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## Short introduction to Apache log4cxx

## Introduction

Apache log4cxx is a logging framework for C++ patterned after Apache log4j . Apache log4cxx uses Apache Portable Runtime for most platform-specific code and should be usable on any platform supported by APR. Apache log4cxx is licensed under the Apache License , an open source license certified by the Open Source Initiative .

Almost every large application includes its own logging or tracing API. Inserting log statements into code is a low-tech method for debugging it. It may also be the only way because debuggers are not always available or applicable. This is usually the case for multithreaded applications and distributed applications at large.

Experience indicates that logging is an important component of the development cycle. It offeres several advantages. It provides precise context about a run of the application. Once inserted into the code, the generation of logging output requires no human intervention. Moreover, log output can be saved in persistent medium to be studied at a later time. In addition to its use in the development cycle, a sufficiently rich logging package can also be viewed as an auditing tool.

Logging does have its drawbacks. It can slow down an application. If too verbose, it can cause scrolling blindness. To alleviate these concerns, log4cxx is designed to be reliable, fast and extensible. Since logging is rarely the main focus of an application, the log4cxx API strives to be simple to understand and to use.

## Loggers, Appenders and Layouts

Log4cxx has three main components: loggers, appenders and layouts. These three types of components work together to enable developers to log messages according to message type and level, and to control at runtime how these messages are formatted and where they are reported.

### Logger hierarchy

The first and foremost advantage of any logging API over plain `std::cout` resides in its ability to disable certain log statements while allowing others to print unhindered. This capability assumes that the logging space, that is, the space of all possible logging statements, is categorized according to some developer-chosen criteria.

Loggers are named entities. Logger names are case-sensitive and they follow the hierarchical naming rule:

Named Hierarchy
  A logger is said to be an ancestor of another logger if its name followed by a dot is a prefix of the descendant logger name. A logger is said to be a parent of a child logger if there are no ancestors between itself and the descendant logger.

For example, the logger named `"com.foo"` is a parent of the logger named `"com.foo.Bar"`. Similarly, `"java"` is a parent of `"java.util"` and an ancestor of `"java.util.Vector"`. This naming scheme should be familiar to most developers.

The root logger resides at the top of the logger hierarchy. It is exceptional in two ways:

1. it always exists,
2. it cannot be retrieved by name.

Invoking the class static log4cxx::Logger::getRootLogger method retrieves it. All other loggers are instantiated and retrieved with the class static log4cxx::Logger::getLogger method. This method takes the name of the desired logger as a parameter. Some of the basic methods in the Logger class are listed below.

      namespace log4cxx {

        class Logger
     {
           public:
           // Creation & retrieval methods:
           static LoggerPtr getRootLogger();
           static LoggerPtr getLogger(const std::string& name);
           static LoggerPtr getLogger(const std::wstring& name);

         }
       }
    //
    //   Use these macros instead of calling Logger methods directly.
    //   Macros will handle char or wchar_t pointers or strings
    //   or most right-hand side expressions of an
    //   std::basic_string::operator<<.
    //
    #define LOG4CXX_TRACE(logger, expression) ...
    #define LOG4CXX_DEBUG(logger, expression) ...
    #define LOG4CXX_INFO(logger, expression) ...
    #define LOG4CXX_WARN(logger, expression) ...
    #define LOG4CXX_ERROR(logger, expression) ...
    #define LOG4CXX_FATAL(logger, expression) ...

Loggers may be assigned levels. The pre-defined levels: TRACE, DEBUG, INFO, WARN, ERROR and FATAL are defined in the `log4cxx::Level` class which provides accessor functions.

If a given logger is not assigned a level, then it inherits one from its closest ancestor with an assigned level. More formally:

Level Inheritance
  The inherited level for a given logger C , is equal to the first non-null level in the logger hierarchy, starting at C and proceeding upwards in the hierarchy towards the `root` logger.

To ensure that all loggers can eventually inherit a level, the root logger always has an assigned level.

Below are four tables with various assigned level values and the resulting inherited levels according to the above rule.

Logger name | Assigned level | Inherited level
--- | --- | ---
root | Proot | Proot
X | none | Proot
X.Y | none | Proot
X.Y.Z | none | Proot
Example 1

In example 1 above, only the root logger is assigned a level. This level value, `Proot`, is inherited by the other loggers `X`, `X.Y` and `X.Y.Z`.

Logger name | Assigned level | Inherited level
--- | --- | ---
root | Proot | Proot
X | Px | Px
X.Y | Pxy | Pxy
X.Y.Z | Pxyz | Pxyz
Example 2

In example 2, all loggers have an assigned level value. There is no need for level inheritence.

Logger name | Assigned level | Inherited level
--- | --- | ---
root | Proot | Proot
X | Px | Px
X.Y | none | Px
X.Y.Z | Pxyz | Pxyz
Example 3

In example 3, the loggers `root`, `X` and `X.Y.Z` are assigned the levels `Proot`, `Px` and `Pxyz` respectively. The logger `X.Y` inherits its level value from its parent `X`.

Logger name | Assigned level | Inherited level
--- | --- | ---
root | Proot | Proot
X | Px | Px
X.Y | none | Px
X.Y.Z | none | Px
Example 4

In example 4, the loggers `root` and `X` and are assigned the levels `Proot` and `Px` respectively. The loggers `X.Y` and `X.Y.Z` inherits their level value from their nearest parent `X` having an assigned level.

Logging requests are made by invoking a method of a logger instance, preferrably through the use of LOG4CXX_INFO or similar macros which support short-circuiting if the threshold is not satisfied and use of the insertion operator (<<) in the message parameter.

       log4cxx::LoggerPtr logger(log4cxx::Logger::getLogger("com.foo"));
       const char* region = "World";
       LOG4CXX_INFO(logger, "Simple message text.");
       LOG4CXX_INFO(logger, "Hello, " << region);
       LOG4CXX_DEBUG(logger, L"Iteration " << i);
       LOG4CXX_DEBUG(logger, "e^10 = " << std::scientific << exp(10.0));
       //
       //  Use a wchar_t first operand to force use of wchar_t based stream.
       //
       LOG4CXX_WARN(logger, L"" << i << L" is the number of the iteration.");

A logging request is said to be enabled if its level is higher than or equal to the level of its logger. Otherwise, the request is said to be disabled. A logger without an assigned level will inherit one from the hierarchy. This rule is summarized below.

Basic Selection Rule
  A log request of level p in a logger with (either assigned or inherited, whichever is appropriate) level q , is enabled if p >= q .

This rule is at the heart of log4cxx. It assumes that levels are ordered. For the standard levels, we have `TRACE < DEBUG < INFO < WARN < ERROR < FATAL`.

Here is an example of this rule.

       // get a logger instance named "com.foo"
       log4cxx::LoggerPtr  logger(log4cxx::Logger::getLogger("com.foo"));

       // Now set its level. Normally you do not need to set the
       // level of a logger programmatically. This is usually done
       // in configuration files.
       logger->setLevel(log4cxx::Level::getInfo());

       log4cxx::LoggerPtr barlogger(log4cxx::Logger::getLogger("com.foo.Bar");

       // This request is enabled, because WARN >= INFO.
       LOG4CXX_WARN(logger, "Low fuel level.");

       // This request is disabled, because DEBUG < INFO.
       LOG4CXX_DEBUG(logger, "Starting search for nearest gas station.");

       // The logger instance barlogger, named "com.foo.Bar",
       // will inherit its level from the logger named
       // "com.foo" Thus, the following request is enabled
       // because INFO >= INFO.
       LOG4CXX_INFO(barlogger. "Located nearest gas station.");

       // This request is disabled, because DEBUG < INFO.
       LOG4CXX_DEBUG(barlogger, "Exiting gas station search");

Calling the `getLogger` method with the same name will always return a reference to the exact same logger object.

For example, in

       log4cxx::LoggerPtr x = log4cxx::Logger::getLogger("wombat");
       log4cxx::LoggerPtr y = log4cxx::Logger::getLogger("wombat");

`x` and `y` refer to exactly the same logger object.

Thus, it is possible to configure a logger and then to retrieve the same instance somewhere else in the code without passing around references. In fundamental contradiction to biological parenthood, where parents always preceed their children, log4cxx loggers can be created and configured in any order. In particular, a "parent" logger will find and link to its descendants even if it is instantiated after them.

Configuration of the log4cxx environment is typically done at application initialization. The preferred way is by reading a configuration file. This approach will be discussed shortly.

Log4cxx makes it easy to name loggers by software component. This can be accomplished by statically instantiating a logger in each class, with the logger name equal to the fully qualified name of the class. This is a useful and straightforward method of defining loggers. As the log output bears the name of the generating logger, this naming strategy makes it easy to identify the origin of a log message. However, this is only one possible, albeit common, strategy for naming loggers. Log4cxx does not restrict the possible set of loggers. The developer is free to name the loggers as desired.

Nevertheless, naming loggers after the class where they are located seems to be the best strategy known so far.

## Appenders and Layouts

The ability to selectively enable or disable logging requests based on their logger is only part of the picture. Log4cxx allows logging requests to print to multiple destinations. In log4cxx speak, an output destination is called an appender. Currently, appenders exist for the console , files , GUI components, remote socket servers, NT Event Loggers , and remote UNIX Syslog daemons. It is also possible to log asynchronously .

More than one appender can be attached to a logger.

The addAppender method adds an appender to a given logger. Each enabled logging request for a given logger will be forwarded to all the appenders in that logger as well as the appenders higher in the hierarchy. In other words, appenders are inherited additively from the logger hierarchy. For example, if a console appender is added to the root logger, then all enabled logging requests will at least print on the console. If in addition a file appender is added to a logger, say C, then enabled logging requests for C and C's children will print on a file and on the console. It is possible to override this default behavior so that appender accumulation is no longer additive by setting the additivity flag to `false`.

The rules governing appender additivity are summarized below.

Appender Additivity
  The output of a log statement of logger C will go to all the appenders in C and its ancestors. This is the meaning of the term "appender additivity".

  However, if an ancestor of logger C , say P , has the additivity flag set to `false`, then C 's output will be directed to all the appenders in C and it's ancestors upto and including P but not the appenders in any of the ancestors of P .

  Loggers have their additivity flag set to `true` by default.

The table below shows an example:

Logger Name | Added Appenders | Additivity Flag | Output Targets | Comment
--- | --- | --- | --- | ---
root | A1 | not applicable | A1 | The root logger is anonymous but can be accessed with the log4cxx::Logger::getRootLogger() method. There is no default appender attached to root.
x | A-x1, A-x2 | true | A1, A-x1, A-x2 | Appenders of "x" and root.
x.y | none | true | A1, A-x1, A-x2 | Appenders of "x" and root.
x.y.z | A-xyz1 | true | A1, A-x1, A-x2, A-xyz1 | Appenders in "x.y.z", "x" and root.
security | A-sec | false | A-sec | No appender accumulation since the additivity flag is set to `false`.
security.access | none | true | A-sec | Only appenders of "security" because the additivity flag in "security" is set to `false`.

More often than not, users wish to customize not only the output destination but also the output format. This is accomplished by associating a layout with an appender. The layout is responsible for formatting the logging request according to the user's wishes, whereas an appender takes care of sending the formatted output to its destination.

The PatternLayout , part of the standard log4cxx distribution, lets the user specify the output format according to conversion patterns similar to the C language `printf` function.

For example, the PatternLayout with the conversion pattern "%r [%t] %-5p %c - %m%n" will output something akin to:

    176 [main] INFO  org.foo.Bar - Located nearest gas station.

The first field is the number of milliseconds elapsed since the start of the program. The second field is the thread making the log request. The third field is the level of the log statement. The fourth field is the name of the logger associated with the log request. The text after the '-' is the message of the statement.

## Configuration

Inserting log requests into the application code requires a fair amount of planning and effort. Observation shows that approximately 4 percent of code is dedicated to logging. Consequently, even moderately sized applications will have thousands of logging statements embedded within their code. Given their number, it becomes imperative to manage these log statements without the need to modify them manually.

The log4cxx environment is fully configurable programmatically. However, it is far more flexible to configure log4cxx using configuration files. Currently, configuration files can be written in XML or in Java properties (key=value) format.

Let us give a taste of how this is done with the help of an imaginary application `MyApp` that uses log4cxx.

    #include "com/foo/bar.h"
    using namespace com::foo;

    // include log4cxx header files.
    #include "log4cxx/logger.h"
    #include "log4cxx/basicconfigurator.h"
    #include "log4cxx/helpers/exception.h"

    using namespace log4cxx;
    using namespace log4cxx::helpers;

    LoggerPtr logger(Logger::getLogger("MyApp"));

    int main(int argc, char **argv)
    {
            int result = EXIT_SUCCESS;
            try
            {
                    // Set up a simple configuration that logs on the console.
                    BasicConfigurator::configure();

                    LOG4CXX_INFO(logger, "Entering application.");
                    Bar bar;
                    bar.doIt();
                    LOG4CXX_INFO(logger, "Exiting application.");
            }
            catch(Exception&)
            {
                    result = EXIT_FAILURE;
            }

            return result;
    }

`MyApp` begins by including log4cxx headers. It then defines a static logger variable with the name `MyApp` which happens to be the fully qualified name of the class.

`MyApp` uses the `Bar` class defined in header file `com/foo/bar.h`.

    //  file com/foo/bar.h
    #include "log4cxx/logger.h"

    namespace com {
       namespace foo {
          class Bar {
              static log4cxx::LoggerPtr logger;

          public:
              void doIt();
           }
        }
    }
    // file bar.cpp
    #include "com/foo/bar.h"

    using namespace com::foo;
    using namespace log4cxx;

    LoggerPtr Bar::logger(Logger::getLogger("com.foo.bar"));

    void Bar::doIt() {
       LOG4CXX_DEBUG(logger, "Did it again!");
    }

The invocation of the BasicConfigurator::configure method creates a rather simple log4cxx setup. This method is hardwired to add to the root logger a ConsoleAppender . The output will be formatted using a PatternLayout set to the pattern "%-4r [%t] %-5p %c %x - %m%n".

Note that by default, the root logger is assigned to `Level::getDebug()`.

The output of MyApp is:

    0    [12345] INFO  MyApp  - Entering application.
    36   [12345] DEBUG com.foo.Bar  - Did it again!
    51   [12345] INFO  MyApp  - Exiting application.

The previous example always outputs the same log information. Fortunately, it is easy to modify `MyApp` so that the log output can be controlled at run-time. Here is a slightly modified version.

    // file MyApp2.cpp

    #include "com/foo/bar.h"
    using namespace com::foo;

    // include log4cxx header files.
    #include "log4cxx/logger.h"
    #include "log4cxx/basicconfigurator.h"
    #include "log4cxx/propertyconfigurator.h"
    #include "log4cxx/helpers/exception.h"

    using namespace log4cxx;
    using namespace log4cxx::helpers;
    // Define a static logger variable so that it references the
    // Logger instance named "MyApp".
    LoggerPtr logger(Logger::getLogger("MyApp"));

    int main(int argc, char **argv)
    {
            int result = EXIT_SUCCESS;
            try
            {
                    if (argc > 1)
                    {
                            // BasicConfigurator replaced with PropertyConfigurator.
                            PropertyConfigurator::configure(argv[1]);
                    }
                    else
                    {
                            BasicConfigurator::configure();
                    }

                    LOG4CXX_INFO(logger, "Entering application.");
                    Bar bar
                    bar.doIt();
                    LOG4CXX_INFO(logger, "Exiting application.");
            }
            catch(Exception&)
            {
                    result = EXIT_FAILURE;
            }

            return result;
    }

This version of `MyApp` instructs `PropertyConfigurator` to parse a configuration file and set up logging accordingly.

Here is a sample configuration file that results in exactly same output as the previous `BasicConfigurator` based example.

    # Set root logger level to DEBUG and its only appender to A1.
    log4j.rootLogger=DEBUG, A1

    # A1 is set to be a ConsoleAppender.
    log4j.appender.A1=org.apache.log4j.ConsoleAppender

    # A1 uses PatternLayout.
    log4j.appender.A1.layout=org.apache.log4j.PatternLayout
    log4j.appender.A1.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

It can be noticed that the PropertyConfigurator file format is the same as log4j.

Suppose we are no longer interested in seeing the output of any component belonging to the `com::foo` package. The following configuration file shows one possible way of achieving this.

    log4j.rootLogger=DEBUG, A1
    log4j.appender.A1=org.apache.log4j.ConsoleAppender
    log4j.appender.A1.layout=org.apache.log4j.PatternLayout

    # Print the date in ISO 8601 format
    log4j.appender.A1.layout.ConversionPattern=%d [%t] %-5p %c - %m%n

    # Print only messages of level WARN or above in the package com.foo.
    log4j.logger.com.foo=WARN

The output of `MyApp` configured with this file is shown below.

    2000-09-07 14:07:41,508 [12345] INFO  MyApp - Entering application.
    2000-09-07 14:07:41,529 [12345] INFO  MyApp - Exiting application.

As the logger `com.foo.Bar` does not have an assigned level, it inherits its level from `com.foo`, which was set to WARN in the configuration file. The log statement from the `Bar::doIt` method has the level DEBUG, lower than the logger level WARN. Consequently, `doIt()` method's log request is suppressed.

Here is another configuration file that uses multiple appenders.

    log4j.rootLogger=debug, stdout, R

    log4j.appender.stdout=org.apache.log4j.ConsoleAppender
    log4j.appender.stdout.layout=org.apache.log4j.PatternLayout

    # Pattern to output the caller's file name and line number.
    log4j.appender.stdout.layout.ConversionPattern=%5p [%t] (%F:%L) - %m%n

    log4j.appender.R=org.apache.log4j.RollingFileAppender
    log4j.appender.R.File=example.log

    log4j.appender.R.MaxFileSize=100KB
    # Keep one backup file
    log4j.appender.R.MaxBackupIndex=1

    log4j.appender.R.layout=org.apache.log4j.PatternLayout
    log4j.appender.R.layout.ConversionPattern=%p %t %c - %m%n

Calling the enhanced MyApp with the this configuration file will output the following on the console.

     INFO [12345] (MyApp2.cpp:31) - Entering application.
    DEBUG [12345] (Bar.h:16) - Doing it again!
     INFO [12345] (MyApp2.cpp:34) - Exiting application.

In addition, as the root logger has been allocated a second appender, output will also be directed to the `example.log` file. This file will be rolled over when it reaches 100KB. When roll-over occurs, the old version of `example.log` is automatically moved to `example.log.1`.

Note that to obtain these different logging behaviors we did not need to recompile code. We could just as easily have logged to a UNIX Syslog daemon, redirected all `com.foo` output to an NT Event logger, or forwarded logging events to a remote log4cxx server, which would log according to local server policy, for example by forwarding the log event to a second log4cxx server.

## Default Initialization Procedure

The log4cxx library does not make any assumptions about its environment. In particular, there are no default log4cxx appenders. Under certain well-defined circumstances however, the static inializer of the `Logger` class will attempt to automatically configure log4cxx.

The exact default initialization algorithm is defined as follows:

1. Set the configurationOptionStr string variable to the value of the LOG4CXX_CONFIGURATION environment variable if set, otherwise the value of the log4j.configuration or LOG4CXX_CONFIGURATION environment variable if set, otherwise the first of the following file names which exist in the current working directory, "log4cxx.xml", "log4cxx.properties", "log4j.xml" and "log4j.properties". If configurationOptionStr has not been set, then disable logging.
2. Unless a custom configurator is specified using the LOG4CXX_CONFIGURATOR_CLASS or log4j.configuratorClass environment variable, the PropertyConfigurator will be used to configure log4cxx unless the file name ends with the ".xml" extension, in which case the DOMConfigurator will be used. If a custom configurator is specified, the environment variable should contain a fully qualified class name of a class that implements the Configurator interface.

## Nested Diagnostic Contexts

Most real-world systems have to deal with multiple clients simultaneously. In a typical multithreaded implementation of such a system, different threads will handle different clients. Logging is especially well suited to trace and debug complex distributed applications. A common approach to differentiate the logging output of one client from another is to instantiate a new separate logger for each client. This promotes the proliferation of loggers and increases the management overhead of logging.

A lighter technique is to uniquely stamp each log request initiated from the same client interaction. Neil Harrison described this method in the book "Patterns for Logging Diagnostic Messages," in Pattern Languages of Program Design 3, edited by R. Martin, D. Riehle, and F. Buschmann (Addison-Wesley, 1997).

To uniquely stamp each request, the user pushes contextual information into the NDC, the abbreviation of Nested Diagnostic Context. The NDC class is shown below.

    namespace log4cxx {
      class NDC {
      public:
        //  pushes the value on construction and pops on destruction.
        NDC(const std::string& value);
        NDC(const std::wstring& value);

        // Remove the top of the context from the NDC.
        static LogString pop();

        // Add diagnostic context for the current thread.
        static void push(const std::string& message);
        static void push(const std::wstring& message);
      }

The NDC is managed per thread as a stack of contextual information. Note that all methods of the `log4cxx::NDC` class are static. Assuming that NDC printing is turned on, every time a log request is made, the appropriate log4cxx component will include the entire NDC stack for the current thread in the log output. This is done without the intervention of the user, who is responsible only for placing the correct information in the NDC by using the `push` and `pop` methods at a few well-defined points in the code. In contrast, the per-client logger approach commands extensive changes in the code.

To illustrate this point, let us take the example of a servlet delivering content to numerous clients. The servlet can build the NDC at the very beginning of the request before executing other code. The contextual information can be the client's host name and other information inherent to the request, typically information contained in cookies. Hence, even if the servlet is serving multiple clients simultaneously, the logs initiated by the same code, i.e. belonging to the same logger, can still be distinguished because each client request will have a different NDC stack. Contrast this with the complexity of passing a freshly instantiated logger to all code exercised during the client's request.

Nevertheless, some sophisticated applications, such as virtual hosting web servers, must log differently depending on the virtual host context and also depending on the software component issuing the request. Recent log4cxx releases support multiple hierarchy trees. This enhancement allows each virtual host to possess its own copy of the logger hierarchy.

## Performance

One of the often-cited arguments against logging is its computational cost. This is a legitimate concern as even moderately sized applications can generate thousands of log requests. Much effort was spent measuring and tweaking logging performance. Log4cxx claims to be fast and flexible: speed first, flexibility second.

The user should be aware of the following performance issues.

1. Logging performance when logging is turned off.
   When logging is turned off entirely or just for a set of levels, the cost of a log request consists of a method invocation plus an integer comparison. The LOG4CXX_DEBUG and similar macros suppress unnecessary expression evaluation if the request is not enabled.
2. The performance of deciding whether to log or not to log when logging is turned on.
   This is essentially the performance of walking the logger hierarchy. When logging is turned on, log4cxx still needs to compare the level of the log request with the level of the request logger. However, loggers may not have an assigned level; they can inherit them from the logger hierarchy. Thus, before inheriting a level, the logger may need to search its ancestors.

   There has been a serious effort to make this hierarchy walk to be as fast as possible. For example, child loggers link only to their existing ancestors. In the `BasicConfigurator` example shown earlier, the logger named `com.foo.Bar` is linked directly to the root logger, thereby circumventing the nonexistent `com` or `com.foo` loggers. This significantly improves the speed of the walk, especially in "sparse" hierarchies.

   The cost of walking the hierarchy is typically 3 times slower than when logging is turned off entirely.

3. Actually outputting log messages
   This is the cost of formatting the log output and sending it to its target destination. Here again, a serious effort was made to make layouts (formatters) perform as quickly as possible. The same is true for appenders.

## Conclusions

Apache Log4cxx is a popular logging package written in C++. One of its distinctive features is the notion of inheritance in loggers. Using a logger hierarchy it is possible to control which log statements are output at arbitrary granularity. This helps reduce the volume of logged output and minimize the cost of logging.

One of the advantages of the log4cxx API is its manageability. Once the log statements have been inserted into the code, they can be controlled with configuration files. They can be selectively enabled or disabled, and sent to different and multiple output targets in user-chosen formats. The log4cxx package is designed so that log statements can remain in shipped code without incurring a heavy performance cost.