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London vs Chicago, Comparative Case Study - Sandro Mancuso and Uncle Bob

My notes on the video series "London vs Chicago TDD styles" by Uncle Bob And Sandro Mancuso

The git repo of the kata is here:

The "starting-point" branch is where both implementations began:

  • The ๐Ÿ‡ฌ๐Ÿ‡ง "openchat-outside-in" branch captures the tomato by tomato history of the London approach.
  • The ๐Ÿ‡บ๐Ÿ‡ธ "openchat-unclebob" branch captures the tomato by tomato history of the Chicago approach.

What I like about Sandro's style ๐Ÿ‘

  • Great confidence with IntelliJ IDE (shortcuts, templates, refactoring moves, etc). BTW as a viewer I would have appreciated a lot to actually see the typed keys, so I would recommend a plugin like Presentation Assistant for the next video :)
  • Almost all IDE commands are given with shortcuts, and most of them are customized because IntelliJ does not assign a shortcut for some specific moves by default (e.g. splitting the window in two panes, moving files to the right/left window with a shortcut, close all files in a pane, etc).
  • Clear ideas on the design and layering of a webapp (where to put things and responsibilities).
  • High frequency of commits.

What I don't fully like about Sandro's style (still need to think about ๐Ÿค”)

  • Having to write all the ITs / ATs before start writing any UT / production code: to use the "ATs as a probe", you have to have the ATs in place before start coding, don't you?
    • BTW this also mean that you actually commit code with red tests (=> broken build): I wonder if this approach could work when working in a team and with a "real project" (something you work on for weeks, months, not days).
  • Mocking Spark objects (like Request or Response) mean you have to know in advance the Spark web framework, because otherwise you could not know which method Spark will really call in the end...
  • Sandro's style is a bit of erratic sometimes, in my view (e.g. jumping from a UT to another, from production code to ATs, filling the holes that you left behind); but if you can control this and keep the focus on what you're doing and what you have to do, his style is quite powerful (maybe I could suggest the use of a TODO list with the next things to do and what has been done).
  • Related to the previous point, I see that Sandro often works and codes with many still unresolved compilation errors, which I understand is part of his flow and approach, but sometimes it seems to me that this goes too far (e.g. you have syntax issues you cannot spot because everything is red...). Again, I think you have to have a strong focus to not get lost while having to deal with many compilation errors.
  • Sandro creates many small helper methods (postsTextFrom, jsonContaining, ...) upfront (they do not even compile at the beginning): this seems coming from his great experience and indeed from his style (he knows he'll need those, so he creates them as early as possible): I tend to prefer to let those helper methods emerge when refactoring with a green bar.
  • I'm not fully convinced by the use of exceptions as a way to control flow... what are the alternatives?

Noteworthy things I noticed


Unit test classes are named with the Should suffix instead of the classic Test suffix.
Test method names follow the snake case convention (both for UTs and ITs).

So, for example, you have: PostServiceShould, LoginAPIShould, UserRepositoryShould (inform_when_a_username_is_already_taken, return_user_matching_valid_credentials, return_all_users, detect_a_following_already_exists, ...).

Integration test classes are named with an IT_ prefix, e.g. IT_RegistrationAPI, IT_UsersAPI.

Extensive use of constants in the tests to stand for fixed values (e.g. USER_ID, USER for an instance of User, POST_TEXT, POST_ID, DATE_TIME, PASSWORD, USER_CREDENTIALS, ...) ("I normally create them as constants because I don't care what they are..." says Sandro)

Runtime exception or Checked exceptions?

Sandro prefers to use checked exceptions for "business-related" error conditions, exceptions from which you could recover from: in those case a checked exceptions force the layer above to deal with that condition.
E.g. The PostsAPI class needs to deal with an InappropriateLanguageException, because it's part of the logic (see the commit).

While he prefers to use runtime exceptions for error conditions you could not recover from.

Sandro prefers concrete classes to interfaces?

It seems Sandro doesn't use interfaces when designing with TDD, meaning that when some new collaborator emerges while writing a unit test, its implementation will be a concrete class, not an interface (a different way is the "interface discovery" approach followed by Steve Freeman and Nat Pryce, explained in their book "Growing Object-Oriented Software, Guided by Tests").
I guess this is due to his "pragmatism" in doing TDD (see also how he avoids baby steps).
This is something I already saw with his way of solving the Bank Account Kata.

Commit style

  • The commit message used by Sandro contains often the name of the class and methods implemented (e.g. UserService.createUser(registrationData) implemented.).
  • The verb is at past tense.

AAA = ARRANGE, ACT, ASSERT and the "backwards" style

"When writing a test, I always start from the bottom: the assertion (this is what I expect to happen)." (=> ASSERT)
"Then I ask: 'what should I need to trigger this?' (=> ACT)."
Then I think at the bare minimum I have to do in order to have all in place (=> ARRANGE)."

The role of the Integration tests / Acceptance Tests in Sandro style

The UTs drive the architecture, the ATs probe the architecture.

"Integration tests / Acceptance Tests act as a probe to where we are and what we have still need to finish, in order to complete the feature. They show me what is left to be tested, and this will guide me to the next step, going inside starting from the outside."

๐Ÿ‡บ๐Ÿ‡ธ Uncle Bob describing Sandro's style: "The ATs will probe how deeply you've gone; the UTs mock things out, while the ATs will block as soon as they hit something that the UTs are mocking and still needs to be implemented."

โ— => "The Acceptance Tests act as a 'north star', a guiding star: they make sure we don't forget anything, because in the ATs I use the real stuff, not mocks."

The AT are used as a probe to measure the completeness of the UT.
Classically, ATs probe the completion of behavior (when we're done). In Sandro style, ATs probe the completion of architecture (what is the next layer to code, what is still missing in the architecture).

๐Ÿ‡บ๐Ÿ‡ธ Uncle Bob on Sandro's style: "You will only write the Unit Tests that cause the ATs to pass, respecting some architectural decisions, the layering and deriving the architecture from the UTs."

๐Ÿ‡บ๐Ÿ‡ธ Uncle Bob: "Here the ATs are testing behaviourally and the UTs are driving the architecture."

Sandro: "The ATs guarantee that the app is providing the expected behaviour to the external world, while through the UTs I derive the design, layer by layer."

ATs vs UTs

"The ATs only tests the happy-path scenarios ('sunny day' scenarios). They're not trying to get all the possible errors and corner cases, because then you would have a big overlap between UTs and ATs, and a combinatory explosion of ATs (see the 'pyramid of tests')."

"In the ATs I want to verify the most important path only, and leave all the other cases to the UTs."

Use objects, avoid primitive obsession!

"When I have two pieces of data that together they mean something (e.g. UserCredentials instead of two strings for username and password), I prefer to create that thing as an Object instead of passing around those two pieces of data."

Notes on "Comparative Case Study Episode 1"

I headed to the starting-point branch.

Notes on the openchat project:

  • Use ReactJS as the framework for the frontend of the chat: this will be the client of the Java backend API.

  • API documented with Swagger - to have the openchat swagger API available on localhost (port 8080), just run docker run --rm -p 8080:8080 -v $PWD:/openchat -e SWAGGER_JSON=/openchat/APIs.yaml swaggerapi/swagger-ui

  • Uses different interesting tools in the Java project:

    • JUnitParams, to have better parametrized tests
    • AssertJ and RestAssured
    • also minimal-json to generate JSON programmatically
    • also BDD Mockito
  • Take a look at how the maven POM is defined, and more generally how the project is organized.

  • See also how Integration Tests (IT) are defined in the POM, and run with a specific maven profile.

Quotes and reflections

๐Ÿ‡ฌ๐Ÿ‡ง "When we call the method under test, you need to understand what is the expected side-effect: what should happen?"

"A test should always have a side-effect: focus on that when you start writing a test."

"I always start with an assertion: what do I want to test? What am I testing?."

"I don't like mocking classes that are not mine, but in this case (the Spark Request class) I don't have an option!."

"In my style of TDD, many design decisions happen and take place in the 'RED' step", which BTW takes quite some time in the Sandro way of doing outside-in TDD (I found this sometimes too long, personally I'm not feeling safe when I stay too much time in a 'RED bar' condition).

"Design in the red" when TDDing on the UsersAPI class

"We already know that the class will violate the SRP and we already know that we'll have to keep separated the delivery mechanism side (which is where UsersAPI belongs) from the domain side (where e.g. you persist the user, generate an ID for the user, validate the registration request, etc), so we feel safe to take those design decisions up-front, instead of delaying those to a later moment (e.g. in the "refactoring" step)."

"It's all about confidence. If I don't know, I take small steps, I put all in the same class and then refactor later, when I'll be GREEN."

UsersAPI is effectively a controller.

"Design in the RED" โ—

UserJson handle the serialization of the User domain class as a JSON string. It has been created in UsersAPI as a static inner class.

"Some people use annotations on their domain objects to be serialized as JSON: I don't like that. I don't want to bind the two things." ๐Ÿ‘

"The JSON belongs to the delivery mechanism, on the contract we have with the external world.
While the User belongs to the domain, and cannot know how will be transformed to be delivered somewhere."

Having annotations for JSON serialization on the User class or having methods like toJSON() binds User and my domain to the delivery mechanism!

Separations of concerns!

"Otherwise, you end up with a domain that breaks persistence and API when you e.g. rename a field, because you bound the domain object to the persistence layer from one side and to the presentation layer to the other side as well..."

"I prefer to take data from the delivery mechanism and extract / convert those data into the domain names / data, I don't want to have the same names and conventions."

Using builders in tests

Builders are used in tests, but not in production code.

There's an IntelliJ shortcut to create a builder for a class.
Sandro then do three more things on the builder:

  • adds a aThing() method to return an instance of say ThingBuilder so that the chaining is then more fluent,
  • renames createThing() into build(),
  • sets a default values for all the fields of Thing so that is safe to use any instance create with the builder.

How should UserService behave when it's not able to create a new user?

  • we could have the service return an optional User
  • we could have the service throw an exception
  • ...

Sandro prefers the second option.

"Mocking is not about testing, it is about designing the collaboration between classes"

Notes on "Comparative Case Study Episode 2"

Accidental duplication vs "real" duplication

Accidental duplication is when code is accidentally duplicated, but may then change for different reasons, so you decide to keep those pieces of code duplicated (e.g. same logic in different APIs that may then change for different reasons).

Pragmatic layering :)

"A repository is a helper class to the service: everyone that wants to access the domain should go through the service. BUT in this simple app it would be a simple delegation to the repo, so we decide pragmatically to skip that delegation and use the repository directly in the LoginAPI."

Notes on "Comparative Case Study Episode 3"

Uses LocalDateTime.of() e DateTimeFormatter.ofPattern()

I don't like the method names prepareOKResponse, prepareErrorResponse.

Uncle Bob seems to prefer to write tests in a "top going forward to the bottom" way (e.g. when creating an object he instantiates its dependencies first, then the object), while Sandro often follow a "bottom backwards to the top" way (e.g. starting from the assertion and going backwards, creating helper methods that still don't exist, creating objects before instantiating their dependencies, ...), like a crayfish moving backward when swimming.

Tomato Gods! :-)

"In this style every test is a design decision"

PostService starts to have too many collaborators (four!).

PostRepository.add(post) method has been left unimplemented intentionally, because we still don't have query methods to fetch posts.

๐Ÿ‡ฌ๐Ÿ‡ง Sandro seems to uses the term inform (e.g. inform_when_a_text_has_inappropriate_language) to name tests that test for boolean methods.

When unit testing LanguageService Sandro and UncleBob say "here we can do some baby-steps / classic TDD approach because we are at the end of the architectural chain".
So, when you don't have any collaborations with other objects it seems natural to use the classic state-based TDD approach.

The API developed so far:

 API        |         Domain                                                                                   |  Infrastructure
UsersAPI    | UserService | UserRepository, User, IdGenerator, RegistrationData, UsernameAlreadyInUseException | UserJson
LoginAPI    |             | UserRepository, User, UserCredentials                                              | 
PostsAPI    | PostService | PostRepository, Post, LanguageService, Clock, InappropriateLanguageException       | PostJson 

Notes on "Comparative Case Study Episode 4"

It seems to me that Sandro sometimes takes a very long step to go from RED to GREEN (e.g. see PostsAPIShould.return_a_json_containing_posts_from_a_given_user), and when the GREEN arrives, it's kinda magic, like you wouldn't want to believe it and you want to review all the code touched by the test to be sure you understand all clearly (and I guess UncleBob has the same feeling :-)

In the test for PostRepository, the add(post) is used as a setup for the test of the query method postsBy(userId).

New requirement!

The post of a given user should be displayed in reverse chronological order! So where should we put this behaviour? In the repository? Somewhere outside it?

return_posts_for_a_given_user_in_reverse_chronological_order seems a very big test, which is verifying two things:

  1. that the repo is able to select the posts of a given user from all the posts
  2. that the repo will fetch those post in a specific order

"The size of the test is related to my confidence"

In Sandro's view, the extension of a test, its size, is all about his confidence: higher confidence means larger steps, lower confidence means more tiny steps and small increments.

Object mothers! => Builder for test!

I found a bit ambiguous to talk about "reverse chronological order" when in fact what the first implementation does is more a "reverse insertion order": what are you doing is fetching the elements (the user posts in this case) in the opposite order of addition, like in a LIFO stack. Maybe it's just a naming issue.

Where this "reverse chronological order" knowledge should be?

We decided to push this logic down to the repository.

"This behaviour (getting all the user posts in reverse chronological order) is not explicit, it's hidden in the PostRepository. It's a compromise."

Alternative: PostRepository returns a list of unordered posts and the PostService, one level higher in the layers, will sort the posts in "reverse chronological order"; so we would move that knowledge up in the service layer.

Another alternative (my two cents here) would be to have the repository expose a finder method with the possibility to specify an ordering strategy (e.g. "reverse chronological order"), so that the sorting implementation would be performed down in the repository while the selection of the strategy (e.g. "reverse chronological order") would be done up in the service.

Uncle Bob's take on this: "The order of the posts is purely a UI issue, has nothing to do with the internal of the application."

Timezone hits everybody :-)

There's a timezone issue! The backend returns the post time with a UTC timezone (+00) while (I'm guessing here) the frontend doesn't handle the post time correctly, probably it doesn't know the backend runs with the Chicago timezone, where they have UTC-06 :-)

The UI filters out the current user from all the users to follow.

Uncle Bob: "Things are getting mechanical. No interesting design decisions showing up."

Following API

Following API: probably I would have preferred a different API, something like

POST /users/{followerId}/followings instead of POST /followings with both the follower and followee ids in the body.

49:47: Why creating up-front the Following DTO while writing the UT for the FollowingAPI class? Where's the need for that coming from? Do you have any clue that you will need that object right now? Moreover, in the UT you'll have to "unwrap" that object to get from it the followerId and the followeeId...

59:00: => Gotcha! The need for the Following DTO emerges from the new version of the UT where we verify the interaction with the UserService!

Designing while writing the test

Again, the RED step seems indeed to take quite a long time (compared to Uncle Bob's golden rule of 'You are not allowed to write any more of a unit test than is sufficient to fail'). "A lot of people may say that this test has too much stuff in it, but this is design, we are designing, 'Designing in the RED'."

"It's not just writing the test, we are designing while we're writing the test"

Notes on "Comparative Case Study Episode 5"

Sandro creates test data with too generic names IMO. Specifically, the example is FOLLOWING in UserServiceShould test: I prefer to use explicit values so that is clear that those are test values, e.g. "anyFollowerId" instead of "followerId".

On UserRepository, I don't like using the same name (add) for the method to add users and for that to add following relationships.

Again, using exceptions to control flow (FollowingAlreadyExistsException): I wonder what alternatives are there to this strategy. Ideas?

Gotta use doThrow when mocking an exception thrown by a void method => gosh!

WIP => 13:00


  • review Sandro's test-and-code flow (e.g. in login and registration APIs)
  • remove the AWT List implementation from the types suggested by IntelliJ
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