Some thoughts and ideas about some of the topics that has been discussed regarding the proposed Boost.Logging library. 1. Enabled/disabled logger, logging macros and efficiency The logging macro BOOST_LOG should accept two arguments instead of one. The logger and the log message. The syntax (1) BOOST_LOG(some_log) << "my message " << foo() << bar(); should be replaced with (2) BOOST_LOG(some_log, "my message " << foo() << bar()); and the macro being defined something like this (3) #define BOOST_LOG(logger, msg) \ if(logger.isEnabled()) { /* code that logs msg */ ... } Thus, disabled loggers will never take longer than simple if-statement, as long as the isEnabled() method is fast, which IMO should take no longer than comparing booleans. In the original solution (1) the method foo() will always be called, which is inefficient when the logger is disabled, in solution (2), foo() will never be called when the logger is disabled. Further more, the macro could insert extra information such as __LINE__, __FILE__ etc. If needed or wanted, the macro could of course be redefined to leave no trace of the logging. I could also envision a few other macros that for example creates a logger object on the stack that logs upon creation and destruction, a special macro to aid developers that are always compiled out of release code. Perhaps such macros is best left to the user of the library although I do think that there are common set of macros that is useful for most applications and situations and thus are good candidates for being defined in the library. 2. Filtering There has been some discussion about the ability of a library to be able to do more filtering than simple log levels, notably by Gennadiy Rozental. Gennadiy writes:
At the bare minimum it should support:
entry level - levels set is ordered set of values indication importance of information provided. Filtering is based on threshold. Examples: DEBUG, INFO, MAJOR
entry category - categories set is a set of unique values indicating kind of information provided Filtering is based on masking. Examples: ARGS, RETURN_VALUE, ERROR_LOG, DATA_FLOW, PROG_FLOW
entry keyword - keyword set is set of user defined keywords (most frequently strings) identifying area of the program. Filtering is based on match of keywords. Keywords usually are used to mark specific part of application.
I agree that it is very important for a logging library to support this. However, I do not think that the solution is for the logging library to be aware of such special values. The library should be as simplistic as possible, with no or few assumptions about its use, at least at the basic "framework level". Only then IMO could the library accodomate various neeeds. Let me explain. If special values are introduced in the library, there will always be applications and users that lack some other special value, perhaps specific to their domain, but even more likely, to their taste and style. My proposed solution is to basically let loggers be somewhat entry (Gennadiy's definition above) unaware. Instead, let the developer be entry aware. By that I mean that the developer should log messages that belong to a specific entry category/level to a specific logger. For example, if there is a need to log and filter a RETUN_VALUE category, then a specific logger, perhaphs named RETURN_VALUE_LOGGER (or whatever), and let the user send log messages to that logger. This could look something like this: BOOST_LOG( RETURN_VALUE_LOGGER, "The return value is: " << value ); This solution also means that you have a different logger for each level, level filtering perhaps being the most common filter. For example: BOOST_LOG( debug, "some debug output " << a << foo() ); BOOST_LOG( error, "Error: " << e.what() ); where 'debug' and 'error' are loggers. Now, the drawback with this approach is that several loggers needs to be defined and configured, but as long as this is simple this should be ok. There should also be default definitions and configurations available from the library, which "sits" on top of the basic library framework. As for entry keywords (see above), I see two interpretations of Gennadiy's idea. (1) A log statement is marked with a keyword. For example: BOOST_LOGX( logger, "user defined keyword", "my log msg " << value ); Or (2), logging is filtered on what strings are contained in the log message. That is, log statements are written in the normal way by the programmer but is filtered by some mechanism where the log message is matched against certain keywords, like the grep command. For interpretation (1): My proposed solution should solve the problem. Declare a specific logger that the programmer send log statements to for the specified keyword. For interpretation (2 ): IMO the filtering mechanism should either sit on top of loggers or possibly in the appenders, or the tools that are used to view logs (e.g. grep and such). The "on top" solution would be to use specicial functions that utilizes the "lower" layer of simple loggers. It should be an extension to the log library and not be part of the core functionality (but could still be provided by a logging library). For example: BOOST_LOGX( logger, Predicate( ... ), "my log msg" << value ); Thus, the approach to filtering, based on Gennadiy's idea of various criteria is to to enable/disable loggers depending on system/users/developers needs'. Hence, enabling/disabling must be simple. Using John Torjo's solution of manipulate_log-functions this ought be relative easy. The programmer should be able to enable/disable loggers on various critiera. One solution for this would be to attach user, as well as common/default library, supplied name-value pairs to loggers and let the manipulate_log functions enable/disable loggers that meet certain criteria for these name-value pairs. For example, when defining which loggers that are available to the programmer (by himself or some other developer), a log level name-value object is attached to the loggers. The logger named 'debug' is attached with a log level of 'debug' (a literal integer value chosen carefully e.g. 0). Later on in the program, the programmer can issue the statement, using the manipulate_log-functions to enable all loggers that have a property name log level with a value greater than 'debug' and so on. Additional benefits of using this clean approach (maybe possible with Johns solution as well (?)) to loggers is that it is quite possible to have debug loggers log in one subsystem while normal logging is done in the rest of the program. Any combination is possible. The major drawback with this clean approach to loggers as I see it is the additional complexity, cognitive burden, for setting up loggers. Part of this problem should be alleviated by the library by providing a default set of loggers for common/simple/I just wanna get started -situations. Cheers, Richard Glanmark The content of this e-mail is intended only for the confidential use of the person(s) to whom it is addressed. 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