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.
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.
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:
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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:
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) ...
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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:
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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 |
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 |
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 |
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 |
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.");
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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.
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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");
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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.
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.
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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 languageprintf 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.
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;
}
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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!");
}
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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;
}
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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.
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:
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.
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.
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.
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.