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From c...@apache.org
Subject cvs commit: jakarta-log4j/src/java/org/apache/log4j concurrency.txt
Date Thu, 10 Jul 2003 08:51:53 GMT
ceki        2003/07/10 01:51:53

  Added:       src/java/org/apache/log4j concurrency.txt
  Just some thoughts on concurrency. Wecan remove this file later on.
  Revision  Changes    Path
  1.1                  jakarta-log4j/src/java/org/apache/log4j/concurrency.txt
  Index: concurrency.txt
  This document describes the concurrency model used in some of the log4j core classes such
as Logger (i.e. Category) and Hierarchy.
  Concurrency primitives in these classes must be studied carefully because concurrency problems
are usually hard to reproduce. 
  Moreover, since log4j is a tool to diagnose problems, the correctness of the diagnosis tool
must be guaranteed. 
  We must also recognize that the data in Logger and Hierarchy are read frequently and written
to rarely. Up to and including log4j 1.2, 
  we did not differentiate between read and write operations. Instead, a coarse object-wide
locks were used which treated reads the same 
  as writes. This serializes some logging operations. A better approach would be to allow
simultaneous reads but exclusive writes.
  For this purpose log4j 1.3 includes a ReaderWriterLock that allow simultaneous reads but
exclusive writes. When readers and writers 
  compete for the lock, preference is given to writers. See o.a.l.helpers.ReaderWriterLock.java
for source code which is remarkably simple.
  The ReaderWriterLock is not reentrant. Thus, we must make sure that a writer owning the
lock does try to reacquire the lock because 
  any such attempt would result in a deadlock. Similarly, it is not safe for a reader owning
the lock to try to reacquire it. This
  condition can be verified only if few simple instructions are allowed between the line acquiring
the lock and the line releasing it. 
  If the code inside a protected block is complex, then either it is wrong or our model needs
to be changed.

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