Bug 28647 - Add "Flush on Level" capability to FileAppender
Summary: Add "Flush on Level" capability to FileAppender
Alias: None
Product: Log4j - Now in Jira
Classification: Unclassified
Component: Appender (show other bugs)
Version: 1.2
Hardware: All All
: P5 enhancement
Target Milestone: ---
Assignee: log4j-dev
Depends on:
Reported: 2004-04-28 03:20 UTC by Simon Dorrat
Modified: 2009-10-10 19:54 UTC (History)
1 user (show)

Performance test results (2.47 KB, text/plain)
2004-05-13 00:12 UTC, Simon Dorrat
Java test harness (3.63 KB, text/plain)
2004-05-13 00:15 UTC, Simon Dorrat

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Description Simon Dorrat 2004-04-28 03:20:14 UTC
New Feature Requested:
I have used logging extensively for many years and tend to have my programs 
log alot of information.  When writing to a file this can cause a performance 
impact due to very frequent disk I/O.  So my own logging libraries I used (in 
C, VB, PL/SQL) always had the ability to configure the level that the log text 
was flushed to disk.  

I have now extended FileAppender to create this feature.  It simply allows you 
to set the level that flushing occurs.  Whenever a log event is logged for 
that level or higher, a flush occurs.  i.e. if the FlushLevel is INFO, then 
when an INFO, WARN, ERROR or FATAL event is logged the stream will be flushed 
to disk. DEBUG messages will accumulate and not flush until the next INFO 
event is logged.

-The performance improvement allows for more detailed logging levels to be 
left permanently on, or to have less impact when they are on. 
- The logging style in most programs (certainly all mine) finish each major 
processing step logging an event at a level higher than (or at least equal to) 
the previously logged events.  Eg an operation "createOrder" may have a number 
of INFO and DEBUG msgs (and soon TRACE!) but would finish on a INFO msg 
like "Successfully created order for 1000 ORCL...".  This pattern guarantees 
that flush of all log event has occurred when the operation finishes, rather 
than waiting for some arbitrary point like a 8K buffer being filled.
-The one downside is that if a program crashes, then any final logged events 
below the threshold that occurred after the last threhold level will not 
appear in the file.  This is normally a small price to pay, as you just then 
lower the flush level and rerun so you get all events flushed.  (This assumes 
the error is repeatable, but with no flushing feature, the lower level events 
wouldnt have been turned on due to the performance hit so you are no worse off 
even if it is not).
NOTE: I have implemented a shutdown hook to address the last issue - i.e. 
automatically flush on VM exit, but that is a Java 1.3 method so cannot be 
used in Log4j as I understand it.

Current Status:
I have written a new appender "LevelFlushedFileAppender" that extends 
FileAppender that does this job.  This works fine and I am happy to submit 
it.  However I think that it would be better to alter the FileAppender itself 
and add the code directly to this, so it could be inherited in all classes 
extending FileAppender.  I am happy to implement it like this also.

Let me know if the feature is accepted or you want further information, and 
what I should do going forward.
Comment 1 Ceki Gulcu 2004-05-05 09:09:57 UTC
Hi Simon,

Have you measured the performance improvements?

I'd be interested in the results with 5 threads performing simultaneous 
logging, of which 80% DEBUGS and 20% INFO. Running the tests on both Windows 
and Linux would be even better.
Comment 2 Simon Dorrat 2004-05-06 02:10:20 UTC
Performance tests I did previously on Windows XP showed the new method was up 
to 20 times faster.  However this was with 1000 writes per flush.  I will 
retest with the ratio you suggested (much more realistic) and more than 1 
Comment 3 Paul Smith 2004-05-06 02:19:34 UTC
This is where having Appender being an interface would be ideal, because then 
one could 'decorate' one appender with another object to provide support code 
such as a flushing function.  Sort of like how the InputStream stuff works.

Lets assume Appender is an interface.  One could then do something like this in 
java code.  Configuration code of course might be more complex to achieve this, 
but certainly possible:

 Appender a = new FlushingAppender(new FileAppender(.....));

In the above example the FlushingAppender implements the Appender interface, 
but for the most part delegates to the internal (file)appender, except for the 
actual append methods where it might buffer events based on some strategy of 
the implementors choosing.  That way we could mix and match strategies.

This is how AsyncAppender kinda works, but I think having Appender as an 
interface is neater.  Large change though.

Comment 4 Simon Dorrat 2004-05-06 03:32:03 UTC

Appender is an interface!

Would this able to be configured via Property files/XML config files?  (Is 
this what you referred to as "more complex config code"?).  Using a decorator 
seems like a useful approach, as it would not only allow many such features to 
be mixed and matched, but also potentially allow to be applied to any 
appender. However since this LevelFlushedFileAppender flushes by calling the 
qw.flush() from the WriterAppender class, it could only be applied to classes 
who are decendants of that class.  To apply to other appenders would require a 
change to the interface and each appender class to support a "flush" method, 
which seems to defeat the purpose of a decorator pattern.

What do you think?
Comment 5 Paul Smith 2004-05-06 03:41:50 UTC
You are of course correct... When I looked in the Hierachy view within Eclipse, 
I keep seeing AppenderSkeleton, and forget that there is actually an interface.

Silly me, particularly as a log4j-dev member... 8-|

Anyway. I was thinking that the appender that is wrapped need not have extend 
from WriterAppender.  The Wrapping appender (FlushAppender in our example) is 
free to determine when to 'flush', which really means 'take any currently 
buffered events and send them to the wrapped appender'.  

This is more a more generic flushing solution but kind of duplicates the work 
that WriterAppender would do.  I guess you could have a FlushingWriterAppender 
wrapper that is only designed to wrap a WriterAppender only, and rather than 
store it's own buffer of events, always writes the event to the internal 
WriterAppender, but waits until the internal strategy determines that a flush 
should occur, and only at that point calls writerAppender.flush().

Does that make sense?  
Comment 6 Simon Dorrat 2004-05-06 04:24:29 UTC
It does make sense.  In the first approach however (wrapper buffering events 
then sending them all at once to the wrapped appender upon flush), you lose 
most of the advantage of buffering.  This is because the wrapped appender will 
still execute all events one by one when 'flushed'.  I.e for a file writer it 
would still be still be one disk i/o per event.

The second strategy was the one I implemented (though via inheritance rather 
than as a decorator). Not convinced either way yet.  Will get the performance 
testing done tonight or tommorow on the current implementation.
Comment 7 Paul Smith 2004-05-06 04:32:30 UTC
Yup, this is where the (smarter) FlushingWriterAppender would not actually 
buffer any events, just write them to the wrapped WriterAppender and 
periodically call writerAppender.flush() as it deems appropriate.

So via the decorator approach, you could have a generic flushing appender (akin 
really to the Async appender), and one optimised to dealing with 

With the decorator approache you could then do something like:

Appender a = new FlushingAppender(new FlushingWriterAppender(new WriterAppender
(...), 50), 500);

Lets say that in this example:

* FlushingAppender: 500 = # ms to buffer events before flushing to it's 
internal appender (FlushingWriterAppender)

* FlushingWriterAppender: 50= # of event objects to write before calling 

Having said all of this though, one does need to think about the risks of some 
events not getting written.  While you may have a JDK hook to listen for a 
shutdown, some applications  won't always exit quite so gracefully.  And this 
is usually when you MOST want to know what was going on......
Comment 8 Simon Dorrat 2004-05-06 06:16:17 UTC
This is why I use the LevelFlushed approach (i.e Flush on level equal to or 
higher than x).  If the error is trapped prior to shutdown, then the log will 
be flushed when an ERROR event is logged.  If the error is not trapped, then 
some low level messages will not be flushed.  But with the existing approach, 
you wouldnt have low level logging enabled anyway (due to performance hit), so 
no difference.  Thats the rationale behind this.  It enables you to leave a 
lower level on permanently than you otherwise would.  So you get the advantage 
of seeing the low level info in all cases bar catastrophic failure.
Comment 9 Scott Deboy 2004-05-06 17:30:53 UTC
Would it make sense to use something like the new ExpressionFilter in this case
instead of only supporting the triggering based on Level?
Comment 10 Simon Dorrat 2004-05-06 23:33:39 UTC
Not sure what the ExpressionFilter is.  Where can I find info on it? I could 
certainly imagine that there could be a number of different approaches to 
triggering the flush that people may want to take, so a flexible framework has 
Comment 11 Scott Deboy 2004-05-07 03:45:58 UTC
See ExpressionFilter here:

And ExpressionRule here:

The ExpressionFilter is an example of how an ExpressionRule can be used to allow
or prevent events from being processed by an appender (and ExpressionRule could
be used similarly in a flush trigger).

Here is an example of how an ExpressionFilter is used to filter events:

   <appender name="STDOUT" class="org.apache.log4j.ConsoleAppender">
      <layout class="org.apache.log4j.SimpleLayout"/>
        <filter class="org.apache.log4j.varia.ExpressionFilter">
          <param name="Expression" value="level > INFO" />
          <param name="AcceptOnMatch" value="true"/>
          <param name="ConvertInFixToPostFix" value="true"/>
        <filter class="org.apache.log4j.varia.DenyAllFilter"/>
Comment 12 Simon Dorrat 2004-05-13 00:12:12 UTC
Created attachment 11527 [details]
Performance test results
Comment 13 Simon Dorrat 2004-05-13 00:15:26 UTC
Created attachment 11528 [details]
Java test harness
Comment 14 Simon Dorrat 2004-05-13 00:16:01 UTC
Performance test results attached as requested.  Also Java test harness. 
Improvement roughly 25%.  System load and differing threading models and disk 
caching between XP and Linux made individual tests hard to compare, but it 
certainly showed a consistent performance improvement.
Comment 15 olivier dupuy 2006-08-23 13:40:08 UTC
Hi all,

I totally aggree with the flushing being tied to the level of the event. This 
totally makes sense and would be good to have it in the default log4J.

As Simon, I also use a shutdown hook in my application to address the issue - 
i.e. automatically flush on VM exit.

The only way I found to flush was to close all the appenders. This is fine as I 
am stopping the application but a way to flush globally all the logs without 
closing the appenders in a very simple way should also be there.
Let's imagine I want to flush all the logs using some admin interface to 
examine them but I don't want to close them even to reopen seconds later (to 
not loose events and getting error messages everywhere while they are created). 
What are my options in this case????
So flushing should be present in each appender that could delegate to sub 
appenders if required.

Olivier DUPUY
Comment 16 Elias Ross 2007-01-29 11:55:34 UTC
I think what should be supported is to enhance WriterAppender so that subclasses
can specify the flushing behavior.  I don't think it's generally a good idea to
code in the policy itself, but it seems fair to allow users to easily extend
existing classes to get the behavior they want.

I thought of adding an Interface, such as "FlushPolicy" but it seemed like overkill.

I propose the following change:
Index: C:/src/workspace/log4j/src/java/org/apache/log4j/WriterAppender.java
--- C:/src/workspace/log4j/src/java/org/apache/log4j/WriterAppender.java
(revision 500611)
+++ C:/src/workspace/log4j/src/java/org/apache/log4j/WriterAppender.java
(working copy)
@@ -372,7 +372,7 @@
-      if (this.immediateFlush) {
+      if (shouldFlush(event)) {
@@ -426,6 +426,15 @@
+  /**
+   * Returns true if an event should be flushed immediately.
+   * This method returns the value of {@link #immediateFlush}.
+   * Subclasses override this behavior to flush only when certain events occur.
+   */
+  protected boolean shouldFlush(LoggingEvent event) {
+    return this.immediateFlush;
+  }


In response to #15, adding a separate API to "flush" appenders really is a
separate issue that should be raised in a different bug.
Comment 17 Curt Arnold 2007-08-22 20:25:32 UTC
*** Bug 43061 has been marked as a duplicate of this bug. ***
Comment 18 olivier dupuy 2009-03-05 08:58:37 UTC
I really hope that you will integrate this in 2.0 as in the comment to my RFE below.
See my follow up comments for a general way to flush whatever the type of logger or appender.


This is really something important for production systems to have a decent flush mechanism with no performance penalty associated to an ongoing flushing.
Comment 19 Mark Thomas 2009-06-10 13:21:40 UTC
Reset assignee
Comment 20 Curt Arnold 2009-10-10 19:54:37 UTC
Committed Elias's recommendation in rev 824003.