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List:       logback-dev
Subject:    [logback-dev] [JIRA] Commented: (LBCLASSIC-272) Automatic MDC
From:       Lóránt_Pintér_(JIRA) <noreply-jira () qos ! ch>
Date:       2011-05-31 14:15:51
Message-ID: 1017000282.1306851351383.JavaMail.ceki () pixie
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    [ http://jira.qos.ch/browse/LBCLASSIC-272?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12023#action_12023 \
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Lóránt Pintér commented on LBCLASSIC-272:
-----------------------------------------

Apparently I was under the wrong impression that Log4J had no MDC inheritance. Alas, \
this is still a problem.

> Automatic MDC inheritance with thread pools cases false data printed in the log
> -------------------------------------------------------------------------------
> 
> Key: LBCLASSIC-272
> URL: http://jira.qos.ch/browse/LBCLASSIC-272
> Project: logback-classic
> Issue Type: Bug
> Components: Other
> Affects Versions: 0.9.27
> Environment: Applies to all Logback versions, specifically tested with Logback \
>                 0.9.27.
> Reporter: Lóránt Pintér
> Assignee: Logback dev list
> Attachments: mdc-test.zip
> 
> 
> MDC inheritance when used in conjunction with a thread pool can turn out to be \
> malicious. Consider the following in a web server where each line in the log \
> contains the current user's name. 1. A web request comes in and processing is \
> started on a thread 2. Authenticate the user (on original thread)
> 3. Put the user name into the MDC (on original thread)
> 4. Create some Runnable and pass it to a thread pool to be run in the background \
> (on thread pool thread) Let's take a look at the logs that are printed from the \
> Runnable created in step 4. Without automatic MDC inheritance (e.g. when using \
> Log4J) these log lines will contain no user name. This can be good or bad, \
> depending on what you want. If you do want the user name in these log lines, you \
> can explicitly transfer the MDC from the originating thread to the thread pool \
> thread. If you don't want to print the user name for background tasks, you are \
> already done. If you have no access to the thread pool or the Runnables, you simply \
> get no MDC information. However, if you do have automatic MDC inheritance (as with \
> Logback), the log lines *will* contain a user name, but not from the originating \
> thread. The thread pool thread will instead inherit its MDC from the first thread \
> that executed a Runnable on it. This can never be what you want. You can correct \
> this by transferring the original MDC manually, or clearing it explicitly inside \
> your Runnable. But if you forget to do this, or you don't have access to the thread \
> pool and the Runnables, you will end up with corrupt data in your logs. The latter \
> can happen if you are using some third-party API that maintains its own thread \
> pool. I can see two ways to fix this issue:
> 1) Make automatic MDC inhertance configurable from logback.xml. This is not a \
> perfect solution: if you mix code that relies on MDC inheritance with code that \
> relies on no MDC inheritance you will have no good value for this setting. 2) \
> Remove the automatic MDC inheritance feature altogether. Instead, Logback (or even \
> better: SLF4J) could provide some utility classes to wrap Runnable's and Callable's \
> in something like this: {code}
> class MdcRetainingRunnable implements Runnable {
> private final Map context;
> public MdcRetainingRunnable {
> this.context = MDC.getCopyOfContextMap();
> }
> @Override
> public void run() {
> Map originalContext = MDC.getCopyOfContextMap();
> MDC.setContextMap(context);
> try {
> runWithMdc();
> } finally {
> MDC.setContextMap(originalContext);
> }
> }
> abstract protected runWithMdc();
> }
> {code}
> I would prefer solution #2 as it provides for the rule of least astonishment.

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