Can unit testing be successfully added into an existing production project? If so, how and is it worth it?

Question

I'm strongly considering adding unit testing to an existing project that is in production. It was started 18 months ago before I could really see any benefit of TDD (face palm), so now it's a rather large solution with a number of projects and I haven't the foggiest idea where to start in adding unit tests. What's making me consider this is that occasionally an old bug seems to resurface, or a bug is checked in as fixed without really being fixed. Unit testing would reduce or prevents these issues occuring.

By reading similar questions on SO, I've seen recommendations such as starting at the bug tracker and writing a test case for each bug to prevent regression. However, I'm concerned that I'll end up missing the big picture and end up missing fundamental tests that would have been included if I'd used TDD from the get go.

Are there any process/steps that should be adhered to in order to ensure that an existing solutions is properly unit tested and not just bodged in? How can I ensure that the tests are of a good quality and aren't just a case of any test is better than no tests.

So I guess what I'm also asking is;

• Is it worth the effort for an existing solution that's in production?
• Would it better to ignore the testing for this project and add it in a possible future re-write?
• What will be more benefical; spending a few weeks adding tests or a few weeks adding functionality?

(Obviously the answer to the third point is entirely dependant on whether you're speaking to management or a developer)

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Reason for Bounty

Adding a bounty to try and attract a broader range of answers that not only confirm my existing suspicion that it is a good thing to do, but also some good reasons against.

I'm aiming to write this question up later with pros and cons to try and show management that it's worth spending the man hours on moving the future development of the product to TDD. I want to approach this challenge and develop my reasoning without my own biased point of view.

Answer 1

• Is it worth the effort for an existing solution that's in production?

Yes!

• Would it better to ignore the testing for this project and add it in a possible future re-write?

No!

• What will be more benefical; spending a few weeks adding tests or a few weeks adding functionality?

Adding testing (especially automated testing) makes it much easier to keep the project working in the future, and it makes it significantly less likely that you'll ship stupid problems to the user.

Tests to put in a priori are ones that check whether what you believe the public interface to your code (and each module in it) is working the way you think. If you can, try to also induce each isolated failure mode that your code modules should have (note that this can be non-trivial, and you should be careful to not check too carefully how things fail, e.g., you don't really want to do things like counting the number of log messages produced on failure, since verifying that it is logged at all is enough).

Then put in a test for each current bug in your bug database that induces exactly the bug and which will pass when the bug is fixed. Then fix those bugs! :-)

It does cost time up front to add tests, but you get paid back many times over at the back end as your code ends up being of much higher quality. That matters enormously when you're trying to ship a new version or carry out maintenance.

Answer 2

• yes, it is. when you start adding new functionality it can cause some old code modification and as results it is a source of potential bugs.
• (see the first one) before you start adding new functionality all (or almost) code (ideally) should be covered by unit tests.
• (see the first and second one) :). a new grandiose functionality can "destroy" the old worked code.

Answer 3

The problem with retrofitting unit tests is you'll realise you didn't think of injecting a dependency here or using an interface there, and before long you'll be rewriting the entire component. If you have the time to do this, you'll build yourself a nice safety net, but you could have introduced subtle bugs along the way.

I've been involved with many projects which really needed unit tests from day one, and there is no easy way to get them in there, short of a complete rewrite, which cannot usually be justified when the code is working and already making money. Recently, I have resorted to writing powershell scripts that exercise the code in a way that reproduces a defect as soon as it is raised and then keeping these scripts as a suite of regression tests for further changes down the line. That way you can at least start to build up some tests for the application without changing it too much, however, these are more like end to end regression tests than proper unit tests.

Answer 4

Yes. No. Adding tests.

Going towards a more TDD approach will actually better inform your efforts to add new functionality and make regression testing much easier. Check it out!

Answer 5

Yes it can: Just try to make sure all code you write from now has a test in place.

If the code that is already in place needs to be modified and can be tested, then do so, but it is better not to be too vigorous in trying to get tests in place for stable code. That sort of thing tends to have a knock-on effect and can spiral out of control.

Answer 6

Whether it's worth adding unit tests to an app that's in production depends on the cost of maintaining the app. If the app has few bugs and enhancement requests, then maybe it's not worth the effort. OTOH, if the app is buggy or frequently modified then unit tests will be hugely beneficial.

At this point, remember that I'm talking about adding unit tests selectively, not trying to generate a suite of tests similar to those that would exist if you had practiced TDD from the start. Therefore, in response to the second half of your second question: make a point of using TDD on your next project, whether it's a new project or a re-write (apologies, but here is a link to another book that you really should read: Growing Object Oriented Software Guided by Tests)

My answer to your third question is the same as the first: it depends on the context of your project.

Embedded within you post is a further question about ensuring that any retro-fitted testing is done properly. The important thing to ensure is that unit tests really are unit tests, and this (more often than not) means that retrofitting tests requires refactoring existing code to allow decoupling of your layers/components (cf. dependency injection; inversion of control; stubbing; mocking). If you fail to enforce this then your tests become integration tests, which are useful, but less targeted and more brittle than true unit tests.

Answer 7

It's absolutely worth it. Our app has complex cross-validation rules, and we recently had to make significant changes to the business rules. We ended up with conflicts that prevented the user from saving. I realized it would take forever to sort it out in the applcation (it takes several minutes just to get to the point where the problems were). I'd wanted to introduce automated unit tests and had the framework installed, but I hadn't done anything beyond a couple of dummy tests to make sure things were working. With the new business rules in hand, I started writing tests. The tests quickly identified the conditions that caused the conflicts, and we were able to get the rules clarified.

If you write tests that cover the functionality you're adding or modifying, you'll get an immediate benefit. If you wait for a re-write, you may never have automated tests.

You shouldn't spend a lot of time writing tests for existing things that already work. Most of the time, you don't have a specification for the existing code, so the main thing you're testing is your reverse-engineering ability. On the other hand, if you're going to modify something, you need to cover that functionality with tests so you'll know you made the changes correctly. And of course, for new functionality, write tests that fail, then implement the missing functionality.

Answer 8

I do agree with what most everyone else has said. Adding tests to existing code is valuable. I will never disagree with that point, but I would like to add one caveat.

Although adding tests to existing code is valuable, it does come at a cost. It comes at the cost of not building out new features. How these two things balance out depends entirely on the project, and there are a number of variables.

• How long will it take you to put all that code under test? Days? Weeks? Months? Years?
• Who are you writing this code for? Paying customers? A professor? An open source project?
• What is your schedule like? Do you have hard deadlines you must meet? Do you have any deadlines at all?

Again, let me stress, tests are valuable and you should work to put your old code under test. This is really more a matter of how you approach it. If you can afford to drop everything and put all your old code under test, do it. If that's not realistic, here's what you should do at the very least

• Any new code you write should be completely under unit test
• Any old code you happen to touch (bug fix, extension, etc.) should be put under unit test

Also, this is not an all or nothing proposition. If you have a team of, say, four people, and you can meet your deadlines by putting one or two people on legacy testing duty, by all means do that.

Edit:

I'm aiming to write this question up later with pros and cons to try and show management that it's worth spending the man hours on moving the future development of the product to TDD.

This is like asking "What are the pros and cons to using source control?" or "What are the pros and cons to interviewing people before hiring them?" or "What are the pros and cons to breathing?"

Sometimes there is only one side to the argument. You need to have automated tests of some form for any project of any complexity. No, tests don't write themselves, and, yes, it will take a little extra time to get things out the door. But in the long run it will take more time and cost more money to fix bugs after the fact than write tests up front. Period. That's all there is to it.

Answer 9

I would like to start this answer by saying that unit testing is really important because it will help you arrest bugs before they creep into production.

Identify the areas projects/modules where bugs have been re-introduced. start with those projects to write tests. It perfectly makes sense to write tests for new functionality and for bug fix.

Is it worth the effort for an existing solution that's in production?

Yes. You will see the effect of bugs coming down and maintenance becoming easier

Would it better to ignore the testing for this project and add it in a possible future re-write?

I would recommend to start if from now.

What will be more benefical; spending a few weeks adding tests or a few weeks adding functionality?

You are asking the wrong question. Definitely, functionality is more important than anything else. But, rather you should ask if spending a few weeks adding test will make my system more stable. Will this help my end user? Will it help a new developer in the team to understand the project and also to ensure that he/she, doesn't introduce a bug due to lack of understanding of the overall impact of a change.

Answer 10

Is it worth the effort for an existing solution that's in production?

Yes. But you don't have to write all unit tests to get started. Just add them one by one.

Would it better to ignore the testing for this project and add it in a possible future re-write?

No. First time you are adding code which breaks the functionality, you will regret it.

What will be more benefical; spending a few weeks adding tests or a few weeks adding functionality?

For new functionality (code) it is simple. You write the unit test first and then the functionality. For old code you decide on the way. You don't have to have all unit tests in place... Add the ones that hurt you most not having... Time (and errors) will tell on which one you have to focus ;)

Answer 11

It's unlikely you'll ever have significant test coverage, so you must be tactical about where you add tests:

• As you mentioned, when you find a bug, it's a good time to write a test (to reproduce it), and then fix the bug. If you see the test reproduce the bug, you can be sure it's a good, alid test. Given such a large portion of bugs are regressions (50%?), it's almost always worth writing regression tests.
• When you dive into an area of code to modify it, it's a good time to write tests around it. Depending on the nature of the code, different tests are appropriate. One good set of advice is found here.

OTOH, it's not worth just sitting around writing tests around code that people are happy with-- especially if nobody is going to modify it. It just doesn't add value (except maybe understanding the behavior of the system).

Good luck!

Answer 12

I've introduced unit tests to code bases that did not have it previously. The last big project I was involved with where I did this the product was already in production with zero unit tests when I arrived to the team. When I left - 2 years later - we had 4500+ or so tests yielding about 33 % code coverage in a code base with 230 000 + production LOC (real time financial Win-Forms application). That may sound low, but the result was a significant improvement in code quality and defect rate - plus improved morale and profitability.

It can be done when you have both an accurate understanding and commitment from the parties involved.

First of all, it is important to understand that unit testing is a skill in itself. You can be a very productive programmer by "conventional" standards and still struggle to write unit tests in a way that scales in a larger project.

Also, and specifically for your situation, adding unit tests to an existing code base that has no tests is also a specialized skill in itself. Unless you or somebody in your team has successful experience with introducing unit tests to an existing code base, I would say reading Feather's book is a requirement (not optional or strongly recommended).

Making the transition to unit testing your code is an investment in people and skills just as much as in the quality of the code base. Understanding this is very important in terms of mindset and managing expectations.

Now, for your comments and questions:

However, I'm concerned that I'll end up missing the big picture and end up missing fundamental tests that would have been included if I'd used TDD from the get go.

Short answer: Yes, you will miss tests and yes they might not initially look like what they would have in a green field situation.

Deeper level answer is this: It does not matter. You start with no tests. Start adding tests, and refactor as you go. As skill levels get better, start raising the bar for all newly written code added to your project. Keep improving etc...

Now, reading in between the lines here I get the impression that this is coming from the mindset of "perfection as an excuse for not taking action". A better mindset is to focus on self trust. So as you may not know how to do it yet, you will figure out how to as you go and fill in the blanks. Therefore, there is no reason to worry.

Again, its a skill. You can not go from zero tests to TDD-perfection in one "process" or "step by step" cook book approach in a linear fashion. It will be a process. Your expectations must be to make gradual and incremental progress and improvement. There is no magic pill.

The good news is that as the months (and even years) pass, your code will gradually start to become "proper" well factored and well tested code.

As a side note. You will find that the primary obstacle to introducing unit tests in an old code base is lack of cohesion and excessive dependencies. You will therefore probably find that the most important skill will become how to break existing dependencies and decoupling code, rather than writing the actual unit tests themselves.

Are there any process/steps that should be adhered to in order to ensure that an existing solutions is properly unit tested and not just bodged in?

Unless you already have it, set up a build server and set up a continuous integration build that runs on every checkin including all unit tests with code coverage.

Train your people.

Start somewhere and start adding tests while you make progress from the customer's perspective (see below).

Use code coverage as a guiding reference of how much of your production code base is under test.

Build time should always be FAST. If your build time is slow, your unit testing skills are lagging. Find the slow tests and improve them (decouple production code and test in isolation). Well written, you should easilly be able to have several thousands of unit tests and still complete a build in under 10 minutes (~1-few ms / test is a good but very rough guideline, some few exceptions may apply like code using reflection etc).

Inspect and adapt.

How can I ensure that the tests are of a good quality and aren't just a case of any test is better than no tests.

Your own judgement must be your primary source of reality. There is no metric that can replace skill.

If you don't have that experience or judgement, consider contracting someone who does.

Two rough secondary indicators are total code coverage and build speed.

Is it worth the effort for an existing solution that's in production?

Yes. The vast majority of the money spent on a custom built system or solution is spent after it is put in production. And investing in quality, people and skills should never be out of style.

Would it better to ignore the testing for this project and add it in a possible future re-write?

You would have to take into consideration, not only the investment in people and skills, but most importantly the total cost of ownership and the expected life time of the system.

My personal answer would be "yes of course" in the majority of cases because I know its just so much better, but I recognize that there might be exceptions.

What will be more benefical; spending a few weeks adding tests or a few weeks adding functionality?

Neither. Your approach should be to add tests to your code base WHILE you are making progress in terms of functionality.

Again, it is an investment in people, skills AND the quality of the code base and as such it will require time. Team members need to learn how to break dependencies, write unit tests, learn new habbits, improve discipline and quality awareness, how to better design software, etc. It is important to understand that when you start adding tests your team members likely don't have these skills yet at the level they need to be for that approach to be successful, so stopping progress to spend all time to add a lot of tests simply won't work.

Also, adding unit tests to an existing code base of any sizeable project size is a LARGE undertaking which requires commitment and persistance. You can't change something fundamental, expect a lot of learning on the way and ask your sponsor to not expect any ROI by halting the flow of business value. That won't fly, and frankly it shouldn't.

Thirdly, you want to instill sound business focus values in your team. Quality never comes at the expense of the customer and you can't go fast without quality. Also, the customer is living in a changing world, and your job is to make it easier for him to adapt. Customer alignment requires both quality and the flow of business value.

What you are doing is paying off technical debt. And you are doing so while still serving your customers ever changing needs. Gradually as debt is paid off, the situation improves, and it is easier to serve the customer better and deliver more value. Etc. This positive momentum is what you should aim for because it underlines the principles of sustainable pace and will maintain and improve moral - both for your development team, your customer and your stakeholders.

Hope that helps

Answer 13

You say you don't want to buy another book. So just read Michael Feather's article on working effectively with legacy code. Then buy the book :)

Answer 14

I'm very fond of Refactor the Low-hanging Fruit as an answer to the question of where to begin refactoring. It's a way to ease into better design without biting off more than you can chew.

I think the same logic applies to TDD - or just unit tests: write the tests you need, as you need them; write tests for new code; write tests for bugs as they appear. You're worried about neglecting harder-to-reach areas of the code base, and it's certainly a risk, but as a way to get started: get started! You can mitigate the risk down the road with code coverage tools, and the risk isn't (in my opinion) that big, anyway: if you're covering the bugs, covering the new code, covering the code you're looking at, then you're covering the code that has the greatest need for tests.

Answer 15

I'll add my voice and say yes, it's always useful!

There are some distinctions you should keep in mind, though: black-box vs white-box, and unit vs functional. Since definitions vary, here's what I mean by these:

• Black-box = tests that are written without special knowledge of the implementation, typically poking around at the edge cases to make sure things happen as a naive user would expect.
• White-box = tests that are written with knowledge of the implementation, which often try to exercise well-known failure points.
• Unit tests = tests of individual units (functions, separable modules, etc). For example: making sure your array class works as expected, and that your string comparison function returns the expected results for a wide range of inputs.
• Functional tests = tests of the entire system all at once. These tests will exercise a big chunk of the system all at once. For example: init, open a connection, do some real-world stuff, close down, terminate. I like to draw a distinction between these and unit tests, because they serve a different purpose.

When I've added tests to a shipping product late in the game, I found that I got the most bang for the buck from white-box and functional tests. If there's any part of the code that you know is especially fragile, write white-box tests to cover the problem cases to help make sure it doesn't break the same way twice. Similarly, whole-system functional tests are a useful sanity check that helps you make sure you never break the 10 most common use cases.

Black-box and unit tests of small units are useful too, but if your time is limited, it's better to add them early. By the time you're shipping, you've generally found (the hard way) the majority of the edge cases and problems that these tests would have found.

Like the others, I'll also remind you of the two most important things about TDD:

1. Creating tests is a continuous job. It never stops. You should try to add new tests every time you write new code, or modify existing code.
2. Your test suite is never infallible! Don't let the fact that you have tests lull you into a false sense of security. Just because it passes the test suite doesn't mean it's working correctly, or that you haven't introduced a subtle performance regression, etc.

Answer 16

You don't mention the implementation language, but if in Java then you could try this approach:

1) In a seperate source tree build regression or 'smoke' tests, using a tool to generate them, which might get you close to 80% coverage. These tests execute all the code logic paths, and verify from that point on that the code still does exactly what it does currently (even if a bug is present). This gives you a safety net against inadvertently changing behaviour when doing the necessary refactoring to make code easily unit testable by hand.

a) Product suggestion - I used to use the free web based product Junit Factory, but sadly it's closed now. However the product lives on in the commercially licenced AgitarOne JUnit Generator at http://www.agitar.com/solutions/products/automated_junit_generation.html

2) For each bug you fix, or feature you add from now on, use a TDD approach to ensure new code is designed to be testable and place these tests in a normal test source tree.

3) Existing code will also likely need to be changed, or refactored to make it testable as part of adding new features; your smoke tests will give you a safety net against regressions or inadvertent subtle changes to behaviour.

4) When making changes (bug fixes or features) via TDD, when complete it's likely the companion smoke test is failing. Verify the failures are as expected due to the changes made and remove the less readable smoke test, as your hand written unit test has full coverage of that improved component. Ensure that your test coverage does not decline only stay the same or increase.

5) When fixing bugs write a failing unit test that exposes the bug first.

Answer 17

If I were in your place, I would probably take an outside-in approach, starting with functional tests that exercise the whole system. I would try to re-document the system's requirements using a BDD specification language like RSpec, and then write tests to verify those requirements by automating the user interface.

Then I would do defect driven development for newly discovered bugs, writing unit tests to reproduce the problems, and work on the bugs until the tests pass.

For new features, I would stick with the outside-in approach: Start with features documented in RSpec and verified by automating the user interface (which will of course fail initially), then add more finely-grained unit tests as the implementation moves along.

I'm no expert on the process, but from what little experience I have I can tell you that BDD via automated UI testing is not easy, but I think it's worth the effort, and probably would yield the most benefit in your case.

Answer 18

When we started adding tests, it was to a ten-year-old, approximately million-line codebase, with far too much logic in the UI and in the reporting code.

One of the first things we did (after setting up a continuous build server) was to add regression tests. These were end-to-end tests.

• Each test suite starts by initializing the database to a known state. We actually have dozens of regression datasets that we keep in Subversion (in a separate repository from our code, because of the sheer size). Each test's FixtureSetUp copies one of these regression datasets into a temp database, and then runs from there.
• The test fixture setup then runs some process whose results we're interested in. (This step is optional -- some regression tests exist only to test the reports.)
• Then each test runs a report, outputs the report to a .csv file, and compares the contents of that .csv to a saved snapshot. These snapshot .csvs are stored in Subversion next to each regression dataset. If the report output doesn't match the saved snapshot, the test fails.

The purpose of regression tests is to tell you if something changes. That means they fail if you broke something, but they also fail if you changed something on purpose (in which case the fix is to update the snapshot file). You don't know that the snapshot files are even correct -- there might be bugs in the system (and then when you fix those bugs, the regression tests will fail).

Nevertheless, regression tests were a huge win for us. Just about everything in our system has a report, so by spending a few weeks getting a test harness around the reports, we were able to get some level of coverage over a huge part of our code base. Writing the equivalent unit tests would have taken months or years. (Unit tests would have given us far better coverage, and would have been far less fragile; but I'd rather have something now, rather than waiting years for perfection.)

Then we went back and started adding unit tests when we fixed bugs, or added enhancements, or needed to understand some code. Regression tests in no way remove the need for unit tests; they're just a first-level safety net, so that you get some level of test coverage quickly. Then you can start refactoring to break dependencies, so you can add unit tests; and the regression tests give you a level of confidence that your refactoring isn't breaking anything.

Regression tests have problems: they're slow, and there are too many reasons why they can break. But at least for us, they were so worth it. They've caught countless bugs over the last five years, and they catch them within a few hours, rather than waiting for a QA cycle. We still have those original regression tests, spread over seven different continuous-build machines (separate from the one that runs the fast unit tests), and we even add to them from time to time, because we still have so much code that our 6,000+ unit tests don't cover.

Answer 19

Im going to raise a few eyebrows here :)

First of all what is your project - if it is a compiler or a language or a framework or anything else that is not going to change functionally for a long time, then I think its absolutely fantastic to add unit tests.

However, if you are working on an application that is probably going to require changes in functionality (because of changing requirements) then there is no point in taking that extra effort.

Why?

1. Unit tests only cover code tests - whether the code does what it is designed to - it is not a replacement for manual testing which anyways has to be done (to uncover functional bugs, usability issues and all other kinds of issues)

2. Unit tests cost time! Now where I come from, that's a precious commodity - and business generally picks better functionality over a complete test suite.

3. If your application is even remotely useful to users, they are going to request changes - so you will have versions that will do things better, faster and probably do new things - there may also be a lot of refactoring as your code grows. Maintaining a full grown unit test suite in a dynamic environment is a headache.

4. Unit tests are not going to affect the perceived quality of your product - the quality that the user sees. Sure, your methods might work exactly as they did on day 1, the interface between presentation layer and business layer might be pristine - but guess what? The user does not care! Get some real testers to test your application. And more often than not, those methods and interfaces have to change anyways, sooner or later.

What will be more benefical; spending a few weeks adding tests or a few weeks adding functionality? - There are hell lot of things that you can do better than writing tests - Write new functionality, improve performance, improve usability, write better help manuals, resolve pending bugs, etc etc.

Now dont get me wrong - If you are absolutely positive that things are not going to change for next 100 years, go ahead, knock yourself out and write those tests. Automated Tests are a great idea for APIs as well, where you absolutely do not want to break third party code. Everywhere else, its just something that makes me ship later!

Answer 20

It depends...
It's great to have unit tests but you need to consider who your users are and what they are willing to tolerate in order to get a more bug-free product. Inevitably by refactoring your code which has no unit tests at present, you will introduce bugs and many users will find it hard to understand that you are making the product temporarily more defective to make it less defective in the long run. Ultimately it's the users who will have the final say...

Answer 21

I'm not a seasoned TDD expert by any means, but of course I would say that it's incredibly important to unit test as much as you can. Since the code is already in place, I would start by getting some sort of unit test automation in place. I use TeamCity to exercise all of the tests in my projects, and it gives you a nice summary of how the components did.

With that in place, I'd move on to those really critical business logic-like components that can't fail. In my case, there are some basic trigometry problems that need to be solved for various inputs, so I test the heck out of those. The reason I do this is that when I'm burning the midnight oil, it's very easy to waste time digging down to depths of code that really don't need to be touched, because you know they are tested for all of the possible inputs (in my case, there is a finite number of inputs).

Ok, so now you hopefully feel better about those critical pieces. Instead of sitting down and banging out all of the tests, I would attack them as they come up. If you hit a bug that's a real PITA to fix, write the unit tests for it and get them out of the way.

There are cases where you'll find that testing is tough because you can't instantiate a particular class from the test, so you have to mock it. Oh, but maybe you can't mock it easily because you didn't write to an interface. I take these "whoops" scenarios as an opportunity to implement said interface, because, well, it's a Good Thing.

From there, I'd get your build server or whatever automation you have in place configured with a code coverage tool. They create nasty bar graphs with big red zones where you have poor coverage. Now 100% coverage isn't your goal, nor would 100% coverage necessarily mean your code is bulletproof, but the red bar definitely motivates me when I have free time. :)

Answer 22

There is so many good answers so I will not repeat their content. I checked your profile and it seems you are C# .NET developer. Because of that I'm adding reference to Microsoft PEX and Moles project which can help you with autogenerating unit tests for legacy code. I know that autogeneration is not the best way but at least it is the way to start. Check this very interesting article from MSDN magazine about using PEX for legacy code.