Monday, July 13, 2009

Blogging about logging

I've just finished a series of changes which were meant to make wxLog less embarrassing and more useful. Of course, wxLog was always meant to be a simple logging framework adapted for typical logging patterns of GUI applications but there is such thing as being too simple and it became apparent since quite some time that wxLog was insufficient for any kind of application using multiple threads or even simply separated in multiple components whose logging should be controlled simultaneously. And as most applications nowadays do use multiple threads, this is a serious limitation indeed.

As an aside, when I realized that the deficiencies of wxLog really prevented it from being useful in the application I was working on, my first idea was not to enhance it but to switch to another, dedicated logging library. But incredibly enough I couldn't find any good candidate: there are tons of libraries based on log4j but translating Java API in C++ is really not a good idea and I hoped to find something more idiomatically C++-ish. So I naturally turned towards Boost and found not one but two libraries named "Boost.Log", with one even confusingly called "Boost.Log v2" despite being older than the other one. Unfortunately, while both of them are undoubtedly great libraries, I was completely overwhelmed by their complexity. They are certainly great and allow some things I wouldn't even think of if I were creating a new logging library from scratch, e.g. a possibility to associate a decrementing counter starting from 100 with step of -5 with every log record which is extremely impressive but also doesn't seem to be especially useful in practice and I'd prefer to just simply use a logging library instead of admiring its marvellous elegance. So I passed them too -- and decided that while wxLog might be too simple, keeping it simple enough was still very important.

With this in mind, I decided to simply fix the few most glaring omissions in wxLog:
  1. Lack of support for logging from threads other than main.
  2. Impossibility to treat logs from different parts of application differently.
  3. Absence of __FILE__, __LINE__ and __FUNCTION__ information.
The first one was already solved for some logging targets, e.g. wxLogWindow was already thread-safe as it collected the messages coming from other threads and really displayed it in its text control only during the idle time from the main thread. All I did was to extend this approach to all log targets by moving its implementation in wxLog itself.

This does introduce a new problem however: as the messages are buffered instead of being output immediately, they could be lost if the program crashes before the main thread has a chance to output them. So I also added a concept of per-thread log targets which can be associated with a single thread only and don't need to do any buffering. Of course, such target can't show messages to the user -- as this can only be done from the main GUI thread -- but it can log them to a file and so a thread can always set up wxLogStderr or a wxLogStream to ensure that its messages are saved in a file as soon as they are output.

On a related note, using wxLogNull (and wxLog::EnableLogging() which it uses internally) now only disables logging for the current thread and not the application as a whole. This makes sense as if you just want to suppress an error message from a wxWidgets function you're going to call, you shouldn't disable all the logs from the other threads of your application which can be doing something completely unrelated while this function is executing. The initial plan was to also add a new way of disabling the logging globally but after thinking about it for quite some time I couldn't find any realistic use case when doing this would be really useful so for now logging can only be enabled thread-wise -- but we can always make it possible to disable it either globally or, which probably makes more sense, on log target basis, if really needed.

The second problem was solved by introducing the notion of "log components". These are simply arbitrary strings which identify the component which logged a message. By default, messages logged by wxWidgets come from the log component "wx" and its subcomponents, that is strings starting with "wx/" like, for example, "wx/net/ftp", while messages generated outside of wxWidgets have empty log component as it's not defined by default. This is already useful as sometimes you may want to treat wxWidgets and your own messages differently, e.g. you could disable all non-error messages from wxWidgets by setting the log level of the "wx" component to wxLOG_Error while keeping all messages, including the debugging ones, from your code enabled. But this feature becomes really useful mostly when you do define your own custom log components. This is done simply by #define-ing wxLOG_COMPONENT before using wxLogXXX() functions. It can be done on the compiler command line (to ensure that the same value is uniformly used everywhere) or inside the source files. In either case you will probably want to use different values for different parts of your application, e.g. "myapp/ui" and "myapp/db" and "myapp/network" and so on. And then you can independently configure the log level for each module and, also importantly, you can distinguish between the messages logged by different components and send them to different final destinations (e.g. database-related messages to one log file and network ones to another) from your overridden wxLog::DoLogRecord().

Finally, to solve the last problem in the list, all wxLogXXX() functions have been replaced by macros with the same names, which allows to record the information about the log message location. It can be retrieved from DoLogRecord() from the wxLogRecordInfo struct passed to it. By default, this information is not used in any of the predefined loggers (yet?) but it's available in case you nee it.

Moreover, in process of doing this, I actually created a relatively generic mechanism for passing arbitrary extra information to the log functions -- but, still remembering my experience of reading Boost.Log documentation, I decided to not make it public for now and to keep things simple.

After all, with the additions mentioned above wxLog is already much more useful and hopefully it's good enough for even complex wxWidgets applications now. And if not, we'd be interested to hear about still missing features, of course, so do have a look at the improved wxLog version in svn trunk and let us know what do you think!
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