[logback-dev] reducing resource contention in logback

[Ceki Gulcu]

Hello all,

Ralph and I had a lengthy chat discussing his
contribution in LBCLASSIC-35

   http://jira.qos.ch/browse/LBCLASSIC-35

For the public record, here is a brief summary of the exchange.

His premise is that by carefully modifying synchronization in the
AppenderBase and Logger classes we can significantly reduce contention
among various threads using logback. The changes must preserve logging
event atomicity. For example, if event0 consist of a message m0 and
exception e0, and event1 consist of message m1, when an appender
writes event0 and event1, possibly sent by different threads, the
output must never be m0,m1,e0 but always m0,e0,m1 or alternatively
m1,m0,e0. In other words, event intermingling is disallowed even if
event ordering is not guaranteed.

Raph modified the Logger class so that iterating on the appenders is
done without locking. He currently uses ConcurrentHashMap. I think
that CopyOnWriteArrayList would also work fine and result in simpler
code.

Ralph has also modified the doAppend method in AppenderBase pushing
the synchronization point to within each derived appender class. Thus
it becomes the responsibility of each appender to manage its own
synchronization.

These changes open the way for threads which support concurrent calls
to be invoked without any contention by multiple threads. Of course,
the underlying resource, where the logging output is eventually
written to, still remains as a contention point.

Cheers,

-- 
Ceki Gülcü
Logback: The reliable, generic, fast and flexible logging framework for Java.
http://logback.qos.ch