Logging

Logging is not really a hard concept. It's just a useful tool to be aware of. You have a project in mind, you have made the first prototype, and you are now ready to remove all of the print statements. But wait! You can do something else. Use logging! Instead of printing all of your checks and what nots to the terminal with commands like println!(), you can use a logging crate, like the defacto standard, tracing, to print messages to a log file, which you can inspect once your program is done running. In some cases this should always be running allowing you to inspect the system after the fact. Logging also has some performance side effects. First of all, anytime you print to the terminal, it involves a system call. As you might recall, anytime we interact with the operating system, like printing to a terminal, it is likely to be costly in terms of performance. If you have ever timed a loop before and after putting in print statements, you'll know what I mean.

If the logging library has been implemented correctly (please don't implement it yourself), it will be keeping most messages in memory, and then once in a while writing a large chunk of text to the output file. Preferably it should do this on another thread to not hold up the rest of the program. If the logging crate is working correctly it should flush all remaining lines in memory to the file once it is about to be dropped/go out of scope/be destroyed.

Levels of logging. Image credit

Logging usually happens at various levels. Levels like debug, warning and error allow you to filter which messages are presented at runtime, which are written to the log file and, when inspecting the log file, you can filter which messages you actually care about.

As an example, at runtime, you might want the logger to only print critical errors to the terminal or to some other user facing display mechanism you have put in place, but once your application is done, you might want to clean up your program by checking that there are of course no errors, but fixing all the code that results in warnings. Logging is definitely a useful tool in most cases, and the performance implications are likely to be neglible, but just like printing, unless you really need it, you probably shouldn't be calling it in a loop by default.