Wednesday, December 28, 2011

A quick one: Testing log messages

I'm writing a talk on unit testing for work, and it reminded me of one of the coolest things I learned from the ZK code base with respect to testing: testing log messages.

You probably don't want to rely too heavily on log messages for testing, but sometimes it's the only indication you have that a certain condition happened. So how do you test it?

    Layout layout = Logger.getRootLogger().getAppender("CONSOLE")
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        WriterAppender appender = new WriterAppender(layout, os);
        Logger zlogger = Logger.getLogger("org.apache.zookeeper");

        try {
 } finally {

        LineNumberReader r = new LineNumberReader(new StringReader(os
        String line;
        Pattern p = Pattern.compile(".*Majority server found.*");
        boolean found = false;
        while ((line = r.readLine()) != null) {
            if (p.matcher(line).matches()) {
                found = true;
                "Majority server wasn't found while connected to r/o server", found);

From ReadOnlyModeTest. Kudos to Patrick Hunt (@phunt), the original author.

12/29 Edit:
Some folks have found this cringe-worthy. I agree. This is not a testing method that should be common in any code base, and for goodness sakes if you can find a different way to ensure something happened, do so. But there are a few times when this kind of test splits the difference between expedience and coverage (ie, I'd rather write a test to validate a log statement then just make the change and observe the log, or refactor the code base to expose some fact that the conditions causing the log were met in order to be able to test it).


  1. That seems....kind of terrifying? Unless you're specifically testing your logging.

  2. Well, you know, sometimes you have behavior that leaves no side-effect evident but a logging statement. Especially in library code. You want to make sure you're exercising the call path that ends in that logging statement, so this is a best last-ditch option.

  3. I wrote an Appender that would stop appending if it wrote more than X bytes in Y seconds, as a hack to prevent our disks from filling up. I tested it the same way. It was the only way I could think to isolate the test.

  4. huh -- I thought I commented on this at the time, I guess it was on Twitter. IMO in this situation it indicates that the condition should have some kind of service metric recorded, alongside the log message; and the presence/absence of that metric is an important thing to test.