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From "Yonik Seeley (JIRA)" <>
Subject [jira] Commented: (LUCENE-1482) Replace infoSteram by a logging framework (SLF4J)
Date Fri, 09 Jan 2009 19:05:59 GMT


Yonik Seeley commented on LUCENE-1482:

I'm not arguing for or against SLF4J at this point, but simply pointing out that I didn't
think it was appropriate to base any analysis on the NOP adapter, which can't be used for
any project already using SLF4J.

I think using a logger to replace the infostream stuff is probably acceptable.  What I personally
don't want to see happen is instrumentation creep/bloat, where debugging statements slowly
make their way all throughout Lucene.

bq. Because I seriously don't understand why you think that checking if debug is enabled can
pose any performance hit, even when used with a real logger.

I've tried to explain - these calls can be costly if used in the wrong place, esp on the wrong
processor architectures.  What appears in inner loop will vary widely by application, and
there are a *ton* of lucene users out there using it in all sorts of ways we can't imagine.
 For example, I'd rather not see debugging in Query/Weight/Scorer classes - for most applications,
query and weight construction won't be a bottleneck, but there are some where it could be
(running thousands of stored queries against each incoming document via memoryindex for example).

> Replace infoSteram by a logging framework (SLF4J)
> -------------------------------------------------
>                 Key: LUCENE-1482
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Shai Erera
>             Fix For: 2.4.1, 2.9
>         Attachments: LUCENE-1482-2.patch, LUCENE-1482.patch, slf4j-api-1.5.6.jar, slf4j-nop-1.5.6.jar
> Lucene makes use of infoStream to output messages in its indexing code only. For debugging
purposes, when the search application is run on the customer side, getting messages from other
code flows, like search, query parsing, analysis etc can be extremely useful.
> There are two main problems with infoStream today:
> 1. It is owned by IndexWriter, so if I want to add logging capabilities to other classes
I need to either expose an API or propagate infoStream to all classes (see for example DocumentsWriter,
which receives its infoStream instance from IndexWriter).
> 2. I can either turn debugging on or off, for the entire code.
> Introducing a logging framework can allow each class to control its logging independently,
and more importantly, allows the application to turn on logging for only specific areas in
the code (i.e., org.apache.lucene.index.*).
> I've investigated SLF4J (stands for Simple Logging Facade for Java) which is, as it names
states, a facade over different logging frameworks. As such, you can include the slf4j.jar
in your application, and it recognizes at deploy time what is the actual logging framework
you'd like to use. SLF4J comes with several adapters for Java logging, Log4j and others. If
you know your application uses Java logging, simply drop slf4j.jar and slf4j-jdk14.jar in
your classpath, and your logging statements will use Java logging underneath the covers.
> This makes the logging code very simple. For a class A the logger will be instantiated
like this:
> public class A {
>   private static final logger = LoggerFactory.getLogger(A.class);
> }
> And will later be used like this:
> public class A {
>   private static final logger = LoggerFactory.getLogger(A.class);
>   public void foo() {
>     if (logger.isDebugEnabled()) {
>       logger.debug("message");
>     }
>   }
> }
> That's all !
> Checking for isDebugEnabled is very quick, at least using the JDK14 adapter (but I assume
it's fast also over other logging frameworks).
> The important thing is, every class controls its own logger. Not all classes have to
output logging messages, and we can improve Lucene's logging gradually, w/o changing the API,
by adding more logging messages to interesting classes.
> I will submit a patch shortly

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