Great Programmers Write Debuggable Code

All programs need some form of logging built in to them, so we can observe what it is doing. This is especially important when things go wrong. One of the differences between a great programmer and a bad programmer is that a great programmer adds logging and tools that make it easy to debug the program when things fail.

When the program works as expected, there is often no difference in the quality of the logging. However, as soon as the program fails, or you get the wrong result, you can almost immediately tell the good programmers from the bad.


For example, a tester came to me with a call case that didn’t work. We looked at the logs, and saw that the problem seemed to be in a neighboring module. A call to the other module to get a list of values simply returned null. When we enabled logging in the neighboring module and re-ran the test case, there was no more information available. No clues to why null was returned – did we supply the wrong arguments, did some external system fail, was there an error in the neighboring module, or what?

When asking the developer responsible for the code, the answer we got was: “Oh, then we have to make a debug version to see what happens”. Fail! It must be possible to get some sense of where the problem is just from the logs. If this had been in a production system, adding a debug version would have been a lot of work. The code needs to include enough information in the log statements so you at least have some idea of why it fails.


One of our products at work finds the cheapest route for the delivery of SMS:s (text messages) to mobile phones. Depending on the current location of the phone, and the operator the destination subscriber belongs to, there are many possible routes to choose from, each with a given cost and other characteristics.  In addition, there can be exceptions that forbid some routes, and promote other routes. There are typically many thousands of routes defined. The system finds the cheapest route in each case, given the constraints, and delivers the SMS.

Now, suppose a certain SMS gets delivered using route A, but we believe it should have used route B. Why was route A chosen? If there is no logging information (other than “route A was chosen”), we are left with hundreds of possible routes, their costs, the exceptions, and a complex algorithm. Good luck figuring out why A was chosen.

In our implementation, the log includes all potential routes, in cost order. As routes are eliminated due to different restrictions, the eliminated routes are listed in the log, together with the reason for elimination. With all the information on the input to the algorithm, as well as the steps taken, listed in the log, it is fairly easy to see why a certain route was picked.


So, why don’t all programmers write debuggable code? I can think of three reasons:

1.      You have to be humble enough to realize that there will be cases when your code doesn’t work as expected. I believe many programmers have a hard time with this.

2.      If you test your own code thoroughly, you will make sure it works (or fails gracefully) in a lot of different scenarios. For each scenario, it is natural to add logging. If you don’t test those cases, you are less likely to add logging there.

3.      Many programmers don’t trouble-shoot their own code in production systems often enough. If you have a problem in a live system, and the logs tell you nothing about why there is a problem, you have a strong incentive to add logging that will actually help you the next time you are in a similar situation.

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