Full Code of arialdomartini/Back-End-Developer-Interview-Questions for AI

Repository: arialdomartini/Back-End-Developer-Interview-Questions
Branch: master
Commit: 5bcf6e9a7eb9
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Directory structure:
gitextract_pmjpqgu3/

├── LICENSE
├── README.md
├── design-patterns/
│   ├── active-record.md
│   ├── dont-repeat-yourself.md
│   ├── inversion-of-control.md
│   └── law-of-demeter.md
└── snippets/
    └── kill-the-if-chain.md

================================================
FILE CONTENTS
================================================

================================================
FILE: LICENSE
================================================
GNU GENERAL PUBLIC LICENSE
                       Version 2, June 1991

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Also add information on how to contact you by electronic and paper mail.

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================================================
FILE: README.md
================================================
Back-End Developer Interview Questions
======================================

This page has been translated to [Chinese](https://github.com/monklof/Back-End-Developer-Interview-Questions) by [monklof](https://github.com/monklof).

I started writing down this list as my personal notes of topics I discussed with colleagues and friends, and that I wanted to deepen...

I'm not a big fan of asking technical questions in job interviews: I rather prefer to sit together with candidates in front of some real code, hands on the keyboard, facing a real problem, and to have a full day of pair programming, hopefully rotating with all the other team members. It is one of the best opportunities to know each others' style and to let candidates know some details about their future job.

Yet, I feel some categories of technical questions could be a good starting point to begin an engaging conversation. 

This repo collects a number of back-end related questions that can be used when vetting potential candidates. It is by no means recommended to use every single question on the same candidate: that would take hours, and would have no sense at all, as they cover a too broad set of topics for a single developer's to possibly know. Browse the section you find more relevant for your context, and pick the questions that give you more ideas on the conversation to have.

This project is admittedly inspired by [Front-end Job Interview Questions](https://github.com/darcyclarke/Front-end-Developer-Interview-Questions) by [@darcyclarke](https://github.com/darcyclarke)

## Where are the answers?
I didn't include any.<br/>
Most of the questions are open-ended, and some of them just don't have a *right* or a *wrong* answer. On the contrary, they are intended to be used as the starting point for a conversation that hopefully tells you more about the person's capabilities than a straight answer would. Personally, I would even choose the questions whose answers are not yet clear to me.

Feel free to open a [Discussion](https://github.com/arialdomartini/Back-End-Developer-Interview-Questions/discussions).





## <a name='toc'>Table of Contents</a>

* [Questions about Design Patterns](#patterns)
  * [Globals Are Evil](#globals-are-evil)
  * [Inversion of Control](#inversion-of-control)
  * [Law of Demeter](#law-of-demeter)
  * [Active-Record](#active-record)
  * [Data-Mapper](#data-mapper)
  * [Billion-Dollar Mistake](#billion-dollar-mistake)
  * [Inheritance vs Composition](#inheritance-vs-composition)
  * [Anti-Corruption Layer](#anti-corruption-layer)
  * [Singleton](#singleton)
  * [Data Abstraction](#data-abstraction)
  * [Don't Repeat Yourself](#dont-repeat-yourself)
  * [Dependency Hell](#dependency-hell)
  * [Goto Is Evil](#goto-is-evil)
  * [Robustness Principle](#robustness-principle)
  * [Separation of Concerns](#separation-of-concerns)

* [Questions about Code Design](#design)
  * [High Cohesion, Loose Coupling](#high-cohesion-loose-coupling)
  * [Index 0](#index-0)
  * [TDD](#tdd)
  * [DRY Violation](#dry-violation)
  * [Cohesion vs Coupling](#cohesion-vs-coupling)
  * [Refactoring](#refactoring)
  * [Code Comments](#code-comments)
  * [Design vs Architecture](#design-vs-architecture)
  * [Early Testing](#early-testing)
  * [Multiple Inheritance](#multiple-inheritance)
  * [Domain Logic in Stored Procedures](#domain-logic-in-stored-procedures)
  * [OOP Took Over the World](#oop-took-over-the-world)
  * [Bad Design](#bad-design)

* [Questions about languages](#languages)
  * [3 Worst Defects](#3-worst-defects)
  * [Functional Programming](#functional-programming)
  * [Closures](#closures)
  * [Generics](#generics)
  * [High-Order Functions](#high-order-functions)
  * [Loops and Recursion](#loops-and-recursion)
  * [Functions as First-Class Citizens](#functions-as-first-class-citizens)
  * [Anonymous Functions](#anonymous-functions)
  * [Static and Dynamic Typing](#static-and-dynamic-typing)
  * [Namespaces](#namespaces)
  * [Language Interoperability](#language-interoperability)
  * [Hate of Java](#hate-of-java)
  * [Good and Bad Languages](#good-and-bad-languages)
  * [Referential Transparency](#referencial-transparency)
  * [Stack and Heap](#stack-and-heap)
  * [Pattern Matching](#pattern-matching)
  * [Exceptions](#exceptions)
  * [Variant and Contravariant Inheritance](#variant-and-contravariant-inheritance)
  * [Constructors and Interfaces](#constructors-and-interfaces)
  * [Node.js](#node-js)
  * [Java and Time Traveling](#java-and-time-traveling)
  * [Eliminate Null](#eliminate-null)

* [Web Questions](#web)
  * [3rd Party Cookies](#3rd-party-cookies)
  * [API Versioning](#api-versioning)
  * [SPAs](#spas)
  * [Statelessness](#statelessness)
  * [REST vs SOAP](#rest-vs-soap)
  * [MVC and MVVM](#mvc-and-mvvm)

* [Databases Questions](#databases)
  * [DB Migrations](#db-migrations)
  * [NULL is special](#null-is-special)
  * [ACID](#acid)
  * [Schema Migrations](#schema-migrations)
  * [Lazy Loading](#lazy-loading)
  * [N+1 Problem](#n1-problem)
  * [Slowest Queries](#slowest-queries)
  * [Normalization](#normalization)
  * [Blue/Green Deployment](#bluegreen-deployment)

* [NoSQL Questions](#nosql)
  * [Eventual Consistency](#eventual-consistency)
  * [CAP Theorem](#cap-theorem)
  * [NoSql](#nosql)
  * [NoSQL and Scalability](#nosql-and-scalability)
  * [Document and Relational DBs](#document-and-relational-dbs)

* [Code Versioning Questions](#codeversioning)
  * [Branching in HG and in Git](#branching-in-hg-and-in-git)
  * [DVCS](dvcs)
  * [GitFlow and GitHubFlow](#gitflow-and-githubflow)
  * [Rebase](#rebase)
  * [Merging in HG and in Git](#merging-in-hg-and-in-git)

* [Questions about Concurrency](#concurrency)
  * [Why?](#why)
  * [Testing Concurrency](#testing-concurrency)
  * [Race Conditions](#race-conditions)
  * [Deadlocks](#deadlocks)
  * [Process Starvation](#process-starvation)
  * [Free Algorithm](#free-algorithm)

* [Questions about Distributed Systems](#distributed)
  * [Testing Distributed Systems](#testing-distributed-systems)
  * [Async Communication](#async-communication)
  * [Pitfalls of RPC](#pitfalls-of-rpc)
  * [Design of Distributed Systems](#design-of-distributed-systems)
  * [Fault Tolerance](#fault-tolerance)
  * [Failures](#failures)
  * [Network Partitions](#network-partitions)
  * [Fallacies of Distributed Computing](#fallacies-of-distibuted-computing)
  * [Request/Reply vs Publish/Subscribe](#requestreply-vs-publishsubscribe)
  * [Implement Transactions](#implement-transactions)

* [Questions about Software Lifecycle and Team Management](#management)
  * [Agility](#agility)
  * [Legacy Code](#legacy-code)
  * [Legacy Code ELI5](#legacy-code-eli5)
  * [Sell me Kanban](#sell-me-kanban)
  * [Agile vs Waterfall](#agile-vs-waterfall)
  * [Death by Meetings](#death-by-meetings)
  * [Late Projects](#late-projects)
  * [Agile Manifesto](#agile-manifesto)
  * [If I were the CTO](#if-i-were-the-cto)
  * [PMs](#pms)
  * [Team Organization](#team-organization)
  * [Turn Over](#turn-over)
  * [Qualities](#qualities)
  * [3 Things About Code](#3-things-about-code)
  * [1 Month's Revolution](#1-months-revolution)

* [Questions about logic and algorithms](#algorithms)
  * [FIFO with LIFO](#fifo-with-lifo)
  * [Stack Overflow](#stack-overflow)
  * [Tail Recursive n!](#tail-recursive-n)
  * [REPL](#repl)
  * [Defragger](#defragger)
  * [Mazes](#mazes)
  * [Memory Leaks](#memory-leaks)
  * [PRNG](#prng)
  * [Garbage Collecting](#garbage-collecting)
  * [Queues](#queues)
  * [Simple Web Server](#simple-web-server)
  * [Sorting Huge Files](#sorting-huge-files)
  * [Duplicates](#duplicates)

* [Questions about Software Architecture](#architecture)
  * [No Cache](#no-cache)
  * [Event-Driven Architecture](#event-driven-architecture)
  * [Readability](#readability)
  * [Emergent and Evolutionary](#emergent-and-evolutionary)
  * [Scale-Out, Scale-Up](#scale-out-scale-up)
  * [Failures User Sessions](#failures-user-sessions)
  * [CQRS](#cqrs)
  * [n-tier](#n-tier)
  * [Scalability](#scalability)
  * [C10K](#c10k)
  * [P2P](#p2p)
  * [CGI](#cgi)
  * [Vendor Lock-in](#vendor-lock-in)
  * [Pub/Sub](#pubsub)
  * [CPUs](#cpus)
  * [Performance](#performance)
  * [DDOS](#ddos)
  * [Performance and Scalability](#performance-and-scalability)
  * [Tight Coupling](#tight-coupling)
  * [Cloud Readiness](#cloud-readiness)
  * [Emergent Architecture](#emergent-architecture)
  * [Design, Architecture, Functionality, Aesthetic](#design-architecture-functionality-aesthetic)

* [Questions about Service Oriented Architecture and Microservices](#soa)
  * [Long-lived Transactions](#long-lived-transactions)
  * [SOA and Micro Services](#soa-and-micro-services)
  * [Versioning and Breaking Changes](#versioning-and-breaking-changes)
  * [Sagas and compensations](#sagas-and-compensations)
  * [Too Micro](#too-micro "Too Micro")
  * [Micro Services Architecture](#micro-services-architecture)

* [Questions about Security](#security)
  * [Security by Default](#security-by-default)
  * [Don't Invent Cryptography](#dont-invent-cryptography)
  * [2-FA](#2-fa)
  * [Confidential Data in Logs](#confidential-data-in-logs)
  * [SQL Injection](#sql-injection)
  * [Detect SQL Injection](#detect-sql-injection)
  * [XSS](#xss)
  * [Cross-Site Forgery Attack](#cross-site-forgery-attack)
  * [HTTPS](#https)
  * [MITM Attack](#mitm-attack)
  * [Stealing Sessions](#stealing-sessions)

* [General Questions](#general)
  * [Why FP?](#why-fp)
  * [Browsers](#browsers)
  * [TCP Sockets](#tcp-sockets)
  * [Encapsulation](#encapsulation)
  * [Real-time systems](#real-time-systems)
  * [Real-time and memory allocation](#real-time-and-memory-allocation)
  * [Immutability](#immutability)
  * [Mutable vs Immutable](#mutable-vs-immutable)
  * [Object-Relational Impedance Mismatch](#object-relational-impedance-mismatch)
  * [Sizing a Cache](#sizing-a-cache)
  * [TCP and HTTP](#tcp-and-http)
  * [Client-Side vs Server-Side](#client-side-vs-server-side)
  * [Reliable and non-reliable channels](#reliable-and-non-reliable-channels)

* [Open Questions](#open)
  * [Resistance to Change](#resistance-to-change)
  * [Threading ELI5](#threading-eli5)
  * [Innovation and Predictability](#innovation-and-predictability)
  * [Good Code](#good-code)
  * [Streaming](#streaming)
  * [1 Week Improvement](#1-week-improvement)
  * [Learnt this week](#learnt-this-week)
  * [Aesthetic](#aesthetic)
  * [Last 5 books](#last-5-books)
  * [Introducing CI/CD](#introducing-cicd)
  * [Reinvent the Wheel](#reinvent-the-wheel)
  * [Not Invented Here](#not-invented-here)
  * [Next Thing to Automate](#next-thing-to-automate)
  * [Coding is Hard](#coding-is-hard)
  * [Green Fields and Brown Fields](#green-fields-and-brown-fields)
  * [Type "Google.com"](#type-googlecom)
  * [While idle](#while-idle)
  * [Unicode](#unicode)
  * [Defending Monoliths](#defending-monoliths)
  * [Professional Developers](#professional-developers)
  * [It's an art](#its-an-art)
  * [People who like this also like...](#people-who-like-this-also-like)
  * [Corporations vs Startups](#corporations-vs-startups)
  * [I'm proud of](#im-proud-of)

* [Questions based on snippets of code](#snippets)
  * [Beware the Closure](#beware-the-closure)
  * [Type Erasure](#type-erasure)
  * [Memory Leak](#memory-leak)
  * [Kill the switch](#kill-the-switch)
  * [Kill the if](#kill-the-if)
  * [Kill the if-chain](#kill-the-if-chain)

* [Bill Gates Style Questions](#billgates)
  * [Mirrors](#mirrors)
  * [Clones](#clones)
  * [Revert](#revert)
  * [Quora](#quora)
  * [Cobol](#cobol)
  * [10 years](#10-years)
  * [Fire me](#fire-me)
  * [From scratch](#from-scratch)
  * [Telling lies](#telling-lies)
  * [Your past self](#your-past-self)




### [[↑]](#toc) <a name='patterns'>Questions about Design Patterns:</a>
#### Globals Are Evil
Why are global and static objects evil? Can you show it with a code example?

#### Inversion of Control
Tell me about Inversion of Control and how it improves the design of code.<br/>
[Resources](design-patterns/inversion-of-control.md)


#### Law of Demeter
The Law of Demeter (the Principle of Least Knowledge) states that each unit should have only limited knowledge about other units and it should only talk to its immediate friends (sometimes stated as "don't talk to strangers").<br/>
Would you write code violating this principle, show why it is a bad design and then fix it?<br/>
[Resources](design-patterns/law-of-demeter.md)

#### Active-Record
Active-Record is the design pattern that promotes objects to include functions such as Insert, Update, and Delete, and properties that correspond to the columns in some underlying database table. In your opinion and experience, which are the limits and pitfalls of the this pattern?<br/>
[Resources](design-patterns/active-record.md)

#### Data-Mapper
Data-Mapper is a design pattern that promotes the use of a layer of Mappers that moves data between objects and a database while keeping them independent of each other and the mapper itself. On the contrary, in Active-Record objects directly incorporate operations for persisting themselves to a database, and properties corresponding to the underlying database tables. Do you have an opinion on those patterns? When would you use one instead of the other?

#### Billion Dollar Mistake
[Tony Hoare](https://en.m.wikipedia.org/wiki/Tony_Hoare) who invented the null reference once said "*I call it my billion-dollar mistake*" since it led to "*innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years*".

Would you discuss the techniques to avoid it, such as the Null Object Pattern introduced by the GOF book, or Option types?

#### Inheritance vs Composition
Many state that, in Object-Oriented Programming, composition is often a better option than inheritance. What's you opinion?

#### Anti-Corruption Layer
What is an Anti-corruption Layer?

#### Singleton
Singleton is a design pattern that restricts the instantiation of a class to one single object. Writing a Thread-Safe Singleton class is not so obvious. Would you try?

#### Data Abstraction
The ability to change implementation without affecting clients is called Data Abstraction. Produce an example violating this property, then fix it.

#### Don't Repeat Yourself
Write a snippet of code violating the Don't Repeat Yourself (DRY) principle. Then, fix it.

#### Dependency Hell
How would you deal with Dependency Hell?

#### Goto is Evil
Is goto evil? You may have heard of the famous paper "Go To Statement Considered Harmful" by Edsger Dijkstra, in which he criticized the use of the `goto` statement and advocated structured programming instead. The use of `goto` has always been controversial, so much that even Dijkstra's letter was criticized with articles such as "'GOTO Considered Harmful' Considered Harmful". What's your opinion on the use of `goto`?

#### Robustness Principle
The robustness principle is a general design guideline for software that recommends "*be conservative in what you send, be liberal in what you accept*". It is often reworded as "*be a tolerant reader and a careful writer*". Would you like to discuss the rationale of this principle?

#### Separation of Concerns
Separation of Concerns is a design principle for separating a computer program into distinct areas, each one addressing a separate concern. There are a lot of different mechanisms for achieving Separation of Concerns (use of objects, functions, modules, or patterns such as MVC and the like). Would you discuss this topic?


### [[↑]](#toc) <a name='design'>Questions about Code Design:</a>

#### High Cohesion, Loose Coupling
It is often said that one of the most important goals in Object-Oriented Design (and code design in general) is to have High Cohesion and Loose Coupling. What does it mean? Why is it that important and how is it achieved?

#### Index 0
Why do array indexes start with '0' in most languages?

#### TDD
How do tests and TDD influence code design?

#### DRY Violation
Write a snippet of code violating the Don't Repeat Yourself (DRY) principle. Then, explain why it is a bad design, and fix it.

#### Cohesion vs Coupling
What's the difference between cohesion and coupling?

#### Refactoring
What is refactoring useful for?

#### Code Comments
Are comments in code useful? Some say they should be avoided as much as possible, and hopefully made unnecessary. Do you agree?

#### Design vs Architecture
What is the difference between design and architecture?

#### Early Testing
In TDD, why are tests written before code?

#### Multiple Inheritance
C++ supports multiple inheritance, and Java allows a class to implement multiple interfaces. What impact does using these facilities have on orthogonality? Is there a difference in impact between using multiple inheritance and multiple interfaces? Is there a difference between using delegation and using inheritance? [This question is from The Pragmatic Programmer, by Andrew Hunt and David Thomas]

#### Domain Logic in Stored Procedures
What are the pros and cons of holding domain logic in Stored Procedures?

#### OOP Took Over the World
In your opinion, why has Object-Oriented Design dominated the market for so many years?

#### Bad Design
What would you do to understand if your code has a bad design?


### [[↑]](#toc) <a name='languages'>Questions about Languages:</a>

#### 3 worst defects
Tell me the 3 worst defects of your preferred language

#### Functional Programming
Why is there a rising interest on Functional Programming?

#### Closures
What is a closure, and what is useful for? What's in common between closures and classes?

#### Generics
What are generics useful for?

#### High-Order Functions
What are higher-order functions? What are they useful for? Write one, in your preferred language.

#### Loops and Recursion
Write a loop, then transform it into a recursive function, using only immutable structures (i.e. avoid using variables). Discuss.

#### Functions as First-Class Citizens
What does it mean when a language treats functions as first-class citizens?
Why is it important that in a language functions are first-class citizens?

#### Anonymous Functions
Show me an example where an anonymous function can be useful.

#### Static and Dynamic typing
There are a lot of different type systems. Let's talk about static and dynamic type systems, and about strong and weak ones. You surely have an opinion and a preference about this topic. Would you like to share them, and discuss why and when would you promote one particular type system for developing an enterprise software?

#### Namespaces
What are namespaces useful for? Invent an alternative.

#### Language Interoperability
Talk about interoperability between Java and C# (in alternative, choose 2 other arbitrary languages)

#### Hate of Java
Why do many software engineers not like Java?

#### Good and Bad Languages
What makes a good language good and a bad language bad?

#### Referential Transparency
Write two functions, one referentially transparent and the other one referentially opaque. Discuss.

#### Stack and Heap
What is a stack and what is a heap? What's a stack overflow?

#### Pattern Matching
Some languages, especially the ones that promote a functional approach, allow a technique called pattern matching. Do you know it? How is pattern matching different from switch clauses?

#### Exceptions
Why do some languages have no exceptions by design? What are the pros and cons?

#### Variant and Contravariant Inheritance
If `Cat` is an `Animal`, is `TakeCare<Cat>` a `TakeCare<Animal>`?

#### Constructors and Interfaces
In Java, C# and many other languages, why are constructors not part of the interface?

#### Node.js
In the last years there has been a lot of hype around Node.js. What's your opinion on using a language that was initially conceived to run in the browser in the backend?

#### Java and time-traveling
* Pretend you have a time machine and pretend that you have the opportunity to go to a particular point in time during Java's (or C#, Python, Go or whatever) history, and talk with some of the JDK architects. What would you try to convince them of? Removing checked exceptions? Adding unsigned primitives? Adding multiple-inheritance?
#### Eliminate Null
Imagine you want to remove the possibility to have null references in your preferred language: how would you achieve this goal? What consequences would this have?


### [[↑]](#toc) <a name='web'>Questions about Web development:</a>

#### 3rd Party Cookies
Why are first-party cookies and third-party cookies treated so differently?

#### API Versioning
How would you manage Web Services API versioning?

#### SPAs
From a backend perspective, are there any disadvantages or drawbacks on the adoption of Single Page Applications?

#### Statelessness
Why do we usually put so much effort for having stateless services? What's so good in stateless code and why and when is statefulness bad?

#### REST vs SOAP
REST and SOAP: when would you choose one, and when the other?

#### MVC and MVVM
In web development, Model-View Controller and Model-View-View-Model approaches are very common, both in the backend and in the frontend. What are they, and why are they advisable?


### [[↑]](#toc) <a name='databases'>Questions about Databases:</a>

#### DB Migrations
How would you migrate an application from a database to another, for example from MySQL to PostgreSQL? If you had to manage that project, which issues would you expect to face?

#### NULL is special
Why do databases treat null as a so special case? For example, why does ```SELECT * FROM table WHERE field = null``` not match records with null ``field`` in SQL?

#### ACID
ACID is an acronym that refers to Atomicity, Consistency, Isolation and Durability, 4 properties guaranteed by a database transaction in most database engines. What do you know about this topic? Would you like to elaborate?

#### Schema Migrations
How would you manage database schema migrations? That is, how would you automate changes to database schema, as the application evolves, version after version?

#### Lazy Loading
How is lazy loading achieved? When is it useful? What are its pitfalls?

#### N+1 Problem
The so called "N + 1 problem" is an issue that occurs when code needs to load the children of a parent-child relationship with a ORMs that have lazy-loading enabled, and that therefore issue a query for the parent record, and then one query for each child record. How to fix it?

#### Slowest Queries
How would you find the most expensive queries in an application?

#### Normalization
In your opinion, is it always needed to use database normalization? When is it advisable to use denormalized databases?

#### Blue/Green Deployment
One of the Continuous Integration's techniques is called Blue-Green Deployment: it consists in having two production environments, as identical as possible, and in performing the deployment in one of them while the other one is still operating, and than in safely switching the traffic to the second one after some convenient testing. This technique becomes more complicated when the deployment includes changes to the database structure or content. I'd like to discuss this topic with you.


### [[↑]](#toc) <a name='nosql'>Questions about NoSQL:</a>

#### Eventual Consistency
What is eventual consistency?

#### CAP Theorem
Brewer's Theorem, most commonly known as the CAP theorem, states that in the presence of a network partition (the P in CAP), a system's designer has to choose between consistency (the C in CAP) and availability (the A in CAP). Can you think about examples of CP, AP and CA systems?

#### NoSQL
How would you explain the recent rise in interest in NoSQL?

#### NoSQL and Scalability
How does NoSQL tackle scalability challenges?

#### Document and Relational DBs
When would you use a document database like MongoDB instead of a relational database like MySQL or PostgreSQL?


### [[↑]](#toc) <a name='codeversioning'>Questions about code versioning:</a>

#### Branching in HG and in Git
Why is branching with Mercurial or git easier than with SVN?
 
#### DVCS
What are the pros and cons of distributed version control systems like Git over centralized ones like SVN?

#### GitFlow and GitHubFlow
Could you describe GitHub Flow and GitFlow workflows?

#### Rebase
What's a rebase?

#### Merging in HG and in Git
Why are merges easier with Mercurial and Git than with SVN and CVS?


### [[↑]](#toc) <a name='concurrency'>Questions about Concurrency:</a>

#### Why?
Why do we need concurrency, anyway? Explain.

#### Testing Concurrency
Why is testing multithreaded/concurrent code so difficult?

#### Race Conditions
What is a race condition? Code an example, using whatever language you like.

#### Deadlocks
What is a deadlock? Would you be able to write some code that is affected by deadlocks?

#### Process Starvation
What is process starvation? If you need, let's review its definition.

#### Free Algorithm
What is a wait free algorithm?


### [[↑]](#toc) <a name='distributed'>Questions about Distributed Systems:</a>

#### Testing Distributed Systems
How would you test a distributed system?

#### Async Communication
When would you apply asynchronous communication between two systems?

#### Pitfalls of RPC
What are the general pitfalls of remote procedure calls?

#### Design of Distributed Systems
If you are building a distributed system for scalability and robustness, what are the different things you'd think of if you are working in a closed and secure network environment versus when you are working in a geographically distributed and public system?

#### Fault Tolerance
How would you manage fault tolerance in a web application? What about in a desktop one?

#### Failures
How would you deal with failures in a distributed system?

#### Network Partitions
Let's talk about the several approaches to reconciliation after network partitions.

#### Fallacies of Distributed Computing
What are the fallacies of distributed computing?

#### Request/Reply vs Publish/Subscribe
When would you use request/reply and when publish/subscribe?

#### Implement Transactions
Suppose the system you are working on does not support transactionality. How would you implement it from scratch?


### [[↑]](#toc) <a name='management'>Questions about Software Lifecycle and Team Management:</a>

#### Agility
What is agility?

#### Legacy Code
How would you deal with legacy code?

#### Legacy Code ELI5
Say I'm your project manager, and I'm no expert in programming. Would you try explaining to me what legacy code is and why should I care about code quality?

#### Sell me Kanban
I'm the CEO of your company. Explain to me Kanban and convince me to invest in it.

#### Agile vs Waterfall
What is the biggest difference between Agile and Waterfall?

#### Death by Meetings
Being a team manager, how would you deal with the problem of having too many meetings?

#### Late Projects
How would you manage a very late project?

#### Agile Manifesto
"*Individuals and interactions over processes and tools*" and "*Customer collaboration over contract negotiation*" comprise half of the values of the Agile Manifesto. Discuss

#### If I were the CTO
Tell me what decisions would you take if you could be the CTO of your Company.

#### PMs
Are program managers useful?

#### Team Organization
Organize a development team using flexible schedules (that is, no imposed working hours) and "take as you need" vacation policy

#### Turn Over
How would you manage a very high turn over and convince developers not to leave the team, without increasing compensation? What could a company improve to make them stay?

#### Qualities
What are the top 3 qualities you look for in colleagues, beyond their code?

#### 3 Things About Code
What are the top 3 things you wish non-technical people knew about code?

#### 1 month's revolution
Imagine your company gives you 1 month and some budget to improve your and your colleagues' daily life. What would you do?


### [[↑]](#toc) <a name='algorithms'>Questions about logic and algorithms:</a>

#### FIFO with LIFO
Make a FIFO queue using only LIFO stacks. Then build a LIFO stack using only FIFO queues.

#### Stack Overflow
Write a snippet of code affected by a stack overflow.

#### Tail Recursive n!
Write a tail-recursive version of the factorial function.
#### REPL
Using your preferred language, write a REPL that echoes your inputs. Evolve it to make it an RPN calculator.

#### Defragger
How would you design a "defragger" utility?

#### Mazes
Write a program that builds random mazes.

#### Memory Leaks
Write a sample program that produces a memory leak.

#### PRNG
Generate a sequence of unique random numbers.

#### Garbage Collecting
Write a simple garbage collection system.

#### Queues
Write a basic message broker, using whatever language you like.

#### Simple Web Server
Write a very basic web server. Draw a road map for features to be implemented in the future.

#### Sorting Huge Files
How would you sort a 10GB file? How would your approach change with a 10TB one?

#### Duplicates
How would you programmatically detect file duplicates?

### [[↑]](#toc) <a name='architecture'>Questions about Software Architecture:</a>

#### No Cache
When is a cache not useful or even dangerous?

#### Event-Driven Architecture
Why does Event-Driven Architecture improve scalability?

#### Readability
What makes code readable?

#### Emergent and Evolutionary
What is the difference between emergent design and evolutionary architecture?

#### Scale-Out, Scale-Up
Scale out vs scale up: how are they different? When to apply one, when the other?

#### Failures User Sessions
How to deal with failover and user sessions?

#### CQRS
What is CQRS (Command Query Responsibility Segregation)? How is it different from the oldest Command-Query Separation Principle?

#### n-tier
The so called "multitier architecture" is an approach to design a client–server system aimed to keep physically and logically separated presentation, application processing, data management and other functions. The most widespread of the multitier architectures is the three-tier architecture. Would you discuss the pros and cons of such an approach?

#### Scalability
How would you design a software system for scalability?

#### C10K
Someone gave the name "The "C10k problem" to the problem of optimising network sockets to handle over 10.000 open connections at once. While handling 10.000 concurrent clients is not the same as handling 10.000 open connection, the context is similar. It's a tough challenge anyway, and no one is expected to know every single detail to solve it. It may be interesting to discuss the strategies you know to deal with that problem. Would you like to try?

#### P2P
How would you design a decentralized (that is, with no central server) P2P system?

#### CGI
You may recall that Common Gateway Interface (CGI) is a standard protocol for web servers to execute programs (CGI scripts) that execute as Command-line programs on a server, and that dynamically generate HTML pages when invoked by a HTTP request. Perl and PHP used to be common languages for such scripts. In CGI, a HTTP request generally causes the invocation of a new process on the server, but FastCGI, SCGI and other approaches improved the mechanism, raising the performance, with techniques such as preforking processes. Can you discuss why CGI became obsolete, and was instead replaced with other architectural approaches?

#### Vendor Lock-in
How would you defend the design of your systems against vendor Lock-in?

#### Pub/Sub
What are the disadvantages of the publish-subscribe pattern at scale?

#### CPUs
What's new in CPUs since the 80s, and how does it affect programming?

#### Performance
In which part of the lifecycle of a software performance should be taken in consideration, and how?

#### DDOS
How could a denial of service arise not maliciously but due to a design or architectural problem?

#### Performance and Scalability
What’s the relationship between performance and scalability?

#### Tight Coupling
When is it OK (if ever) to use tight coupling?

#### Cloud Readiness
What characteristic should a system have to be cloud ready?

#### Emergent Architecture
Does unity of design imply an aristocracy of architects? Putting it simple: can good design emerge from a collective effort of all developers?

#### Design, Architecture, Functionality, Aesthetic
What's the difference between design, architecture, functionality and aesthetic? Discuss.



### [[↑]](#toc) <a name='soa'>Questions about Service Oriented Architecture and Microservices:</a>

#### Long-lived Transactions
Why, in a SOA, long-lived transactions are discouraged and sagas are suggested instead?

#### SOA and Micro Services
What are the differences between SOA and microservice?

#### Versioning and Breaking Changes
Let's talk about web services versioning, version compatibility and breaking changes.

#### Sagas and compensations
What's the difference between a transaction and a compensation operation in a saga, in SOA?

#### Too Micro
When is a microservice too micro?

#### Micro Services Architecture
What are the pros and cons of microservice architecture?


### [[↑]](#toc) <a name='security'>Questions about Security:</a>
#### Security by Default
How do you write secure code? In your opinion, is it one of the developer's duties, or does it require a specialized role in the company? And why?

#### Don't Invent Cryptography
Why is it said that cryptography is not something you should try to invent or design yourself?

#### 2-FA
What is two factor authentication? How would you implement it in an existing web application?

#### Confidential Data in Logs
If not carefully handled, there is always a risk of logs containing sensitive information, such as passwords. How would you deal with this?

#### SQL Injection
Write down a snippet of code affected by SQL injection and fix it.

#### Detect SQL Injection
How would it be possible to detect SQL injection via static code analysis? I don't expect you to write an algorithm capable of doing this, as it is probably a huge topic, but let's discuss a general approach.

#### XSS
What do you know about Cross-Site Scripting? If you don't remember it, let's review online its definition and let's discuss about it.

#### Cross-Site Forgery Attack
What do you know about Cross-Site Forgery Attack? If you don't remember it, let's review online its definition and let's discuss about it.

#### HTTPS
How does HTTPS work?

#### MITM Attack
What's a man-in-the-middle Attack, and why does HTTPS help protect against it?

#### Stealing Sessions
How can you prevent the user's session from being stolen? Chances are you remember what session or cookie hijacking is, otherwise let's read its Wikipedia page together.


### [[↑]](#toc) <a name='general'>General Questions:</a>

#### Why FP?
Why does functional programming matter? When should a functional programming language be used?

#### Browsers
How do companies like Microsoft, Google, Opera and Mozilla profit from their browsers?

#### TCP Sockets
Why does opening a TCP socket have a large overhead?

#### Encapsulation
What is encapsulation important for?

#### Real-time systems
What is a real-time system and how is it different from an ordinary system?

#### Real-time and memory allocation
What's the relationship between real-time languages and heap memory allocation?

#### Immutability
Immutability is the practice of setting values once, at the moment of their creation, and never changing them. How can immutability help write safer code?

#### Mutable vs Immutable
What are the pros and cons of mutable and immutable values.

#### Object-Relational Impedance Mismatch
What's the Object-Relational impedance mismatch?

#### Sizing a Cache
Which principles would you apply to define the size of a cache?

#### TCP and HTTP
What's the difference between TCP and HTTP?

#### Client-Side vs Server-Side
What are the tradeoffs of client-side rendering vs. server-side rendering?

#### Reliable and non-reliable channels
How could you develop a reliable communication protocol based on a non-reliable one?




### [[↑]](#toc) <a name='open'>Open Questions:</a>

#### Resistance to Change
Why do people resist change?

#### Threading ELI5
Explain threads to your grandparents

#### Innovation and Predictability
As a software engineer you want both to innovate and to be predictable. How those two goals can coexist in the same strategy?

#### Good Code
What makes good code good?

#### Streaming
Explain streaming and how you would implement it.

#### 1 Week Improvement
Say your company gives you one week you can use to improve your and your colleagues' lifes: how would you use that week?

#### Learnt this week
What did you learn this week?

#### Aesthetic
There is an aesthetic element to all design. The question is, is this aesthetic element your friend or your enemy?

#### Last 5 books
List the last 5 books you read.

#### Introducing CI/CD
How would you introduce Continuous Delivery in a successful, huge company for which the change from Waterfall to Continuous Delivery would be not trivial, because of the size and complexity of the business?

#### Reinvent the Wheel
When does it make sense to reinvent the wheel?

#### Not Invented Here
Let's have a conversation about "*reinventing the wheel*", the "*not invented here syndrome*" and the "*eating your own food*" practice

#### Next Thing to Automate
What's the next thing you would automate in your current workflow?
#### Coding is Hard
Why is writing software difficult? What makes maintaining software hard?

#### Green Fields and Brown Fields
Would you prefer working on green field or brown field projects? Why?

#### Type "Google.com"
[What happens when you type google.com into your browser and press enter?](https://github.com/alex/what-happens-when)

#### While idle
What does an operating system do when it has got no custom code to run, and therefore it looks idle? I would like to start a discussions about interrupts, daemons, background services, polling, event handling and so on.

#### Unicode
Explain Unicode and database transactions to a 5 year old child.

#### Defending Monoliths
Defend the monolithic architecture.

#### Professional Developers
What does it mean to be a "professional developer"?

#### It's an art
Is developing software Art, Engineering, Crafts or Science? Your opinion.

#### People who like this also like...
"People who like this also like... ". How would you implement this feature in an e-commerce shop?

#### Corporations vs Startups
Why are corporations slower than startups in innovating?

#### I'm proud of
What have you achieved recently that you are proud of?



### [[↑]](#toc) <a name='snippets'>Questions about snippets of code:</a>

#### Beware the Closure
What's the output of this Javascript function?
```javascript
function hookupevents() {
  for (var i = 0; i < 3; i++) {
    document.getElementById("button" + i)
      .addEventListener("click", function() {
        alert(i);
      });
  }
}
```

#### Type Erasure
About Type Erasure, what's the output of this Java snippet, and why?
```java
ArrayList<Integer> li = new ArrayList<Integer>();
ArrayList<Float> lf = new ArrayList<Float>();
if (li.getClass() == lf.getClass()) // evaluates to true
  System.out.println("Equal");
```

#### Memory Leak
Can you spot the memory leak?
```java
public class Stack {
    private Object[] elements;
    private int size = 0;
    private static final int DEFAULT_INITIAL_CAPACITY = 16;

    public Stack() {
        elements = new Object[DEFAULT_INITIAL_CAPACITY];
    }

    public void push(Object e) {
        ensureCapacity();
        elements[size++] = e;
    }

    public Object pop() {
        if (size == 0)
            throw new EmptyStackException();
        return elements[--size];
    }

    /**
     * Ensure space for at least one more element, roughly
     * doubling the capacity each time the array needs to grow.
     */
    private void ensureCapacity() {
        if (elements.length == size)
            elements = Arrays.copyOf(elements, 2 * size + 1);
    }
}
```

#### Kill the witch
`if`s and in general conditional statements lead to procedural and imperative programming. Can you get rid of this `switch` and make this snippet more object oriented?

```java
public class Formatter {

    private Service service;

    public Formatter(Service service) {
        this.service = service;
    }

    public String doTheJob(String theInput) {
        String response = service.askForPermission();
        switch (response) {
        case "FAIL":
            return "error";
        case "OK":
            return String.format("%s%s", theInput, theInput);
        default:
            return null;
        }
    }
}
```

#### Kill the if
Can you get rid of these `if`s and make this snippet of code more object oriented?

```java
public class TheService {
    private final FileHandler fileHandler;
    private final FooRepository fooRepository;

    public TheService(FileHandler fileHandler, FooRepository fooRepository) {
        this.fileHandler = fileHandler;
        this.fooRepository = fooRepository;
    }

    public String Execute(final String file) {

        final String rewrittenUrl = fileHandler.getXmlFileFromFileName(file);
        final String executionId = fileHandler.getExecutionIdFromFileName(file);

        if (executionId.equals("") || rewrittenUrl.equals("")) {
            return "";
        }

        Foo knownFoo = fooRepository.getFooByXmlFileName(rewrittenUrl);

        if (knownFoo == null) {
            return "";
        }

        return knownFoo.DoThat(file);
    }
}
```

#### Kill the if-chain
How would you refactor this code?

```js
function()
{
    HRESULT error = S_OK;

    if(SUCCEEDED(Operation1()))
    {
        if(SUCCEEDED(Operation2()))
        {
            if(SUCCEEDED(Operation3()))
            {
                if(SUCCEEDED(Operation4()))
                {
                }
                else
                {
                    error = OPERATION4FAILED;
                }
            }
            else
            {
                error = OPERATION3FAILED;
            }
        }
        else
        {
            error = OPERATION2FAILED;
        }
    }
    else
    {
        error = OPERATION1FAILED;
    }

    return error;
}
```
[Resources](snippets/kill-the-if-chain.md)

### [[↑]](#toc) <a name='billgates'>Bill Gates Style Questions:</a>
This section collects some weird questions along the lines of the [Manhole Cover Question](https://en.wikipedia.org/wiki/Microsoft_interview#Manhole_cover_question).

#### Mirrors
What would happen if you put a mirror in a scanner?

#### Clones
Imagine there's a perfect clone of yourself. Imagine that that clone is your boss. Would you like to work for him/her?

#### Revert
Interview me.

#### Quora
Why are Quora's answers better than Yahoo Answers' ones?

#### Cobol
Let's play a game: defend Cobol against modern languages, and try to find as many reasonable arguments as you can.

#### 10 years
Where will you be in 10 years?

#### Fire me
You are my boss and I'm fired. Inform me.

#### From scratch
I want to refactor a legacy system. You want to rewrite it from scratch. Argument. Then, switch our roles.

#### Telling lies
Your boss asks you to lie to the company. What's your reaction?

#### Your past self
If you could travel back in time, which advice would you give to your younger self?


================================================
FILE: design-patterns/active-record.md
================================================
# Active Record

Related topics suggested by [Krzysztof Grzybek](https://github.com/krzysztof-grzybek)

- How hard is to test code that uses Active Record? How is domain model tied or independent from data layer?
- What's the relation between objects implementing Active Record and the Single Responsibility Principle?
- How easy is to hit the database multiple times (e.g. in foreach loop) because of the leaking abstraction?


================================================
FILE: design-patterns/dont-repeat-yourself.md
================================================
# Don't Repeat Yourself

## [Krzysztof Grzybek](https://github.com/krzysztof-grzybek)

Code violating the DRY principle:
```javascript
class Employee {
    calculateSalaryNet() {
        return this.hoursWorked * this.hourlyWage;
    }

    calculateSalaryGross() {
        return this.hoursWorked * this.hourlyWage + TAX;
    }
}
```

Fixed code:
```javascript
class Employee {
    calculateSalaryNet() {
        return this.hoursWorked * this.hourlyWage;
    }

    calculateSalaryGross() {
        return this.calculateSalaryNet() + TAX;
    }
}
```


================================================
FILE: design-patterns/inversion-of-control.md
================================================
# Inversion of Control

## Links

* [InversionOfControl - Fowler](https://martinfowler.com/bliki/InversionOfControl.html)
* [Inversion of Control Containers and the Dependency Injection pattern - Fowler](https://martinfowler.com/articles/injection.html)
* [DIP in the Wild - Brett L. Schuchert](https://martinfowler.com/articles/dipInTheWild.html)

## Related topics

* Topics that might be confused with each other
  - Inversion of Control
  - Dependency Inversion Principle (from SOLID)
  - Dependency Injection

They are all related, but in fact separate.

### Relation with Hollywood Principle
For some the Hollywood Principle (or Law) is just a synonym of Inversion of Control; some others see subtle differences. See  [Confusion between Inversion of Control and Hollywood Principle](https://stackoverflow.com/questions/43786221/confusion-between-inversion-of-control-and-hollywood-principle).

In this context, another related topic is the difference between libraries and frameworks, as it is stated that frameworks follow the Hollywood Principle (see [What is the difference between a framework and a library?](https://stackoverflow.com/questions/148747/what-is-the-difference-between-a-framework-and-a-library)).


================================================
FILE: design-patterns/law-of-demeter.md
================================================
# Law of Demeter

* [Law of Demeter - Wikipedia](https://en.wikipedia.org/wiki/Law_of_Demeter)
* [Law of Demeter - WikiWikiWeb](https://wiki.c2.com/?LawOfDemeter)
* [Introducing Demeter and its Laws](http://www.bradapp.net/docs/demeter-intro.html)

## Mnemonic for the Law of Demeter
The post [Visualization Mnemonics for Software Principles](http://www.daedtech.com/visualization-mnemonics-for-software-principles) by [Erik Dietrich](https://github.com/erikdietrich) provides a very effective trick to understand (and never forget anymore) what the Law of Demeter is. An Italian translation is available [here](https://github.com/arialdomartini/mnemonics/blob/law-of-demeter/README-italian.md).


================================================
FILE: snippets/kill-the-if-chain.md
================================================
# Kill the if-chain

The nested structure could be unrolled, as suggested by [Vahid Najafi](https://github.com/vahidvdn)

```js
function()
{
    HRESULT error = S_OK;

    if( !SUCCEEDED(Operation1() ) return OPERATION1FAILED;
    if( !SUCCEEDED(Operation2() ) return OPERATION2FAILED;
    if( !SUCCEEDED(Operation3() ) return OPERATION3FAILED;
    if( !SUCCEEDED(Operation3() ) return OPERATION3FAILED;
    if( !SUCCEEDED(Operation4() ) return OPERATION4FAILED;

}
```

Actually, I feel there is way more space for refactoring, but it all depends on the economic of the projects.

It is worth to discuss the smells and design flaws of that code, to set up the refactoring goals.

## Smells
I see these problems:

* The code violates some of the [SOLID principles][2]. It surely violates the [Open Closed Principle][3], as it is not possible to extend it without changing its code. E.g., adding a new operation would require adding a new `if`/`else` branch;
* It also violate the [Single Responsibility Principle][4]. It just does too much. It performs error checks, it's responsible to execute all the 4 operations, it contains their implementations, it's responsible to check their results and to chain their execution in the right order;
* It violates the [Dependency Inversion Principle][5], because there are dependencies between high-level and low-level components;
* It has a horrible [Cyclomatic complexity][6]
* It exhibits high coupling and low cohesion, which is exactly the opposite of [what is recommended][7];
* It contains a lot of code duplication: the function `Succeeded()` is repeated in each branch; the structure of `if`/`else`s is replicated over and over; the assignment of `error` is duplicated.
* It could have a pure functional nature, but it relies instead on state mutation, which makes reasoning about it not easy.
* There's an empty `if` statement body, which might be confusing.

## Refactoring
Let's see what could be done.<br/>
Here I'm using a C# implementation, but similar steps can be performed with whatever language.<br/>
I renamed some of the elements, as I believe honoring a naming convention is part of the refactoring.

```csharp
  internal class TestClass
    {
        HResult SomeFunction()
        {
            var error = HResult.Ok;

            if(Succeeded(Operation1()))
v            {
                if(Succeeded(Operation2()))
                {
                    if(Succeeded(Operation3()))
                    {
                        if(Succeeded(Operation4()))
                        {
                        }
                        else
                        {
                            error = HResult.Operation4Failed;
                        }
                    }
                    else
                    {
                        error = HResult.Operation3Failed;
                    }
                }
                else
                {
                    error = HResult.Operation2Failed;
                }
            }
            else
            {
                error = HResult.Operation1Failed;
            }

            return error;
        }

        private string Operation1()
        {
            // some operations
            return "operation1 result";
        }
        private string Operation2()
        {
            // some operations
            return "operation2 result";
        }
        private string Operation3()
        {
            // some operations
            return "operation3 result";
        }
        private string Operation4()
        {
            // some operations
            return "operation4 result";
        }

        private bool Succeeded(string operationResult) =>
            operationResult == "some condition";
    }

    internal enum HResult
    {
        Ok,
        Operation1Failed,
        Operation2Failed,
        Operation3Failed,
        Operation4Failed,
    }
}
```

For the sake of simplicity, I supposed each operation returns a string, and that the success or failure is based on an equality check on the string, but of course it could be whatever. In the next steps, it would be nice if the code is independent from the result validation logic.

### Step 1
It would be nice to start the refactoring with the support of some test harness.

```csharp
public class TestCase
{
    [Theory]
    [InlineData("operation1 result", HResult.Operation1Failed)]
    [InlineData("operation2 result", HResult.Operation2Failed)]
    [InlineData("operation3 result", HResult.Operation3Failed)]
    [InlineData("operation4 result", HResult.Operation4Failed)]
    [InlineData("never", HResult.Ok)]
    void acceptance_test(string failWhen, HResult expectedResult)
    {
        var sut = new SomeClass {FailWhen = failWhen};

        var result = sut.SomeFunction();

        result.Should().Be(expectedResult);
    }
}
```

Our case is a trivial one, but being the quiz supposed to be a job interview question, I would not ignore it.


### Step 2
The first refactoring could be getting rid of the mutable state: each if branch could just return the value, instead of mutating the variable `error`. Also, the name `error` is misleading, as it includes the success case. Let's just get rid of it:

```csharp
HResult SomeFunction()
{
    if(Succeeded(Operation1()))
    {
        if(Succeeded(Operation2()))
        {
            if(Succeeded(Operation3()))
            {
                if(Succeeded(Operation4()))
                    return HResult.Ok;
                else
                    return HResult.Operation4Failed;
            }
            else
                return HResult.Operation3Failed;
        }
        else
            return HResult.Operation2Failed;
    }
    else
        return HResult.Operation1Failed;
}
```

We got rid of the empty `if` body, making in the meanwhile the code slightly easier to reason about.

### Step 3
If now we invert each `if` statement (the step suggested by Sergio)

```csharp
internal HResult SomeFunction()
{
    if (!Succeeded(Operation1()))
        return HResult.Operation1Failed;

    if (!Succeeded(Operation2()))
        return HResult.Operation2Failed;

    if (!Succeeded(Operation3()))
        return HResult.Operation3Failed;

    if (!Succeeded(Operation4()))
        return HResult.Operation4Failed;

    return HResult.Ok;
}
```

we make it apparent that the code performs a chain of executions: if an operation succeeds, the next operation is invoked; otherwise, the chain is interrupted, with an error. The GOF [Chain of Responsibility Pattern][8] comes to mind. 

### Step 4
We could move each operation to a separate class, and let our function receive a chain of operations to execute in a single shot. Each class would deal with its specific operation logic (honoring the Single Responsibility Principle).

```csharp
internal HResult SomeFunction()
{
    var operations = new List<IOperation>
    {
        new Operation1(),
        new Operation2(),
        new Operation3(),
        new Operation4()
    };

    foreach (var operation in operations)
    {
        if (!_check.Succeeded(operation.DoJob()))
            return operation.ErrorCode;
    }

    return HResult.Ok;
}
```

We got rid of the `if`s altogether (but one).

Notice how:

* The interface `IOperation` has been introduced, which is a preliminary move to decouple the function from the operations, complying the with the Dependency Inversion Principle;
* The list of operations can easily be injected into the class, using the [Dependency Injection][9].
* The result validation logic has been moved to a separate class `Check`, injected into the main class (Dependency Inversion and Single Responsibility are satisfied).

```csharp
internal class SimpleStringCheck : IResultCheck
{
    private readonly string _failWhen;

    public Check(string failWhen)
    {
        _failWhen = failWhen;
    }

    internal bool Succeeded(string operationResult) =>
        operationResult != _failWhen;
}

```

We gained the ability to switch the check logic without modifying the main class (Open-Closed Principle).

Each operation has been moved to a separate class, like:

```csharp
internal class Operation1 : IOperation {
    public string DoJob()
    {
        return "operation1 result";
    }

    public HResult ErrorCode => HResult.Operation1Failed;
}
```

Each operation knows its own error code. The function itself became independent from it.

### Step 5
There is something more to refactor on the code

```csharp
foreach (var operation in operations)
{
    if (!_check.Succeeded(operation.DoJob()))
        return operation.ErrorCode;
    }

    return HResult.Ok;
}
```

* First, it's not clear why the case `return HResult.Ok;` is handled as a special case: the chain could contain a terminating operation never failing and returning that value. This would allow us to get rid of that last `if`.

* Second, our function still has 2 responsibility: to visit the chain, and to check the result.

An idea could be to encapsulate the operations into a real chain, so our function could reduce to something like:

```
return operations.ChainTogether(_check).Execute();
```

We have 2 options:

* Each operation knows the next operation, so starting from operation1 we could execute the whole chain with a single call;
* Operations are kept unaware of being part of a chain; a separate, encapsulating structure adds to operations the ability to be executed in sequence.

I'm going on with the latter, but that's absolutely debatable. I'm introducing a class modelling a ring in a chain, moving the code away from our class:

```csharp
internal class OperationRing : IRing
{
    private readonly Check _check;
    private readonly IOperation _operation;
    internal IRing Next { private get; set; }

    public OperationRing(Check check, IOperation operation)
    {
        _check = check;
        _operation = operation;
    }

    public HResult Execute()
    {
        var operationResult = _operation.DoJob();

        if (_check.Succeeded(operationResult))
            return Next.Execute();

        return _operation.ErrorCode;
    }
}
```

This class is responsible to execute an operation and to handle the execution to the next ring if it succeeded, or to interrupt the chain returning the right error code.

The chain will be terminated by a never-failing element:

```csharp
internal class AlwaysSucceeds : IRing
{
    public HResult Execute() => HResult.Ok;
}
```

Our original class reduces to:

```csharp
internal class SomeClass
{
    private readonly Check _check;
    private readonly List<IOperation> _operations;

    public SomeClass(Check check, List<IOperation> operations)
    {
        _check = check;
        _operations = operations;
    }

    internal HResult SomeFunction()
    {
        return _operations.ChainTogether(_check).Execute();
    }
}
```

In this case, `ChainTogether()` is a function implemented as an extension of `List<IOperation>`, as I don't believe that the chaining logic is responsibility of our class.


## That's not the *right* answer

It's absolutely debatable that the responsibilities have been separated to the most appropriate classes. For example: 

* is chaining operations a task of our function? Or should it directly receive the chained structure? 
* why the use of an enumerable? As Robert Martin wrote in "Refactoring: Improving the Design of Existing Code": enums are code smells and should be refactored to polymorphic classes;
* how much is too much? Is the resulting design too complex? Does the complexity of the whole application need this level of modularisation?

Therefore, I'm sure there are several other ways to refactor the original function.


  [1]: https://stackoverflow.com/users/125816
  [2]: https://en.wikipedia.org/wiki/SOLID
  [3]: https://en.wikipedia.org/wiki/Open%E2%80%93closed_principle
  [4]: https://en.wikipedia.org/wiki/Single_responsibility_principle
  [5]: https://en.wikipedia.org/wiki/Dependency_inversion_principle
  [6]: https://en.wikipedia.org/wiki/Cyclomatic_complexity
  [7]: https://en.wikipedia.org/wiki/Cohesion_(computer_science)
  [8]: https://en.wikipedia.org/wiki/Chain-of-responsibility_pattern
  [9]: https://en.wikipedia.org/wiki/Dependency_injection
  [10]: https://refactoring.guru/smells/primitive-obsession