Logging with Dancer2::Logger::Log4perl


It's late summer 2018, and there has been a hole in the logging ecosystem of Dancer2 for some time now - we've been missing a Log4perl plugin. There are certainly a lot of great logging options available (Dancer2::Logger::LogAny, Dancer2::Logger::Syslog, and Dancer2::Logger::LogReport to name a few), and a couple of them even include appenders for Log4perl. In looking at our own needs, however, they all seemed to be overkill.

At $work, everything is based on Log4perl (and, in our client-facing Java apps, Log4j), so there has never been need for another logger. We'd been happily using Dancer::Logger::Log4perl for years, and wanted to continue to use something familar to us. Before investing some time in writing our own Log4perl plugin for Dancer2, we expanded our search beyond metacpan, and were rewarded in doing so.

We stumbled across a project from Ryan Larscheidt and Jon Miner at the University of Wisconsin. They wrote a Log4perl plugin for Dancer2 with the intent of releasing it, but as their priorities and projects shifted, it never made its way to CPAN. I used to go to MadMongers (Madison Perl Mongers), so before long, I was able to track them down and they gave me their blessing to release it.

A little packaging and a few tests later, Dancer2::Logger::Log4perl was on its way to CPAN!


First, we need to create a basic Log4perl configuration. Create a new Dancer2 application (I'll call mine TestLog4perl), and in the application directory create a log4perl.conf file with the following:

log4perl.rootLogger              = DEBUG, LOG1

log4perl.appender.LOG1           = Log::Log4perl::Appender::File
log4perl.appender.LOG1.filename  = logs/mylog.log
log4perl.appender.LOG1.mode      = append
log4perl.appender.LOG1.layout    = Log::Log4perl::Layout::PatternLayout
log4perl.appender.LOG1.layout.ConversionPattern = %d %p %m %n

The first one defines our root application logger. The first parameter after the equals sign says what is the minimum level we should log. Since we are saying the minimum log level should be DEBUG, any messages from Dancer2 itself, and anything logged at the core level will be ignored.

After the minimum log level is a comma-separated list of appenders to write to. For now, we will create a single appender named LOG1 (we will see how to add a second appender below). This will write to a file in the logs/ directory named mylog.log, using the Log::Log4perl::Appender::File appender. When the app is started, it will append to an existing log file of that name (or create the file if necessary), and will write to it with a format specified by Log::Log4perl::Layout::PatternLayout.

Each appender can have its own configuration directives. Please consult the pod for each appender for a list of its configuration parameters.

Next, we have to tell our application that we are using Dancer2::Logger::Log4perl as our preferred logger. Edit your environments/development.yml file, and comment out the logger: "console" line. Replace it with the following:

logger: log4perl
log: core
         config_file: log4perl.conf

This tells Dancer2 to use Dancer2::Logger::Log4perl as its logging engine, and to send all levels of message to it. Finally, Log4perl should look for its configuration file in the root application directory in a file called log4perl.conf.


Using Log4perl is simple: use the logging keywords you are already familiar with in Dancer:

get '/' => sub {
    debug "I'M IN UR INDEX";
    template 'index' => { 'title' => 'TestLog4perl' };

Start your application and visit http://localhost:5000/. You will see the following in your logs/mylog.log file:

2018/12/18 21:36:02 DEBUG I'M IN UR INDEX

Hey, I can't see my log messages on the screen!

That's because we didn't add a screen appender! With Log4perl, adding another appender is easy. Let's add another section to our log4perl.conf file:

log4perl.appender.SCREEN         = Log::Log4perl::Appender::Screen
log4perl.appender.SCREEN.stderr  = 0
log4perl.appender.SCREEN.layout  = Log::Log4perl::Layout::PatternLayout
log4perl.appender.SCREEN.layout.ConversionPattern = %m %n

This creates another appender named SCREEN. We then need to tell our root logger to use this appender as well:

log4perl.rootLogger = DEBUG, LOG1, SCREEN

Now, restart your application, and visit a route that has logging installed, and you will see your log message not only goes to the logs/mylog.log file, but also displays on the console running your application. Easy!

Some Gotchas

There are a couple of important nuances you should be aware of:

  • Environment configuration replaces logging configuration

    If you put your logging configuration in config.yml rather than one of your environment-specific configuration files (such as environments/development.yml), you stand a good chance of not using the logging configuration you think you are using. The default configuration file for the development environment, for example, logs only to the console. If you put your Log4perl configuration in config.yml and don't change your development configuration file, your Log4perl configuration will be passed over for the default console logger.

    From my own experience, always configure your logger in your environment-specific configuration, unless you use the same configuration across all environments (I don't endorse this practice).

  • Core level messages are passed as log level trace, but will not be passed unless Dancer2's log level is core.

    Since core doesn't have a good corresponding level in Log4perl, core level messages are sent over to Log4perl at the trace log level. This only happens when you set Dancer2's log level in your config.yml file to core however. So your preferred log level setting is respected, even if core level messages have to be reported at a different level.

  • log should be set a lower priority than the lowest priority as set in your Log4perl configuration.

    If it isn't, the log messages will not be passed to Log4perl.


If Log4perl is all the logging you need in your Dancer2 applications, then Dancer2::Logger::Log4perl is well worth a look. It gives you much of the functionality available to Log4perl while using the logging syntax built into Dancer2. This article should give you sufficient grounding to get going with Log4perl in your Dancer2 applications.

Further Reading


This article has been written by Jason Crome (CromeDome) for the Perl Dancer Advent Calendar 2018.


No copyright retained. Enjoy.

Jason A. Crome