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From: John Torjo (john.lists_at_[hidden])
Date: 2005-11-16 10:07:24

> 1) A given message is directed to a particular named log, from where it is sent
> to 1 or more named appenders. It is easy to connect/disconnect appenders to
> logs. It is also easy to set the log level. However, I find the log level
> facility useless, because in general I want to filter the messages to a
> particular appender, not globally - if one of my appenders is the system error
> log, I always want errors, all errors, and nothing else, going to it. If I have
> some sort of console debug output, I might want to turn on debug level output
> to it alone (in general from only a few logs). I can do this reasonably well if
> I connect the "system error log" to "*.error" and adjust what goes to
> the "debug console" by specifying eg. "" output to go there, or
> more likely "*" and turning off "" if it is too noisy. If
> some library author decides that log levels are the way to go, I can't get my
> app to direct that libraries errors to the error log. Alternatively, if I write
> a library, and I want to ensure that it is useable in this scenario and in some
> level-based logging scheme, I would presumably need to log an error with a
> level of err to a log called "mylib.error" and require that the app using mylib
> connect appenders to "mylib.*" and set levels on "mylib.*" if the app is

I see what you mean. I believe that Gennady's solution of filtering is
the way to go. It is very powerful - it took me a while to get it, but
now I understand ;)

> Extending logger_and_bool to logger_and_predicate and the appropriate c'tors
> for the keeper/enabled_log would allow the predicate to perform checks that
> require access to the logger itself as well. These tests are (potentially)
> heavyweight but so long as a simple call to BOOST_LOG simply bypasses them
> there is no cost to the user who doesn't need them.

Yup, you're right.

> 2) While the library aims to offer a customisation point through replacing the
> manager type, the manager concept is un(der)documented, and the various
> interfaces that depend on the manager type are too closely coupled to allow
> much customisation anyway. Note that this may be ok, if the design of the
> standard manager is flexible enough without extensive customisation, and any
> remaining customisation points/concepts are clearly documented. In any case,
> the use of the customisation needs to be illustrated by example (ie. an
> alternate manager replacing all the default interfaces).

Yes, I definitely need more docs.

> 3) Closely related to the above, I would like to see cleaner separation between
> the components that make up this library. This is something that is hard to do
> when the implementation is a feature driven moving target (I've been trying to
> do it on and off). This separation needs to maintain or increase the existing
> loose coupling between compilation units that use the library to log, those
> that set/modify filtering and those that do actual output of logged messages,
> while reflecting that separation in the implementation. Currrently appenders do
> this, but the logging and filtering is too internally intertwined. If the
> library formalised that separation allowing each module to be customised,
> library code could use logging independent of how the application (or other
> libraries) used it/controlled it (and without forcing library recompilation).

Yes, as I will redesign it, this will definitely happen.

> 5) While applying modifiers to logs rather than to appenders is (potentially)
> more efficient as it avoids doing the modification N times for N appenders, in
> reality I have yet to encounter a case where the log format wasn't associated
> with the destination, rather than the source (ie. a log file should have a
> consistent format). An extreme example is when the log is a structured log such
> as the windows event log. A simple example is the newline modifier. If an
> appender is expecting to receive log messages it is up to the appender to
> delimit the messages in the appropriate way - be that by packing it into a
> syslog UDP packet or just sticking a newline on the end. I don't think the
> newline modifier should exist at all, but in other cases it does make sense to
> include log specific information. I'm not sure that for typical use where N is
> small that the efficency gains are likely to be significant, but it would also
> be possible to improve efficiency issue in cases where it is a real concern by
> using a multiplexing appender (that forwarded formatted output multiple "real"
> appenders).

I still like the modifier concept, but I do see your point.

In the future, I'd like to provide modifiers, which I think are useful
impelmented alone (like: newline, prepend_time, etc.), BUT, have the
appenders call them, if needed.

> The modifier concept also blurs the line between insertion of information and
> formatting - if a log should always contain some particular information at the
> start of it what is wrong with just writing it to the enabled_logger's stream
> when it is created. This also leverages all the built-in and extended output
> and formatting facilities of std streams (including rather more
> internationalisation support than found in strings). This fits in with the user
> extensible logger attributes, predicates etc above. I would like to see 2
> points at which the content can be modified - when the stream is created, so
> that informartion that should be a prefix on all logged messages via a
> particular log, regardless of destination can be inserted there, and another
> associated with the appender, rather than any particular log, so that any
> content that must always be inserted in messages to that log can be
> inserted/formatted there.

Yes, when I'll redesign the lib, I'll make it possible to format the
string at any time.

Thanks for the review.


John Torjo,    Contributing editor, C/C++ Users Journal
-- "Win32 GUI Generics" -- generics & GUI do mix, after all
-- - Sky's the limit!
-- - Click, Build, Run!

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