1.0.4 Log4r Manual

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1.0.4 Log4r Manual
#### Table of Contents Scope of This Manual What Log4r Is Out of the Box Overview Levels Loggers Outputters Formatters Configuration Remote Logging The Art of Logging Avoiding Parameter Evaluation How Many? Where To? Where From? Who's Talking? The Null Logger Gotchas Performance The Project Itself #### Scope of this Manual Most of the API documentation resides in the Log4r RDoc API, so this manual will be brief and targeted to people starting to learn about Log4r or who want to see what Log4r has to offer. Besides the general overview, a section called The Art of Logging provides tricks and tips in using Log4r efficiently. Click on the section title to go back to the Table of Contents at any time. #### What Log4r Is Log4r is a comprehensive and flexible logging library written in Ruby for use in Ruby programs. It features a heiarchial logging system of any number of levels, custom level names, logger inheritance, multiple output destinations, execution tracing, custom formatting, thread safteyness and more. Log4r is an adherent to the philosophy of logging using simple print statements. What Log4r adds to this philosophy is a flexible way of controling the information being logged. Log information can be sent to any kind of destination and with varying degrees of importance and in a variety of formats. Log4r is designed so that logging statements can remain in production code at almost no extra cost, all that's needed is a switch to turn logging off. Log4r intends to be easy to use and configure, no matter the complexity. Casual scripts can use Log4r right away with minimal configuration, while complicated applications can set up an XML configuration using a concise XML schema. Log4r comes with comprehensive documentation and plenty of examples, making it a breeze to learn. Furthermore, Log4r attempts to abide by the Principle of Least Surprise, which means that it works as intended at all points. While Log4r is interpreted, it attempts to achieve optimal performance and scale well. Already, plans are being made to write the performance-critical components as a C extension to Ruby. Log4r was inspired by and provides much of the features of the Apache Log4j project, but is not a direct implementation or clone. Aside from superficial similarities, the projects are not related in any way and the code base is completely distinct. Log4r was developed without even looking at the Apache Log4j code. Log4r is an Open Source project and intends to remain that way. The Log4r license is similar to the Ruby Language license. It resides on this page and in the distribution in a file #### Out of the Box Here's an example of how to use Log4r right away.
    require 'log4r'
    include Log4r

    # create a logger named 'mylog' that logs to stdout
    mylog = Logger.new 'mylog'
    mylog.outputters = Outputter.stdout

    # Now we can log.
    def do_log(log)
      log.debug "This is a message with level DEBUG"
      log.info "This is a message with level INFO"
      log.warn "This is a message with level WARN"
      log.error "This is a message with level ERROR"
      log.fatal "This is a message with level FATAL"
    end
    do_log(mylog)
The output will look something like this:
    DEBUG mylog: This is a message with level DEBUG
     INFO mylog: This is a message with level INFO
     WARN mylog: This is a message with level WARN
    ERROR mylog: This is a message with level ERROR
    FATAL mylog: This is a message with level FATAL
For ease of access, the logger is stored in a hashtable keyed to its name:
    mylog = Logger['mylog']        # Get our logger back
Suppose we want to turn off `DEBUG`, `INFO` and `WARN` messages and see only `ERROR` and `FATAL`. To do this, we set the level threshold for mylog to `ERROR`:
    mylog.level = ERROR
Running `do_log(mylog)` yields:
    ERROR mylog: This is a message with level ERROR
    FATAL mylog: This is a message with level FATAL
#### Overview We will now go over the components of Log4r. A summary of each is provided, and links to the Log4r RDoc API are provided for further perusal. #### Levels Log4r uses a heiarchial system of logging. That is, certain log events can have a higher priority than other log events. Hence, one can control how much information one wants to log by adjusting the level threshold of logging. By default, the logging levels and priorities are:
    DEBUG < INFO < WARN < ERROR < FATAL
In the previous section, we saw how setting the level to `ERROR` prevented messages with levels `DEBUG`, `INFO` and `WARN` from showing up. The names and numbers of these levels are configurable. You can have any number of levels and name them whatever you wish. The logging methods we saw in the last section will be named after the custom levels. Log4r adjusts itself to suit your needs. To find out more about levels, please see rdoc/files/log4r_rb.html. #### Loggers The principle interface in Log4r is a Logger. Loggers have one logging method for each level and any number of output destinations. A logger's level threshold and output destinations may be changed dynamically. Loggers are stored within a Repository for retrieval at any time. Loggers provide all kinds of data for logging: the log message itself, the line number and file it was called in, a timestamp, the log priority, and so on. Loggers can inherit other Loggers. Inheritance means that a Logger initially adopts the characteristics of its parent if none are specified. A Logger's level is inherited once, and a Logger will write to its parents output destinations as well as its own. This behavior is optional, but allows one to structure a powerful, and easily configurable logging system. To find out more about loggers, please see rdoc/files/log4r/logger_rb.html. #### Outputters An output destination (file, raw IO, stdout, etc.) is represented by an Outputter object. An Outputter has a particular means of formatting data (Formatter) and has a level threshold of its own. Outputters, like Loggers, are stored in a repository and can be retrieved and manipulated at any time. Every outputter is thread-safe, meaning that multiple threads can log to the same Outputter without worrying about race conditions. There is a growing collection of outputters provided: raw IO, to stdout, to stderr, to files (including one that splits and zips up logs periodically), to syslog and to an email address. If a specialized Outputter is needed, one can be created from scratch in almost no time, thanks to the ease of extending log4r outputters, the well documented code and the open source license. To find out more about outputters, please see rdoc/files/log4r/outputter/outputter_rb.html. #### Formatters A Formatter is responsible for rendering a log message into an output format. Several Formatters are provided, including the powerful PatternFormatter. PatternFormatter uses sprintf-like directives to format log messages and eliminates the need for custom Formatters. To find out more about formatters, please see rdoc/files/log4r/formatter/formatter_rb.html. To find out more about PatternFormatter, please see rdoc/files/log4r/formatter/patternformatter_rb.html. #### Configuration Configuring Log4r is accomplished via the Configurator class. Configurator allows one to set custom levels and load up XML configurations. The XML grammar used by Log4r is extremely flexible and can accomodate the configuration of custom Outputters and Formatters with no extra work. That is, if you create a custom Outputter, you don't need to write any special code for it to be configured with XML. That's automatically taken care of by some very powerful reflection code within Log4r. To find out more about configuration, please see rdoc/files/log4r/configurator_rb.html. #### Remote Logging It is possible to send log events from an Outputter to a Logger over a network. This is accomplished using the distributed Ruby library ROMP, a subclass of Logger called LogServer, and a RemoteOutputter. To find out more about remote logging, please see rdoc/files/log4r/logserver_rb.html Alternatively, one can just send log reports via email using EmailOutputter. To find out more about EmailOutputter, please see rdoc/classes/Log4r/EmailOutputter.html #### The Art of Logging Log4r in itself does not automatically enable people to understand logging, however it does provide tools to assist in The Art of Logging. We will now cover some of the techniques in this art and how to use Log4r to accomplish them. #### Avoiding Parameter Evaluation Suppose we have a complex structure and don't have the time to make a special `to_s` method. When we want to log the contents of the object, we end up doing something like this:
    log.debug( myobj.collect{|e| e.collect{|p| p.to_s}} )
It is expensive to do this because every time the debug method is called, the parameters passed to it will be evaluated. Because this is a feature of Ruby, setting the logger to `OFF` will not prevent the evaluation. There are two ways to get around parameter evaluation. The first is to perform a simple if condition:
    if log.debug?
      log.debug( myobj.collect{|e| e.collect{|p| p.to_s}} )
    end
Here we are introduced to `log.debug?`, which is called a query method. It returns true if `DEBUG` messages are being logged, otherwise it returns false. Query methods are very cheap to invoke and are a great way to encapsulate complext logging statements. The query methods, like the logging ones, are named after the levels, but with a question mark at the end. As another example, `log.info?` will find out if `INFO` is being logged and so on. The second way around parameter evaluation is to pass a block to the logging method:
    log.debug { myobj.collect{|e| e.collect{|p| p.to_s} }
The block will be evaluated if and only if the logger is capable of handling `DEBUG` log events. #### How Many? How many loggers should one have? Only experience can tell, but a general rule of thumb is to create one static logger per class and one per service or state. When dealing with a large number of loggers, logger inheritance and additivity can help organize what gets logged and to where. The configuration possibilities in Log4r are uncountable and can sometimes be daunting. It's best to start with something simple and evolve it over time. Log4r's XML configuration can assist in that process by providing an easy to read configuration grammar. The XML can be edited directly or with an XML visualization tool. #### Where To? Log4r lets one associate any number of Outputters to a Logger. Logger additivity enables propagation of a log event upwards in the logger heiarchy. The outputters themselves can have their own level thresholds. Unlike normal loggers, Outputters can log at certain specific log levels. this allows one to channel particular data to a particular output. All things considered, log4r offers tremendous flexibility in deciding what gets logged where. #### Where From? Want to find out where a particular log statement came from? Loggers have tracers which record the call stack when turned on:
    Logger['mylog'].trace = true
The trace is then accesible by a Formatter. #### Who's Talking? If there are many loggers that use logger inheritance, it's occasionally a good idea to show the full ancestry of a logger in the log statement. Here's how to set up PatternFormatter to show the full ancestry of a logger in a logging statement (in XML):
    <formatter type="PatternFormatter">
      <!-- %C shows full ancestry -->
      <pattern>[%l %C] %m</pattern>
    </formatter>
For a logger named 'me' with ancestors 'cain::grandpa::pa', it will produce:
    [DEBUG cain::grandpa::pa::me] Log message
#### The Null Logger In addition to being the parent of all loggers, Logger.root is a null object. That means that it does absolutely nothing when its log methods are invoked. Its query methods always return false and it has no outputters. It is useful to turn loggers off from within code:
    noisylog = Logger.root
    noisy.debug "This won't do anything"
#### Gotchas If you are using Log4r, there are a few gotchas that you should be aware of: Logger levels can be dynamically redefined, but the change won't be noticed by any children. That is, if you set `root.level=OFF` after defining some loggers, none of the loggers will change their level to `OFF`. There is a good reason for this behavior. Dynamically redefining levels, tracing or additivity is expensive. It's best to set up all your loggers in a config script and avoid making dynamic changes to Log4r objects. The dynamism is most useful for debugging and development, where local control of logging is a great convenience. When an `IOOutputter`'s `IO` is closed, the `IOOutputter` changes its level to `OFF` #### Performance Profiling has revealed that log4r is typically an order of magnitude or two slower than log4j. However, this is still damn fast! In particular, if a logger is set to `OFF`, the overhead of checking to see if a log event should be logged nearly vanishes. This was accomplished by dynamically redefining the unloggable logging methods to do nothing. In the future, Log4r's performance critical features will be written as a C extension to Ruby. It will still be optional, but it will be available for those who absolutely need to squeeze every last ounce of performance out of Log4r. (No longer in the works, unfortunately.) #### The Project Itself Log4r was initially developed by one person and is slowly but surely gaining popularity and new developers. Please refer to the contact page for more information on the developers.

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