Gennadiy Rozental wrote:
BTW before I start, I recalled couple more issues I forgot to mention in original post:
1. Design: Lightweight interface Log library brings a lot of dependencies. In some case I do not want this if
Care to exemplify? What are the dependencies?
I need to use the library in header. IMO log needs to support/implement some lightweight debug abstract interface Something like:
struct DebugLog { virtual void log( string const& ) = 0; virtual void log( int ) = 0; };
struct Log : DebugLog { ... };
Now I could use DebugLog interface without need to know an implementation. It comes with the price of virtual call. I primarily use this for debug only logging.
Anybody else wants this?
2. Design: Enter/Exit function scope support I believe it's required for log library to provide some kind of support for scope tracking. It could be an addon to core functionality to it needs to be configurable.
Added to my TODO list.
3. Design: hexdump I believe log library needs to provide support for dumping memory in hex format on log file. It could be an addon to core functionality.
I can add this, even though I'm not sure why you'd want this.
4. Design: logged assert I believe log library needs to provide support for custom assert. Such assert would log into log file and invoke system assert depends in assert level. It could be an addon to core functionality.
Well, I want to adapt SMART_ASSERT to allow for this functionality.
5. Implementation/Design: log entry data copying. Modifiers implemented by applying modifier for string that represent entry data. This model is inefficient in following regards: a) you need to make a copy of original entry data for every output. b) you need to prepend the strings in most cases - which is inefficient operation. c) you couldn't share the work for different outputs - you need to repeat it for every string
IMO better model would be to apply modifiers to outputs instead.
Yes, I will consider this. IMO, is better to have a working model, which can be optimized later.
Actually since macros are not primary interface in different subsystems log may look different. Couple examples:
1. LOG( INFO, PROG_FLOW, "SocketLib" ) << "Connection esteblished to: " << address; 2. LOG( DEBUG, RETURN_VALUE ) << "Fetched value: " << v; Here a LOG macro refer to some keyword by name. Usually I have keyword per file, per library or per class. 3. DEBUG_LOG( DATA_FLOW ) << "Input msg: " < msg; Here DEBUG_LOG automatically apply DEBUG level and refer to keyword by name
This IMO is too much to type, and I personally woudn't use it in real-life apps.
Also multithreaded application should support filtering based on thread id.
And what's stopping you from making an appender that does just this?
// trivial example struct only_for_thread_id { only_for_thread_id(int tid, appender_func forward_to) : tid(tid), forward_to(forward_to) {} void operator()(const std::string&,const std::string & msg) { if ( ::GetCurrentThreadId() == tid) forward_to(msg); } int tid; appender_func forward_to; }
You must be kidding, right? I want filtering on log level. So that entry appear or not appear in all outputs.
Please read what you just wrote a couple of lines above: "filtering based on thread id."
2. Configuration support Submitted solution supports configuration in a form of arbitrary modifiers that user could register in log. While this looks like very inflexible it's also as very inconvenient. In most cases I prefer log to be configured at run time from within configuration file. his would be very cumbersome with modifiers model. Not only each modifier needs to be added separately. I also
Did you take a look at Darryl's proposal? http://torjo.com/code/lib/alternate_manipulating.html)
And how that has anything to do with what I am talking about? It's just a twist of the same unacceptable interface.
Unacceptable to you, yes.
need to keep track of it's name to be able to remove it and it's index to know how modifiers are ordered. This ability to configure log remotely in many locations makes it impossible to really know what and how is written into log. My preferred way to configure the log is with use of configuration string. First of all it's simplify an interface and you immediately see how your log entries going to look like. Here is an example: "keyword=*,-ACD;categ=prog_flow,return value,-details;l=debug;track=on;roll=10000;prefix=file ( line ) - time :;timeformat=%s:%m".
You can implement Configuration Support on top of the current library.
I do not see how it possible and why should I do this on top of useless API.
Funny, but I think your last statement is quite useless.
And besides, if you want to configure logging, you'll still need to identify each log, appender and manipulator by a string.
Why is that? I do not want or need to know log name to configure it. I have an instance.
Because when you have some setting in the configuration file, you need whom it refers to. Therefore, you need to have some log-to-string correspondence.
4. Internationalization support I do see some vague traces in code that indicate that author new this needs to be supported. But since documentation have exactly zero information and I couldn't figure out how to do it myself, I assume that library doesn't support this. IMO it's required for industrial strength solution.
How many times have you needed internationalization support logging?
Me, personally - never. But may be the fact that I work in domestic department of American company could be accounted for that. But for the boost level library this is a showstopper. Internationalization is not an afterthought.
Still what do you understand by internationalization? It can deal with Unicode, if this is what you want...
6. Misfeature: pre-init cashing Library starts with cashing turned on to support pre-init logging. IMO this is too dangerous. Now developer needs to remember how many log entries are printed. Otherwise some of them will just start disappearing (could be different in every run). And it may start happening only at the user site. Much more reliable solution is to use some kind of startup log that does not require configuration.
And where would that log write to? What if it's very important data? To console? What if you don't have console?
I think I covered that in my last statement.
Well, I don't think it's covered.
And lets face it -- when one starts using a logging lib, will at least trace one call to the debug lib. If he sees the line doesn't show up anywhere, he'll most likely read the docs. If you take a look at the
I don't see how reading the docs will help. Solution is dangerous in it's nature.
When you'll come up with something better, I'll use it. Until then...
Basic Usage, you'll see the 'flush_log_cache()' call.
Besides, even if you don't call it and you write 256 messages to a log, the caching will automatically turn itself off and flush. The same
And what if log is not configured yet? What will happened? We will loose all those 256 messages or program will crash?
Ok, I have not handled this case yet. I need to handle it -- have a "safe log", where I will write if program exists with cache still turned on, and logs have not been initialized yet.
happens if the application ends, and caching was still on.
7. Misfeature: log hierarchies I personally never have a need for that and have some difficulties to imagine why would anyone have. You could probably have it on top of existent solution as add-on. If it doesn't require any new interfaces (or some
Have you heard of modules, and sub-modules? What if each module/sub-module was to have a log to write to?
I primarily use one file for everything. Why would I want 10 different files? So to figure out what happened I will need to jump through 10
You can have multiple logs that output to the same file.
different output comparing timestamps? (*if* they are printed). I do *rarely* use more than additional logs for information dedicated to different target audience (operation departments, end users). But this is not driven by module/submodule hierarchy. To mark the modules I use much better idiom - keyword (all within bounds of same log).
I'm not even gonna ask why yours is such a better idiom...
8. Misfeature: compile-time log support As it clear from above IMO log is (almost) never always enabled (and in any case there is a way to implement this without any additional support). And always disabled log could be implemented on macro level. Based on this I
Please enlighten me: how can you implement a disabled log at macro level?
Numerous ways. Here is from top of my head:
struct nil_stream {}
template<typename T> nil_stream const& operator<<( nil_stream const& ns, T const&) { return ns; }
#define LOG( .... ) if(true) {} else nil_stream() <<
Amazing... You can turn on/off ***all*** logging... Wow! And to think that BOOST_LOG allows you to turn on/off certain logs -- allowing for some information to be logged while what's not important is turned off... Your solution is indeed much better... Not to say that if you write something like: LOG << some_lengthy_function(); some_lengthy_function() will still be executed, even though logging is disabled...
II. Implementation flaws ------------------
2. Multithreading support implementation a) For some unclear reason author chose to reimplement some of Boost.Threads functionality in some places (and in some places Boost.Thread is used always). I believe log library shouldn't do this.
It was requested -- some users wanted to remove dependency on Boost.Thread.
Well it still wrong. You couldn't justify that by user's request. BTW you do depend on Boost.Threads still.
??? If you use ts_appender(), yes. Otherwise, not.
4. collection in use appender_array and modifier_array are better be implemented using lists
Have you done some tests, or is this just a gut feeling?
I did not provide any tests, did you? And it's not a gut feeling. In places where it's important you never need random access.
It's not about random access. It's about so many tests have shown that std::vector is faster than std::list, especially when the vectors are small - in the case of logger_impl.
5. exception support
try { LOG << "aaa" << foo() } catch(...) { LOG << "exception" }
This construct doesn't seems generate what is expected if foo() throws an exception
What is expected?
Try to run this and you will see.
It tries to print as much as possible from the original "LOG << "aaa" << foo()" expression, and then it prints "exception". What do you expect?
5. appender_array appender_array seems to be defined in 2 placed
Yes, once in the log_impl's class definition, and once in namespace boost { namespace logging { namespace {
Why would you give the same name to 2 different things?
It's about context. Why not?
If we do why all the appenders are so heavy?
I don't understand what you mean.
So costly to copy.
How would you know?
9. Time string cashing prepend_time and prepend_time_strf calls are not cashed. It's a lot of work for nothing.
Please explain.
You may have 1000 messages per second. And you format time string for every single one.
Ok, you're right. I can improve on that.
10. write_msg locking write_msg shouldn't lock on appenders writing. Each appender should be locked independently instead.
It's because appenders/modifiers could be added/deleted while someone is writing a message to the log. Thus, it needs to be thread-safe.
You need to find an alternative solution. Every output has to be locked independently. This is one of the "premature pessimization" you force on the user.
I'll think about it. Best, John -- John Torjo, Contributing editor, C/C++ Users Journal -- "Win32 GUI Generics" -- generics & GUI do mix, after all -- http://www.torjo.com/win32gui/surfaces.html - Sky's the limit! -- http://www.torjo.com/cb/ - Click, Build, Run!