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From "Dominik Psenner" <dpsen...@gmail.com>
Subject AW: AppenderSkeletion lock in DoAppend
Date Wed, 04 Mar 2015 14:51:45 GMT
A log event might be badly programmed and recursive by definition. Think of two objects that
try each to log while trying to format each other:

 

A {

DoWork() {

   Log(“A: {0}”, B)

}

}

B {

DoWork () {

   Log(“B: {0}”, A)

}

}

 

I might be wrong, but something like that does the recursive guard detect and resolve.

 

For the other question you’ll have to accept my apologies because I can’t answer the question
„how to write code“. :)

 

Von: Gert Kello [mailto:gert.kello@gmail.com] 
Gesendet: Mittwoch, 04. März 2015 15:16
An: Log4NET User
Betreff: Re: AppenderSkeletion lock in DoAppend

 

But what about this m_recursiveGuard? Is the recursion protection needed? Perhaps it is, if
log saving code logs something by itself? (Like warning that database operation takes longer
than expected)?

Well, as I plan to make the database saving asyn I need to tackle this problem anyway... I
think I would like to log the performance problem. But not when the problem occurs during
saving log about the same performance problem. Is there any guideline how to write such code?
Something like using OnlyOnceErrorHandler or is there anything similar for warnings?

 

Gert

 

On 4 March 2015 at 15:38, Dominik Psenner <dpsenner@gmail.com <mailto:dpsenner@gmail.com>
> wrote:

Hi,

 

from what I can recall it could happen that the formatted representation of a logging event
gets mixed up with data from other events and thus the characters streamed to a sink become
garbage (i.e. a file, console, ..) if this lock is removed.

 

Internally a lot of things are cached to improve performance and these caches actually require
proper locking.

 

But my memory might be wrong, so feel free to remove the lock and let several threads log
events to your appender to see if it can handle it properly. :)

 

Cheers

 

Von: Gert Kello [mailto:gert.kello@gmail.com <mailto:gert.kello@gmail.com> ] 
Gesendet: Mittwoch, 04. März 2015 09:28
An: log4net-user@logging.apache.org <mailto:log4net-user@logging.apache.org> 
Betreff: AppenderSkeletion lock in DoAppend

 

Hi.

I'm trying to create a database appender which high throughput... I looked at code in AppenderSekeletion.DoAppend()
method and saw following comment:

    public void DoAppend(LoggingEvent loggingEvent)
    {
      // This lock is absolutely critical for correct formatting
      // of the message in a multi-threaded environment.  Without
      // this, the message may be broken up into elements from
      // multiple thread contexts (like get the wrong thread ID).

      lock (this)
      {

I would like to remove this lock from my code but there's a couple of issues I do not understand:

1. As I do not know the internals of log4net well enough I do not understand why lock is important
for message formatting? Is it because the Layout.Format is not supposed to be thread safe?
Or is it because the same instance of m_renderWriter could be used by multiple threads (well,
usage of m_renderWriter is protected by another lock, added later. But AFAIK the RenderLoggingEvent(LoggingEvent
loggingEvent) is still not 100% thread safe)

2. The comment does not mention that lock is crucial for m_recursiveGuard to work correctly.
That's my main complaint: I almost overlooked the issue of potentially skipped logging events.

3. What about implementations of IFilter.Decide? Are those (supposed to be) thread safe?

Gert

 


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