The Appenders: Less spoken but powerful aspect of logger package

Logging is one of the fundamental and very important features of an application that helps both developers and maintainers in debugging any issues either during development or in production.

In Tcl, as part of tcllib, we have log and logger packages that support logging facility. A lot of information available on wiki and their reference documentation.

In this article, I would like to highlight one of the less spoken but a powerful aspect of logger package, that is, appender.

An appender is the one that does the actual job of appending the log message to its destination. You can notice that, by default, logger package logs the messages onto standard output / console. This is because, the logger package by default is configured with an appender called "console".

You can change this default behavior of logging messages to  standard output by configuring the logger package with a different appender. To do this configuration you call the proc: 

::logger::utils::applyAppender -appender appenderType  ? options... ?

In addition to "console", there is also an appender provided by logger package is: colorConsole. By invoking applyAppender as:

::logger::utils::applyAppender -appender colorConsole

you can see that your log messages are not just sent to standard output, but they are also formatted with colors.

Now, comes the secret.... an undocumented feature of logger package.... is that there is an another appender available in the package called fileAppend. This appender sends the log messages to a given output channel. You can use this appender to log messages to a file on the file system...something that all applications in the production will need. To configure the logger package with fileAppend appender, you invoke:

set chan [open /log/file/path]
fconfigure $chan -buffering none
::logger::utils::applyAppender -appender fileAppend -outputChannel  $chan

While there are only 3 appenders provided by logger package, you can write your own appenders and make them part of logger package. We will see how to write your own appenders in a different article.

One can use the applyAppender command to configure specific set of services and / or specific set of logger object instances (returned by logger::init command) and / or for specific set of log levels. This makes application's logging facility to be highly configurable with any level of granularity that you want. You can enable logging for only some parts of the application, different parts of the application being able to send messages to different destinations as per the configuration and different parts of the application logging messages at different log levels.

That is the power of logger package in tcllib.