How do you differentiate log4j sessions in a log file from copies of the same web-app?

Question

There is only one file. And it is written simultaneously as web app copies run.

How do you filter only one session log messages from other log lines?

Answer 1

You could set a context message including the identifier of the specific app instance using org.apache.log4j.NDC, like this:

String appInstanceId = "My App Instance 1";
org.apache.log4j.NDC.push(appInstanceId);
// handle request
org.apache.log4j.NDC.clear();

You can set up the context during the initialization of your web app instance, or inside the doPost() method of your servlets. As its name implies, you can also nest contexts within contexts with multiple push calls at different levels.

See the section "Nested Diagnostic Contexts" in the Log4J manual.

Answer 2

Here is a page that sets up an MDC filter for web-app -> http://rtner.de/software/MDCUserServletFilter.html

Being a servlet filter it will free you from managing MDC/NDC in each of your servlets.

Of course, you should modify it to save information more pertinent to your web-app.

Answer 3

Using a servlet filter with either NDC or MDC information is the best way I've seen. A quick comparison of the two is available at http://wiki.apache.org/logging-log4j/NDCvsMDC.

I've found MDC has worked better for me in the past. Remember that you'll need to update your log4j properties file to include whichever version you prefer (pattern definitions at http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/PatternLayout.html).

A full example of configuring MDC with a servlet filter is available at http://veerasundar.com/blog/2009/11/log4j-mdc-mapped-diagnostic-context-example-code/.

A slightly easier to configure, but significantly inferior option: You could opt to just print out the thread ID (via the properties file) for each request and make sure that the first thing you log about each request is a session identifier. It isn't as proper (or useful), but it can work for low-volume applications.

Answer 4

If you want to differentiate sessions in the same application then the MDC is the way to go. But if you want to differentiate the web applications writing to the same file, then MDC won't help because it works on a thread basis. In such case I used to make my own appender which knows which application instance it serves. This can be done through appender configuration properties. Such appender would stick application name into each logging event as a property before writing it into the media, and then you can use a layout to show this property value in the text file it writes to. Using MDC in such case won't work because every thread will have to MDC.put(applicationName) and that is quite ugly. MDC is only good for single process, not for several processes. If someone knows the other way, I'd like to hear.