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From Luc Maisonobe <>
Subject Re: [Math] Utilitzation of SLF4J?
Date Sat, 26 Sep 2015 17:07:53 GMT
Le 26/09/2015 18:42, Gilles a écrit :
> On Sat, 26 Sep 2015 09:03:06 -0700, Phil Steitz wrote:
>> On 9/26/15 4:56 AM, Thomas Neidhart wrote:
>>> On 09/26/2015 01:11 PM, Gilles wrote:
>>>> On Sat, 26 Sep 2015 09:53:30 +0200, Thomas Neidhart wrote:
>>>>> On 09/26/2015 02:33 AM, Gilles wrote:
>>>>>> On Fri, 25 Sep 2015 16:52:26 -0700, Hasan Diwan wrote:
>>>>>>> On 25 September 2015 at 16:47, Gilles <>
>>>>>>> wrote:
>>>>>>>> On Fri, 25 Sep 2015 17:30:33 +0200, Thomas Neidhart wrote:
>>>>>>>>> On Fri, Sep 25, 2015 at 5:09 PM, Gilles
>>>>>>>>> <>
>>>>>>>>> wrote:
>>>>>>>>> On Fri, 25 Sep 2015 07:28:48 -0700, Phil Steitz wrote:
>>>>>>>>>> On 9/25/15 7:03 AM, Gilles wrote:
>>>>>>>>>>> On Fri, 25 Sep 2015 15:54:14 +0200, Thomas Neidhart
>>>>>>>>>>>> Hi Ole,
>>>>>>>>>>>>> for a start, I think you are asking the
wrong question.
>>>>>>>>>>>>> First of all we need to agree that we
want to add some kind of
>>>>>>>>>>>>> logging
>>>>>>>>>>>>> facility to CM.
>>>>>>>>>>>>> If the outcome is positive, there are
a handful of
>>>>>>>>>>>>> alternatives,
>>>>>>>>>>>>> some of
>>>>>>>>>>>>> them more viable than slf4j in the context
of CM (e.g. JUL or
>>>>>>>>>>>>> commons-logging).
>>>>>>>>>>>> Could someone summarize why those alternatives
were deemed
>>>>>>>>>>>> "more
>>>>>>>>>>>> viable"?
>>>>>>>>>>>> btw. the same discussion has been done for
other commons
>>>>>>>>>>>>> components as
>>>>>>>>>>>>> well, and the result usually was: do
not add logging
>>>>>>>>>>>> What was the rationale?
>>>>>>>>>>> Look at the archives.  We have discussed this
multiple times
>>>>>>>>>>> in the
>>>>>>>>>>> past in [math] and each time came to the conclusion
that Thomas
>>>>>>>>>>> succinctly states above.  What has changed now?
>>>>>>>>>> We also discussed several times to stick with Java
>>>>>>>>>> Fortunately, that has changed. [Although sticking
with Java 7 is
>>>>>>>>>> still
>>>>>>>>>> a bad decision IMHO.]
>>>>>>>>>> As for logging, IIRC, the sole argument was "no dependency"
>>>>>>>>>> because
>>>>>>>>>> (IIRC) of the potential "JAR hell".
>>>>>>>>> that's not correct. The decision to not include any
>>>>>>>>> dependencies has
>>>>>>>>> nothing to do with "JAR hell".
>>>>>>>> Although I can't find it now, I'm pretty sure that I more
than once
>>>>>>>> got such an answer.
>>>>>>>> In order to prevent JAR hell, commons components strictly
>>>>>>>> to the
>>>>>>>>> "Versioning guidelines" [1]
>>>>>>>> I can't see how it relates.
>>>>>>>> But if you mean that no JAR hell can emerge from using a
>>>>>>>> framework,
>>>>>>>> then that's good news.
>>>>>>>> The no-dependency rule is more related to the proposal of
>>>>>>>> component,
>>>>>>>>> see [2]
>>>>>>>> Thanks for the reminder; in that document, we read:
>>>>>>>>   (1) Scope of the Package
>>>>>>>>    [...]
>>>>>>>>    5. Limited dependencies. No external dependencies beyond
>>>>>>>> components and the JDK
>>>>>>>> So we are fine if use "Log4j 2" as kindly offered by Gary.
>>>>> log4j is not a commons component btw.
>>>> Too bad for me. :-/
>>>> Case resolved, then, by the argument of authority?
>>> I just pointed out that log4j is not a commons component and did not
>>> imply anything else.
>>>> "Commons" is OK but not another Apache project, by virtue of a
>>>> document that still refers to "JDK 1.2", "CVS", "Bugzilla" (not to
>>>> mention that the "scope" of CM currently goes well beyond "the most
>>>> common practical problems not immediately available in the Java
>>>> programming language")...
>>>> What's the _technical_ rationale for accepting this dependency and
>>>> not accepting that dependency?
>>>>> I have not seen a single example of a useful logging message that
>>>>> could
>>>>> be added to commons-math, but we are already discussing which
>>>>> framework
>>>>> to use.
>>>> If it is not useful to you, why would you conclude that it is not
>>>> useful to others?
>>>> At the cost of repeating myself, once more, the use-case is not
>>>> primarily about debugging CM, but sometimes one could need to assess
>>>> how a "non-obvious" CM algorithm responds to an application's request.
>>>> I've clearly expressed that use-case in a previous message.
>>>> Another example: I have a class that wraps a CM root solver; it is
>>>> stuffed with log statements because the message contained in the
>>>> "NoBracketingException" was utterly insufficient (and plainly
>>>> misleading due the default formatting of numbers) to figure out why
>>>> certain calls succeeded and others not.
>>>> It's a problem (or a limitation) in the application, but in the
>>>> absence of other clues, tracing the solver could help figure out a
>>>> workaround.
>>>> The alternative to the "logging" approach, would have been to include
>>>> a precondition check before calling the solver, that would in effect
>>>> duplicate the bracketing check done inside the solver. Given the vast
>>>> amount of cases where the code ran smoothly, this is clearly a
>>>> sub-optimal solution as compared to turning logging on and rerun the
>>>> case that led to a crash.
>>>> What can I say more about the usefulness (for a "low-tech" person
>>>> like me) than the intro here:
>>>> ?
>>>>> The examples with println debugging are not valid imho, because how do
>>>>> you know in advance what you will need to log in order to successfully
>>>>> debug some piece of code and such low-level information should not be
>>>>> captured in logs anyway.
>>>> Why are there several log levels?  Low-level info can be routed to
>>>> "DEBUG" or "TRACE".
>>>> As Ole put it quite eloquently, logging is a safety net that we hope
>>>> we'll never need, until we do.
>>>> Each layer of an application has its own notion of what is the
>>>> appropriate log level. What is "INFO" for some low-level library
>>>> will very probably not be so for most applications that use the
>>>> library.
>>>> Setting levels per package or class takes care of that: it's the
>>>> library's *user* who chooses what is useful in the current situation,
>>>> not the library's developer.
>>>> In the context of that asynchronous collaboration, the role of the
>>>> library's developer is to carefully choose what *could* be
>>>> interesting, if the need should arise.
>>>> So, can we eventually discuss the _technical_ arguments against
>>>> logging inside CM, rather than personal opinion?
>>> again, what I want to see is an example what *should* be logged in the
>>> case of an algorithm. Take the LevenbergMarquardtOptimizer as an
>>> example:
>>>  * what did you log using System.out.println()?
>>>  * the algo computes a lot of internal data, which of these is
>>> interesting for debugging problems or for general logging?
>>>  * there are various branches the algo can take, are just some
>>> interesting to log, or all of them?
>>> the use-cases presented so far were mainly about debugging specific
>>> problems, and I am *strongly* against adding logging information just
>>> for this purpose as you are clearly facing a dilemma here:
>>>  you have to log *everything* an algo does as otherwise you might miss
>>> the part that creates problems
>>>  but logging everything is not useful for a standard user of the library
>>> so it contradicts the original proposal to include logging
>>> Again, CM is not an application where you need to log what it is doing,
>>> but a bunch of algorithms and utility methods to perform certain
>>> calculations. I fail to see the need to add logging. What could be
>>> useful, and we had requests like that in the past, is to observe the
>>> state of a certain algorithm and to decide how to proceed in certain
>>> cases.
>>> That is useful for users.
>>> Another useful addition would be to add more aggressive assertions. If
>>> one user encounters a problem, he/she could run the application with
>>> assertions enabled and spot potential problems e.g. due to wrong input.
>>> Logging is a solution for a non existing problem imho.
>>> Logging will not avoid the need to debug CM in case of problems imho.
>> +1
>> The other thing I would add is that the one place where it does make
>> sense to dump text is in exception error messages, which is a place
>> where I think we could really improve things.  Fortunately, that is
>> fairly easily done.
>> I have seen nothing in this thread to convince me that adding
>> logging in [math] will be net positive for either those of us who
>> maintain the component or for users.  If we are not providing clear
>> exception error messages and/or APIs (with complete documentation)
>> so that users can understand what they need to debug their
>> applications, then we should focus on solving those problems.
>> Phil
> First, you carefully do not reply to any of the concrete arguments
> given in this thread, second you give a conclusion to an issue not
> reported in this thread: exceptions and logging do not provide the
> same service.
> At least, I'd wish that people sharing their own opinion (it's
> nothing more since _zero_ technical argument against logging have
> been put forth) stop taking the collective "users" on their side.
> As for *actual* users/maintainers, Ole and I have a need, while
> Thomas and you haven't.  Those are all the facts that exist until
> now.
> In such a situation, what do we do as a project; maintain the status
> quo, or try for a change?
> On numerous occasions over the years, the status quo was enforced;
> and I don't see that it benefited the project in terms of new
> contributors.
> So I'm +1 for trying to change, for a change.

I think one thing has been written in this thread that is worth
noting and could be an intermediate position.

It seems to me one place where we could get some useful information,
and provide it to users is for iterative algorithms (both optimizers
and solvers have already been mentioned, we could add ode integrators
as well to this). For such algorithms, having some way to monitor how
the iterations perform seem an improvement. An observer pattern as
proposed a few days ago for this kind of algorithms would be fine.
Once again, something simple and that does not attempt to be hyper
generic but rather taylored to the algorithms (i.e. most probably
different observer interfaces for different algorithms types).

This intermediate position would provide something to both users
and developers, and it would not attempt to log everything and
add a dependency (I am probably the one who opposed to logging on
the grounds of dependencies).

best regards,

> Gilles
>>> Thomas
>>>>>>>> My long-standing mentioning of slf4j was only because of
>>>>>>>> "weightlessness" (thanks to the no-op implementation of its
>>>>>>>> If "Log4j 2" has followed this path, good for everyone.
>>>>>>>> No objection, then?
>>>>>>> I'm still not clear what log4j 2 adds -- most Apache java projects
>>>>>>> seem to
>>>>>>> use log4j 1.2, seems to work well. -- H
>>>>>> I can only answer about "slf4j" where the "f" stands for facade:
>>>>>> "only"
>>>>>> an API, with bridges to several logging frameworks (log4j, logback,
>>>>>> etc.).
>>>>>> The separation of concerns (API vs one of several implementations
>>>>>> choose from)
>>>>>> allows the top-level application to uniformly configure logging or
>>>>>> disable it
>>>>>> completely (if choosing the "no-op" implementation).
>>>>> That is virtually true for all logging frameworks, including log4j,
>>>>> slf4j, commons-logging.
>>>> Has it always been true?
>>>> I'm certainly no expert; I only try to stay clear of tools about which
>>>> people complain a lot.  A few years ago, that was the case of jcl and
>>>> jul as compared to slf4j.
>>>> Gilles
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