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Apache log4cxx
Version 0.12.1
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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:
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:
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.
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.
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.
Calling the getLogger method with the same name will always return a reference to the exact same logger object.
For example, in
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.
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 up to 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:
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.
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.
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:
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.
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.
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.
The output of MyApp configured with this file is shown below.
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.
Calling the enhanced MyApp with the this configuration file will output the following on the console.
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.
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.
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.
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.
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.
Sometimes, you may want to remove all log statements from your program, either for speed purposes or to remove sensitive information. This can easily be accomplished at build-time when using the standard LOG4CXX_[level] macros (LOG4CXX_TRACE, LOG4CXX_DEBUG, LOG4CXX_INFO, LOG4CXX_WARN, LOG4CXX_ERROR, LOG4CXX_FATAL).
Log statements can be removed either above a certain level, or they can be disabled entirely.
For example, if we want to remove all log statements within our program that use the LOG4CXX_[level] family of macros, add a preprocessor definition LOG4CXX_THRESHOLD set to 50001 or greater. This will ensure that any log statement that uses the LOG4CXX_[level]-macro will be compiled out of the program. To remove all log statements at DEBUG or below, set LOG4CXX_THRESHOLD to a value between 10001-20000.
The levels are set as follows:
| Logger Level | Integer Value |
|---|---|
| TRACE | 5000 |
| DEBUG | 10000 |
| INFO | 20000 |
| WARN | 30000 |
| ERROR(1) | 40000 |
| FATAL | 50000 |
(1) The LOG4CXX_ASSERT macro is the same level as LOG4CXX_ERROR
Note that this has no effect on other macros, such as using the LOG4CXX_LOG, LOG4CXX_LOGLS, or LOG4CXX_L7DLOG family of macros.
Often, the data that needs to be logged is not just standard data types (such as int, string, etc), but amalgamations of those types in a data structure such as a class or struct. In order to log these custom types, simply override an operator<< function, the same as if you would print the custom type to std::cout. This can be accomplished by doing the following:
This will output data similar to the following:
One issue with utilizing log4cxx and its ostream style of logging is that log statements can be very awkward if you need to precisely format something:
This leads to very awkward code to read and write, especially as iostreams don't support positional arguments at all.
In order to get around this, one popular library(that has been standardized as part of C++20) is {fmt}. Supporting positional arguments and printf-like formatting, it makes for much clearer code like the following:
Note that log4cxx does not include a copy of {fmt}, so you must include the correct headers and linker flags in order to use the LOG4CXX_[level]_FMT family of macros.
As with the standard logger macros, these macros will also be compiled out if the LOG4CXX_THRESHOLD macro is set to a level that will compile out the non-FMT macros.
A full example can be seen in the src/examples/cpp/format-string.cpp file.
When dealing with large amounts of logging information, it can be useful to filter on messages that we are interested in. This filtering only takes places after determining that the level of the current logger would log the message in the first place. When defining filters, note that they can only be defined on a per-appender basis, they do not globally affect anything.
The filtering system is similar in concept to Linux iptables rules, in that there is a chain of filters that can accept a log message, deny the log message, or pass the message on to the next filter. Accepting a log message means that the message will be logged immediately without consulting other filters. Denying has the opposite affect, immediately dropping the log message and not consulting any other filters.
See the documentation for Filter for some more information, or view a configuration sample.
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.
1.8.13