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Application logging
METAMOD logs events that happen in the system. The logging of these events can be used for IT operations and simplifying development.
Logging framework
To provide scalable and standardised logging, METAMOD uses the Log::Log4perl framework in Perl code and the Log4php framework in PHP code. Both of these frameworks are ports of the log4j logging framework for Java.
It is important to understand how the log4p* frameworks work before you start using them. In particular it is important to understand how categories, levels and appenders work. This article gives a good introduction to the central concepts of the frameworks: http://www.perl.com/pub/a/2002/09/11/log4perl.html
Configuration
Log::Log4perl and log4php is configured using configuration files. The configuration files used to by the two frameworks are similar to each other, but not identical. For this reason METAMOD uses a “meta” configuration file that contains the configuration for both frameworks. The meta configuration is parsed by the script update_logger_config.pl that will create log4php-config.ini and log4perl-config.ini.
The meta configuration contains lines that start with either log4all, log4php or log4perl. All lines starting with log4all will inserted in the configuration for both PHP and Perl. The log4php and log4perl lines will only be inserted into the configuration file for the respective framework.
Levels
METAMOD uses the following log levels with the following definition.
- FATAL: requires instant human assistance (IT operations). For instance full disks, lack of connection to database servers, file servers or LDAP.
- ERROR: serious error, but isolated to current task/request. Does not affect entire METAMOD instance. IT operations should be notified.
- WARNING: Might be an error, but not necessarily. Should be looked at to see if it is an actual problem.
- INFO: Used to track main application use in production. For instance tracking user log in, uploads, metadata parsing info and other statistics
- DEBUG: Free to be used by the developers as they see fit during development. DEBUG logging should rarely be used in production.
Categories
METAMOD uses the following categories for log messages. The categories correspond with the main parts of the application.
- Metamod
- search
- harvest
- oai-pmh
- upload
- base
- adm
- common
Appenders
By default METAMOD uses two appenders. One for logging just FATAL and ERROR messages and one for logging all messages in the same file.
METAMOD configuration variables
The following configuration variables in the METAMOD configuration file is used to configure the logging.
- LOG4PERL_CONFIG: The configuration file used to configure Log::Log4perl
- LOG4PERL_WATCH_TIME: The number of seconds between each time Log::Log4perl should check for changes to the log configuration.
- LOG4PHP_CONFIG: The configuration file used to configure log4php.
Runtime configuration changes
The configuration files will be watched for changes so it is possible to change the logging configuration at runtime without restarting the application. This is for instance useful for turning on debugging in a production system.
Monitoring
The monitoring of logs is the responsibility of IT operations and is not implemented by METAMOD it self.