Intermittent log4net RollingFileAppender locked file issue

Question

We are seeing an intermittent issue on development and production machines whereby our log files are not getting logged to.

When running in development and debugging using Visual Studio we get the following log4net error messages in the VS output window:

log4net:ERROR [RollingFileAppender] Unable to acquire lock on file C:\folder\file.log.
The process cannot access the file 'C:\folder\file.log' because it is being used by another process.

log4net:ERROR XmlConfigurator: Failed to find configuration section 'log4net' in the application's .config file.
Check your .config file for the  and  elements.
The configuration section should look like:
<section
    name="log4net"
    type="log4net.Config.Log4NetConfigurationSectionHandler,log4net" />

Our current workaround for the issue is to rename the last log file. We would of course expect this to fail (due to the aforementioned file lock), but it normally doesn't. Once or twice the rename has failed due to a lock from the aspnet_wp.exe process.

Our log4net configuration section is shown below:

<log4net>
  <appender name="RollingLogFileAppender" type="log4net.Appender.RollingFileAppender">
    <file value="C:\folder\file.log"/>
    <appendToFile value="true" />
    <datePattern value="yyyyMMdd" />
    <rollingStyle value="Date" />
    <maximumFileSize value="10MB" />
    <maxSizeRollBackups value="100" />
    <layout type="log4net.Layout.PatternLayout">
      <header value="[Header]&#xA;"/>
      <footer value="[Footer]&#xA;"/>
      <conversionPattern value="%date %-5level %logger ${COMPUTERNAME} %property{UserHostAddress} [%property{SessionID}] - %message%newline"/>
    </layout>
  </appender>
  <root>
    <level value="INFO"/>
    <appender-ref ref="RollingLogFileAppender"/>
  </root>
</log4net>

As mentioned, we are seeing this intermittently on machines, but once the issue happens it persists.

Answer 1

Try adding

<lockingModel type="log4net.Appender.FileAppender+MinimalLock" />

to your <appender /> element. There is some performance impact because this means that log4net will lock the file, write to it, and unlock it for each write operation (as opposed to the default behavior, which acquires and holds onto the lock for a long time).

One implication of the default behavior is that if you're using it under a Web site that is being executed under multiple worker processes running on the same machine, each one will try to acquire and hold onto that lock indefinitely, and two of them are just going to lose. Changing the locking model to the minimal lock works around this issue.

(When debugging, ungraceful terminations and spinning up lots of new worker processes is exactly the type of thing that's likely to happen.)

Good luck!