How to customize log message using application data in log4Perl using MDC

In log messages it is often need to pass application data which is common to a set of log messages. One example is to log a requestID of an API call which uniquely identifies all log messages belongs to processing a particular API call.

One approach is to pass the requestID in all log messages by appending the log message with requestID.

Another approach is to use log4Perl::MDC (Mapped Diagnostic Context). Using MDC we can store Thread Specific data and which can be accessed by logger. The thread specific data of a parent thread is accessible to its child thread as well.

Below program explains this.

  use Log::Log4perl;  
  Log::Log4perl::init("log4perl.conf");  
  method();  
  sub method {  
  Log::Log4perl::MDC->put("requestId","req00008");  
 

  my $logger = Log::Log4perl->get_logger("");  

  $logger->info("Received a request");  
  $logger->debug("Received a request");  
  }  

Here we have set a Thread specific data key value pair using Log::Log4perl::MDC->put(). 

Logger will use the following configuration to print requestId from MDC to log message.

 log4perl.rootLogger=TRACE, errorlog  
 log4perl.appender.errorlog=Log::Log4perl::Appender::File  
 log4perl.appender.errorlog.filename=c:/perl/log-dir/error.log  
 log4perl.appender.errorlog.mode=append  
 log4perl.appender.errorlog.Threshold = INFO  
 log4perl.appender.errorlog.layout=Log::Log4perl::Layout::PatternLayout  
 log4perl.appender.errorlog.layout.ConversionPattern=%d{ISO8601}|%X{requestId}|%C|%m%n  

%X{requestId} - indicates that the information will be obtained from MDC with key ='requestId'

The log message will be as below.

2015-08-05 15:32:45,766|req00008|main|Received a request