Eradicating Non-Determinism in Tests
An automated regression suite can play a vital role on a software project, valuable both for reducing defects in production and essential for evolutionary design. In talking with development teams I've often heard about the problem of non-deterministic tests - tests that sometimes pass and sometimes fail. Left uncontrolled, non-deterministic tests can completely destroy the value of an automated regression suite. In this article I outline how to deal with non-deterministic tests. Initially quarantine helps to reduce their damage to other tests, but you still have to fix them soon. Therefore I discuss treatments for the common causes for non-determinism: lack of isolation, asynchronous behavior, remote services, time, and resource leaks. I've enjoyed watching ThoughtWorks tackle many difficult enterprise applications, bringing successful deliveries to many clients who have rarely seen success. Our experiences have been a great demonstration that agile methods, deeply controversial and distrusted when we wrote the manifesto a decade ago, can be used successfully. There are many flavors of agile development out there, but in what we do there is a central role for automated testing. Automated testing was a core approach to Extreme Programming from the beginning, and that philosophy has been the biggest inspiration to our agile work. So we've gained a lot of experience in using automated testing as a core part of software development. Automated testing can look easy when presented in a text book. And indeed the basic ideas are really quite simple. But in the pressure-cooker of a delivery project, trials come up that are often not given much attention in texts. As I know too well, authors have a habit of skimming over many details in order to get a core point across. In my conversations with our delivery teams, one recurring problem that we've run into is tests which have become unreliable, so unreliable that people don't pay much attention to whether they pass or fail. A primary cause of this unreliability is that some tests have become non-deterministic. A test is non-deterministic when it passes sometimes and fails sometimes, without any noticeable change in the code, tests, or environment. Such tests fail, then you re-run them and they pass. Test failures for such tests are seemingly random. Non-determinism can plague any kind of test, but it's particularly prone to affect tests with a broad scope, such as acceptance or functional tests. Why non-deterministic tests are a problem Non-deterministic tests have two problems, firstly they are useless, secondly they are a virulent infection that can completely ruin your entire test suite. As a result they need to be dealt with as soon as you can, before your entire deployment pipeline is compromised. I'll start with expanding on their uselessness. The primary benefit of having automated tests is that they provide bug detection mechanism by acting as regression tests[1]. When a regression test goes red, you know you've got an immediate problem, often because a bug has crept into the system without you realizing. Having such a bug detector has huge benefits. Most obviously it means that you can find and fix bugs just after they are introduced. Not just does this give you the warm fuzzies because you kill bugs quickly, it also makes it easier to remove them since you know the bug got in with the last set of changes that are fresh in your mind. As a result you know where to look for the bug, which is more than half the battle in squashing it. The second level of benefit is that as you gain confidence in your bug detector, you gain the courage to make big changes knowing that when you goof, the bug detector will go off and you can fix the mistake quickly. [2] Without this teams are frightened to make the changes code needs in order to be kept clean, which leads to a rotting of the code base and plummeting development speed. The trouble with non-deterministic tests is that when they go red, you have no idea whether its due to a bug, or just part of the non-deterministic behavior. Usually with these tests a non-deterministic failure is relatively common, so you end up shrugging your shoulders when these tests go red. Once you start ignoring a regression test failure, then that test is useless and you might as well throw it away. Indeed you really ought to throw a non-deterministic test away, since if you don't it has an infectious quality. If you have a suite of 100 tests with 10 non-deterministic tests in them, than that suite will often fail. Initially people will look at the failure report and notice that the failures are in non-deterministic tests, but soon they'll lose the discipline to do that. Once that discipline is lost, then a failure in the healthy deterministic tests will get ignored too. At that point you've lots the whole game and might as well get rid of all the tests. Quarantine My principal aim in this article is to outline common cases of non-deterministic tests and how to eliminate the non-determinism. But before I get there I offer one piece of essential advice: quarantine your non-deterministic tests. If you have non-deterministic tests keep them in a different test suite to your healthy tests. That way you'll you can continue to pay attention to what's going on with your healthy tests and get good feedback from them. Place any non-deterministic test in a quarantined area. (But fix quarantined tests quickly.) Then the question is what to do with the quarantined test suites. They are useless as regression tests, but they do have a future as work items for cleaning up. You should not abandon such tests, since any tests you have in quarantine are not helping you with your regression coverage. A danger here is that tests keep getting thrown into quarantine and forgotten, which means your bug detection system is eroding. As a result it's worthwhile to have a mechanism that ensures that tests don't stay in quarantine too long. I've come across various ways to do this. One is a simple numeric limit: e.g. only allow 8 tests in quarantine. Once you hit the limit you must spend time to clear all the tests out. This has the advantage of batching up your test-cleaning if that's how you like to do things. Another route is to put a time limit on how long a test may be in quarantine, such as no longer than a week. The general approach with quarantine is to take the quarantined tests out of the main deployment pipeline so that you still get your regular build process. However a good team can be more aggressive. Our Mingle team puts its quarantine suite into the deployment pipeline one stage after its healthy tests. That way it can get the feedback from the healthy tests but is also forced to ensure that it sorts out the quarantined tests quickly. [3] Lack of Isolation In order to get tests to run reliably, you must have clear control over the environment in which they run, so you have a well-known state at the beginning of the test. If one test creates some data in the database and leaves it lying around, it can corrupt the run of another test which may rely on a different database state. Therefore I find it's really important to focus on keeping tests isolated. Properly isolated tests can be run in any sequence. As you get to larger operational scope of functional tests, it gets progressively harder to keep tests isolated. When you are tracking down a non-determinism, lack of isolation is a common and frustrating cause. Keep your tests isolated from each other, so that execution of one test will not affect any others. There are a couple of ways to get isolation - either always rebuild your starting state from scratch, or ensure that each test cleans up properly after itself. In general I prefer the former, as it's often easier - and in particular easier to find the source of a problem. If a test fails because it didn't build up the initial state properly, then it's easy to see which test contains the bug. With clean-up, however, one test will contain the bug, but another test will fail - so it's hard to find the real problem. Starting from a blank state is usually easy with unit tests, but can be much harder with functional tests [4] - particularly if you have a lot of data in a database that needs to be there. Rebuilding the database each time can add a lot of time to test runs, so that argues for switching to a clean-up strategy. One trick that's handy when you're using databases, is to conduct your tests inside a transaction, and then to rollback the transaction at the end of the test. That way the transaction manager cleans up for you, reducing the chance of errors[5]. Another approach is to do a single build of a mostly-immutable starting fixture before running a group of tests. Then ensure that the tests don't change that initial state (or if they do, they reverse the changes in tear-down). This tactic is more error-prone than rebuilding the fixture for each test, but it may be worthwhile iff it takes too long to build the fixture each time. Although databases are a common cause for isolation problems, there are plenty of times you can get these in-memory too. In particular be aware with static data and singletons. A good example for this kind of problem is contextual environment, such as the currently logged in user. If you have an explicit tear-down in a test, be wary of exceptions that occur during the tear-down. If this happens the test can pass, but cause isolation failures for subsequent tests. So ensure that if you do get a problem in a tear-down, it makes a loud noise. Some people prefer to put less emphasis on isolation and more on defining clear dependencies to force tests to run in a specified order. I prefer isolation because it gives you more flexibility in running subsets of tests and parallelizing tests. Asynchronous Behavior Asynchrony is a boon that allows you to keep software responsive while taking on long term tasks. Ajax calls allow a browser to stay responsive while going back to the server for more data, asynchronous message allow a server process to communicate with other system without being tied to their laggardly latency. But in testing, asynchrony can be curse. The common mistake here is to throw in a sleep: //pseudo-code makeAsyncCall; sleep(aWhile); readResponse; This can bite you two ways. First off you'll want to set the sleep time to long enough that it gives plenty of time to get the response. But that means that you'll spend a lot of time idly waiting for the response, thus slowing down your tests. The second bite is that, however long you sleep, sometimes it won't be enough. There will be some change in environment that will cause you to exceed the sleep - and you'll get false failure. As a result I strongly urge you to never use bare sleeps like this. Never use bare sleeps to wait for asynchonous responses: use a callback or polling. There are basically two tactics you can do for testing an asynchronous response. The first is for the asynchronous service to take a callback which it can call when done. This is the best since it means you'll never have to wait any longer than you need to [6]. The biggest problem with this is that the environment needs to be able to do this and then the service provider needs to do it. This is one of the advantages of having the development team integrated with testing - if they can provide a callback then they will. The second option is to poll on the answer. This is more than just looking once, but looking regularly, something like this //pseudo-code makeAsyncCall startTime = Time.now; while(! responseReceived) { if (Time.now - startTime > waitLimit) throw new TestTimeoutException; sleep (pollingInterval); } readResponse The point of this approach is that you can set the pollingInterval to a pretty small value, and know that that's the maximum amount of dead time you'll lose to waiting for a response. This means you can set the waitLimit very high, which minimizes the chance of hitting it unless something serious has gone wrong. [7] Make sure you use a clear exception class that indicates this is a test timeout that's failing. This will help make it clear what's gone wrong should it happen, and perhaps allow a more sophisticated test harness to take account of this information in its display. The time values, in particular the waitLimit, should never be literal values. Make sure they are always values that can be easily set in bulk, either by using constants or set through the runtime environment. That way if you need to tweak them (and you will) you can tweak them all quickly. All this advice is handy for async calls where you expect a response from the provider, but how about those where there is no response. These are calls where we invoke a command on something and expect it to happen without any acknowledgment. This is the trickiest case since you can test for your expected response, but there's nothing to do to detect a failure other than timing-out. If the provider is something you're building you can handle this by ensuring the provider implements some way of indicating that it's done - essentially some form of callback. Even if only the testing code uses it, it's worth it - although often you'll find this kind of functionality is valuable for other purposes too[8]. If the provider is someone else's work, you can try persuasion, but otherwise may be stuck. Although this is also a case when using Test Doubles for remote services is worthwhile (which I'll discuss more in the next section). If you have a general failure in something asynchronous, such that it's not responding at all, then you'll always be waiting for timeouts and your test suite will take a long time to fail. To combat this it's a good idea to use a smoke test to check that the asynchronous service is responding at all and stop the test run right away if it isn't. Gerard Meszaros's book, xUnit Test Patterns, contains lots of good patterns for constructing tests. You can also often side-step the asynchrony completely. Gerard Meszaros's Humble Object pattern says that whenever you have some logic that's in a hard-to-test environment, you should isolate the logic you need to test from that environment. In this case it means put most of the logic you need to test in a place where you can test it synchronously. The asynchronous behavior should be as minimal (humble) as possible, that way you don't need that much testing of it. Remote Services Sometimes I'm asked if ThoughtWorks does any integration work, which I find somewhat amusing since there's hardly any project we do that doesn't involve a fair bit of integration. By their nature, enterprise applications involve a great deal of combining data from different systems. These systems are maintained by other teams operating to their own schedules, teams that often use a very different software philosophy to our heavily test-driven agile approach. Testing with such remote systems brings a number of problems, and non-determinism is high on the list. Often remote systems don't have test system we can call, which means hitting a live system. If there is a test system, it may not be stable enough to provide deterministic responses. In this situation it's vital to ensure determinism, so it's time to reach for a Test Double - a component that looks like the remote service, but is really just a pretend version that mimics the remote system's behavior. The double needs to be setup so that provides the right kind of response in interaction with our system, but in a way we control. In this manner we can ensure determinism. Using a double has a downside, in particular when we are testing across a broad scope. How can we be sure that the double behaves in the same way that remote system does? We can tackle this again using tests, a form of test that I call Integration Contract Tests. These run the same interaction with the remote system and the double, and check that the two match. In this case 'match' may not mean coming up with the same result (due to the non-determinisms), but results that share the same essential structure. Integration Contract Tests need to be run frequently, but not part of our system's deployment pipeline. Periodic running based on the rate of the change of the remote system is usually best. For writing these kinds of test doubles, I'm a big fan of Self Initializing Fakes - since these are very simple to manage. Some people are firmly against using Test Doubles in functional tests, believing that you must test with real connection in order to ensure end-to-end behavior. While I sympathize with their argument, automated tests are useless if they are non-deterministic. So any advantage you gain by talking to the real system is overwhelmed by the need to stamp out non-determinism[9]. Time Few things are more non-deterministic than a call to the system clock. Each time you call it, you get a new result, and any tests that depend on it can thus change. Ask for all the todos due in the next hour, and you regularly get a different answer[10]. The most important thing here is to ensure that you always wrap the system clock with routines that can be replaced with a seeded value for testing. A clock stub can be set to particular time and frozen at that time, allowing your tests to have complete control over its movements. That way you can synchronize your test data to the values in the seeded clock.[11][12] Always wrap the system clock, so it can be easily substituted for testing. One thing to watch with this, is that eventually your test data might start having problems because it's too old, and you get conflicts with other time based factors in your application. In this case you can move the data, and your clock seeds to new values. When you do this, ensure that this is the only thing you do. That way you can be sure that any tests that fail are due to time-movement in the test data. Another area where time can be a problem is when you rely on other behaviors from the clock. I once saw a system that generated random keys based on clock values. This systems started failing when it was moved to a faster machine that could allocate multiple ids within a single clock tick.[13] I've heard so many problems due to direct calls to the system clock that I'd argue for finding a way to use code analysis to detect any direct calls to the system clock and failing the build right there. Even a simple regex check might save you a frustrating debugging session after a call at an ungodly hour. Resource Leaks If your application has some kind of resource leak, this will lead to random tests failing, since it's just which test causes the resource leak to go over a limit that gets the failure. This case is awkward because any test can fail intermittently due to this problem. If it isn't a case of one test being non-deterministic then resource leaks are a good candidate to investigate. By resource leak, I mean any resource that the application has to manage by acquiring and releasing. In non-memory-managed environments, the obvious example is memory. Memory-management did much to remove this problem, but other resources still need to be managed, such as database connections. Usually the best way to handle these kind of resources is through a Resource Pool. If you do this then a good tactic is to configure the pool to a size of 1 and make it throw an exception should it get a request for a resource when it has none left to give. That way the first test to request a resource after the leak will fail - which makes it a lot easier to find the problem test. This idea of limiting resource pool sizes, is about increasing constraints to make errors more likely to crop up in tests. This is good because we want errors to show in tests so we can fix them before they manifest themselves in production. This principle can be used in other ways too. One story I heard was of a system which generated randomly named temporary files, didn't clean them up properly, and crashed on a collision. This kind of bug is very hard to find, but one way to manifest it is to stub the randomizer for testing so it always returns the same value. That way you can surface the problem more quickly.
April 14, 2011
by Martin Fowler
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