04-06 06:42:35,274] com.codahale.logula.examples.ThingDoer: About to do a thing.
INFO  [2010-04-06 06:42:35,274] com.codahale.logula.examples.ThingDoer: Doing a thing
WARN  [2010-04-06 06:42:35,275] com.codahale.logula.examples.ThingDoer: Doing a thing
ERROR [2010-04-06 06:42:35,275] com.codahale.logula.examples.ThingDoer: This may get ugly.
FATAL [2010-04-06 06:42:35,275] com.codahale.logula.examples.ThingDoer: The thing has gone horribly wrong.
! java.lang.RuntimeException: oh noes!
! 	at scala.Predef$.error(Predef.scala:74)
! 	at com.codahale.logula.examples.ThingDoer.run(ExampleLoggingRun.scala:16)
! 	at com.codahale.logula.examples.ExampleLoggingRun$.main(ExampleLoggingRun.scala:40)
! 	at com.codahale.logula.examples.ExampleLoggingRun.main(ExampleLoggingRun.scala)
! 	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
! 	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
! 	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
! 	at java.lang.reflect.Method.invoke(Method.java:597)
! 	at sbt.Run.run0(Run.scala:60)
! 	at sbt.Run.execute$1(Run.scala:47)
! 	at sbt.Run$$anonfun$run$2.apply(Run.scala:50)
! 	at sbt.Run$$anonfun$run$2.apply(Run.scala:50)
! 	at sbt.TrapExit$.executeMain$1(TrapExit.scala:33)
! 	at sbt.TrapExit$$anon$1.run(TrapExit.scala:42)
! 

A few items of note:

• All timestamps are in UTC and ISO 8601 format. This really should be OK with you.
• You can grep for messages of a specific level really easily: tail -f logula.log | grep '^WARN'
• You can grep for messages from a specific class or package really easily: tail -f logula.log | grep 'ThingDoer'
• You can even pull out full exception stack traces, plus the accompanying log message: tail -f logula.log | grep -B 1 '^\!'
• If you squint, you can still make out the actual log messages.

License

Copyright (c) 2010-2011 Coda Hale

Published under The MIT License, see LICENSE