January 20, 2011

State of the Union: Application Logging

Over the last week, I've been asked to help investigate some issues with a new obfuscation process that our builds will now go through before deployment (we can save the rants about obfuscating code for some other time). Per the norm for this activity, the test group ran into issues in a section of code and needed help investigating. As I dug into the issue, I turned to the log files and found that they were absolutely worthless. While we suspected that the obfuscation processor renamed or removed a class, we couldn't find the telltale signs of such an activity - no exceptions at all.

After an hour tracing the source code, I found what seems to be a common pattern in the code:

    // do something potentially dangerous
catch( Exception e )
    jobDone( 567, "bad request" );

What's wrong here? The code does prevent an exception from killing the long running server process and it does fulfill the component API by returning an error code. But what was the root cause of the error? This is a simple oversight that I see way too much. This code was written with the assumption that a message will be logged before throwing an exception but the code encountering an error isn't something written by us. It's the spring framework and that code doesn't know anything about our logging configuration. Instead of being able to point at the problem and fix it, I needed to add some logging code, spin another build and then rerun the test. We eventually saw the error (turns out the bean definitions no longer matched the byte code), but this highlighted some very real problems with logging in server side application development.

After years of trying to work with applications there seem to be some consistent problems that always occur. Here's a quick a dirty laundry list that I've seen over the years and I'm sure that I've contributed to this list more that I'd like to admit.

  • Never Contains What You Need

No matter how much we try, it seems that the one critical piece of information about the memory state is never logged out in enough detail. A lot of good developers go 95% of the way, but until we get those crystal balls delivered we're never going to know what would be useful when fighting fires.

  • Contain Too Much Of What You Don't Need

The knee jerk reaction to first point becomes that we just dump more and more information into the log files since we can't figure out what's going on. Things like "Entering method doSomething()", "Returning from method doSomething()" or "Read 7612 bytes of data from socket connection, haveMore = true" aren't very useful the vast majority of the time. Instead they provide a lot of clutter, which brings up the next point.

  • Needle In A Hay Stack

Once you migrate the code from a single dev desktop machine to a production server handling 100,000 concurrently, the verbosity of the logging system becomes your friememy. It's going to be useful but how the heck are you going to notice those really important messages that are keys to solving issues or, worse yet, knowing that you have a user who is having a bad product experience. I've recently read about some people using Hadoop to process their log messages due to the volume of data that's being produced.

  • Coarse Grained Controls

To help cope with the problems of too much data, we assign some coarse grained log levels to each message in the hope that we can suppress some things we want to see on low volume systems but not on high volume production boxes. To me, this doesn't make sense. The additional information about state is a debug level message because when I'm debugging an error I'd like to see it. So do I log this at error level or at debug level? And if I'm logging at error level, what about the debug messages that occurred before? Were they generated on the way to getting into an error state or for some other request? Simple levels like debug, info and error aren't expressive enough to help.

  • Still Prone To Human Mistakes

Finally, the whole process is still very human driven. Each framework requires a developer to come up with the log message, provide the relevant information and then invoke the logging process when it's necessary. If the developer forgets or some additional logging is needed to troubleshoot a problem? You're out of luck and you'll need to change the code and go through another deployment.

In the last 10 years, the industry has made great strides improving web frameworks, scalable data access solutions and large volume data processing but we haven't moved very far in terms of helping ourselves with better logging. What we need is a logging framework that provides a rich declarative structure for defining some more complex logging rules and isn't dependent on developers to remember to invoke it throughout their code. Stay tuned for some random thoughts on what I think that might look like in practice.

Tags: logging architecture idea
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Dan Hable