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From "Ebersole, Steven" <steven.ebers...@vignette.com>
Subject RE: NDC , Servlets and thread safety
Date Wed, 06 Nov 2002 23:26:27 GMT
Actually we ran into this same issue on our weblogic deployment.  It is
because requests are handled by threads retrieved from a thread pool.

However, the real reason for this "bleed" was that NDC.pop() was not always
being called.  Make sure NDC.pop() and NDC.remove() are done in a finally
blocks in your code.


    |-----Original Message-----
    |From: Ceki Gülcü [mailto:ceki@qos.ch]
    |Sent: Wednesday, November 06, 2002 10:36 AM
    |To: Log4J Users List
    |Subject: Re: NDC , Servlets and thread safety
    |At 16:23 06.11.2002 +0000, you wrote:
    |>This may have been asked before but I cant find it in the 
    |archives. I am
    |>trying to use the log4j NDC class to log and distinguish 
    |between multiple
    |>client requests coming into a single servlet instance (we 
    |are using the
    |>frontcontroller pattern). We issue a NDC.push() at the 
    |start of the doPost
    |>method passing in the session ID and a NDC.pop() at the 
    |end of the doPost()
    |>As the servlet operates in the same thread and doPost() 
    |is not synchronized
    |>isn't it quite possible that two users could get into the 
    |same method and
    |>effectively push() two lots of information into the NDC 
    |so that the output
    |>from log4j contains session ids from BOTH of the clients.
    |That is not possible. The NDC is managed on a per thread 
    |basis. Different 
    |threads have different NDCs.
    |>Apologies if I'm being stupid but if this is a problem has anyone
    |>encountered this before and have an elegant solution?
    |There can't be a solution to a non-existent problem.
    |>By the way we don't really want to make the servlet implement the
    |>SingleThreadModel or synchronize the doGet, doPost 
    |methods as we believe
    |>this would kill the application.
    |You do not need to use SingleThreadModel or synchronize 
    |the doGet, doPost 
    |However, you should make sure to call NDC.remove from time 
    |to time. Also 
    |check out the MDC class.
    |BTW, please do not cross post to log4j-user *and* 
    |log4j-dev. The next time 
    |I won't be answering.
    |>Thanks in advance of any comments
    |>Graham Mead
    |TCP implementations will follow a general principle of 
    |robustness: be
    |conservative in what you do, be liberal in what you accept from
    |others. -- Jon Postel, RFC 793
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