DDD

Question 1

What is DDD? And what are the main advantages?

Answer 1

DDD - is Domain-Driven Design

Question 2

What is Bounded Context?

Answer 2

Bounded context is a logical boundary.

Question 3

What is domain?

Answer 3

Domain is a representation of our reality in the code.

Question 4

What is tactical pattern?

Answer 4

Focus on items that help you build the model or building block of domain model. ( Entity, DesingPattern )

Question 5

What is value object?

Answer 5

It’s just a class which responsible for some behavior ( for example Time) Think of them as adjectives in your domain. Value Objects have No identity. Something simular to pure function.

Question 6

What is Entity?

Answer 6

Domain obejct with unique identity ( Value object with id Model in ActiveRecord ). Usually has mutable state ( you can change name for examlple)

Question 7

What is aggregate?

Answer 7

It’s a set of composed domain object defining single consistency unit. One aggregate -> One transaction.

Aggregate has one EntryPoint - Root ( Only using this root outer objects can get access to it)

Entities can be grouped into aggregates. For instance, order is an entity, but it can also be an aggregate, which includes two entities: order and order item ( record and its associations )

Question 8

What is domain event?

Answer 8

• It’s a way to communicate between bounded context

- An object that is used to record a event related to model activity within the system.

Question 9

What is CQS?

Answer 9

Command–Query Separation (CQS).

Every method should either be a command that performs an action, or a query that returns data to the caller, but not both.

Question 10

How to split rails models into bounded context?

Answer 10

We can use modules.

admin/product
user/product
sale/product

class Admin::Product < ApplicationRecord
end

Question 11

What can helps us to split big model into the smallest one?

Answer 11

• fields_name ( first_address, second_address)
• prefix/postfix
• some fields updates in one transaction always

Question 12

Can you provide examples of value objects?

Answer 12

• Time
• Money
• Date
• Point
• Location

Question 13

Can you provide examples of Entities?

Answer 13

• User
• Customer
• Account
• Banknote ( with serial number)

Question 14

Can you provide examples of Aggregates?

Answer 14

• Unit

- CreateCar ( many action, authrizers, policies, forms inside)

Question 15

What the service mean in DDD?

Answer 15

This is a logic which gather some agregates together to perform some action ( Unit as example in our project )

Question 16

What the command object mean in DDD?

Answer 16

Commands are structures that contain all the attributes necessary to perform an operation in your
system and describe the intention. ( Something like a form with Virtus on our project)

Question 17

What is Repositories in DDD?

Answer 17

The DDD meaning of a repository is a service that uses a global interface to provide access to all entities and value objects that are within a particular aggregate collection. ( Something simular to query? )

Question 18

What is Command Bus in DDD?

Answer 18

The command bus matches commands to handlers.

SubmitOrderCommand => method(:submit),
ExpireOrderCommand => method(:expire),
CancelOrderCommand => method(:cancel),
ShipOrderCommand => method(:ship),

I see only one advatadges right now. This allow to set a strict border between clasess/units. You can set restriction to run only action from command bus for example and in this case you won’t be able to run any other action ( outside of this context for example)

Command bus as routing layer but on application level

Question 19

What rules you should use to select a correct name for domain events?

Answer 19

• Something that alredy happened
• Should be named in past tense…
• Represents a state change,
• Try to use names with business value ( not only CRUD )

Question 20

What can we do with Domain Events?

Answer 20

• persist in database / event store

- publish it for subscribers using a pub/sub mechanism

Question 21

What is Event Handler?

Answer 21

This is a logic that receives events and rect to them somehow.

Question 22

What is the diffirence betweeen Sync and Async events handlers?

Answer 22

• Sync events handlers ( Run event_handler/Subscriber directly after sending event)
• Async event handlers save events to the MessageQueue ( Sidekiq, DelayJob)

Question 23

Can you give me example of breaking CQS?

Answer 23

when method implement action and actially reaturn changed value

p. next # 1
p. next # 2

The example above don’t break this principle

p. next # nil; # command
p. current # 1 # query
p. next # nil
p. current # 2

Question 24

What is CQRS?

Answer 24

The Command Query Responsibility Segregation (CQRS) pattern separates read and create/update/delete operations for a data store. The main advantage is reading perfomance improvement.

We can compare it with CRUD. Query it’s READ in CRUD. Command is CREATE, UPDATE, DELETE, or any other operation.

Question 25

What is a process manager?

Answer 25

This is something that can help you orgnaize communication between context. Something simular to state machine ( For example -> Buy ticket -> Pay money -> Gerate PDF)

Question 26

What 4 main layer has layer architecture?

Answer 26

• User Interface ( Presentation layer ) Show info to user
• Application layer - Select which model/domain we should use
• Domain/ Model layer - main layer ( logic/state)
• Infrastructure layer - technically hepls implement domain layer

Question 27

What is Smart UI?

Answer 27

When you don’t have any layers and all business logic is present inside User Interface.

Question 28

What is factory?

Answer 28

A program element that is RESPONSIBLE for creating objects is called a FACTORY.

Question 29

What is specification (by restriction)?

Answer 29

It’s the same as validator. Just strictly show the object limit.

Question 30

What is specification (select by request)?

Answer 30

Provides you data for you model object. Can use combination of aggregates/queries. In most cases soemthing very specific.

Question 31

What is specification by ( generate by request )

Answer 31

It’s a generator which takes params and add additionla logic to this param. When you create something -> you automaticly creates some environment around this somehow.

Question 32

What is side effect free function?

Answer 32

Функции без побочных эффектов. This is a function which don’t produce enxpected behavior ( just calculate and return something without changing argument). Separate function into 2 group ( which one is modified something and just return a value )
[].map

Question 33

What is assertetion?

Answer 33

Методы, изменяющие состояние системы, можно характеризовать с помощью УТВЕРЖДЕНИЙ.

It’s something that can update argument during the calculation.
[].map! ( Or something related to spec strict rules )

Question 34

What is CLOSURE OF OPERATIONS?

Answer 34

Замкнутость операций. When type of returned value is always equal to type of arguments. ( 1 + 1 = 2 always)

Question 35

What is shared kernel?

Answer 35

This is a common area between 2 contexts.

Question 36

What is context map?

Answer 36

All contexts in your project and their itteraction with each other.

Question 37

What is CUSTOMER/SUPPLIER approach?

Answer 37

It’s a relation when somthing from high level depends on something from low level.

Question 38

What is Distillation?

Answer 38

Distillation is the process of identifying the domain, decoupling it, isolating it, clarifying it and making it explicit, allowing us to focus on what is more relevant and having it maintainable.

Question 39

What is core domain?

Answer 39

It’s the most import part of application.

Question 40

What is core domain?

Answer 40

It’s the most import part of application. Without this part your app can’t exist at all.

Question 41

What is generic subdomain?

Answer 41

This is a part of application which just add some additional logic to the main core domain.( i18n, parsing)