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Solidity Engineer Interview Tutorial

Welcome to the interview tutorial for the Solidity Engineer position. This interview is divided into three sessions based on the candidate's experience level: Junior, Middle, and Senior. Each session contains both theory questions and practical coding tasks to be completed or optionally attempted by the candidate.

Reason for Creating This Repository

I created this repository because I noticed that many HR professionals are often confused about the technical requirements and standards needed for the Solidity Engineer position. Therefore, I wanted to create a comprehensive and accessible resource to help both HR professionals and candidates understand what is expected during an interview.

Interview Sessions

1. Junior Solidity Engineer

Goal: To test the candidate’s basic understanding of Solidity, smart contract structure, and ability to complete simple tasks.

Theory Questions:

  • What is a smart contract in the context of blockchain? Explain how it works and its functions.
  • What are the differences between public, internal, and private modifiers in Solidity?
  • Explain what Gas is and how Gas works during transaction execution in Ethereum.
  • What are require() and assert() in Solidity? Explain the differences and their usage.

Practical Tasks (Mandatory):

  1. Create a simple smart contract that allows users to store and retrieve an integer (uint256).

    • Functions: storeNumber(uint256 _number) to store a number, and getNumber() to retrieve the stored number.
    • Unit Testing: Write unit tests to verify that numbers can be stored and retrieved correctly.
    • Documentation: Provide comments in your code explaining each function's purpose.

  2. Implement a function that allows users to reset the stored number to zero.

    • Function: resetNumber().
    • Unit Testing: Write tests to ensure that the reset functionality works as expected.
    • Documentation: Include explanations of how this function interacts with other functions.

  3. Add an event that logs when a number is stored.

    • Event: NumberStored(uint256 indexed _number).
    • Unit Testing: Test that the event is emitted correctly when a number is stored.
    • Documentation: Explain the significance of events in smart contracts.

2. Middle Solidity Engineer

Goal: To assess the candidate’s deeper understanding of smart contracts, gas optimization, access control, and library usage.

Theory Questions:

  • Explain the concept of Inheritance in Solidity and provide an example of its usage.
  • What is a Reentrancy Attack and how can it be prevented in smart contracts?
  • How is Gas optimization achieved in Solidity contracts? Provide tips and tricks to reduce gas consumption.
  • What is a fallback function and how is it used in a smart contract?

Practical Tasks (Mandatory):

  1. Create a smart contract that stores balances in the form of ERC20 tokens using OpenZeppelin's ERC20 implementation. Allow users to transfer tokens and track their balances.

    • Functions: transfer(address _to, uint256 _amount) and balanceOf(address _account).
    • Unit Testing: Write tests to verify correct token transfers and balance tracking.
    • Documentation: Document each function's purpose and how it interacts with OpenZeppelin's ERC20 library.

  2. Implement a function that allows users to approve another address to spend tokens on their behalf using OpenZeppelin’s approve method.

    • Function: approve(address _spender, uint256 _amount).
    • Unit Testing: Ensure that approval works correctly by testing various scenarios.
    • Documentation: Explain how approvals work within ERC20 standards.

  3. Create a simple voting mechanism that allows token holders to vote on proposals where each vote is weighted by the amount of tokens held.

    • Features: vote(uint256 proposalId) where each vote counts based on token balance.
    • Unit Testing: Write tests to validate that votes are counted correctly based on token holdings.
    • Documentation: Provide an overview of how voting rights are determined by token ownership.

3. Senior Solidity Engineer

Goal: To evaluate the candidate’s ability to design complex smart contract systems, manage high-level security, and implement efficient patterns.

Theory Questions:

  • What is Delegatecall and how does it work in smart contracts?
  • Explain how Oracles work in the Ethereum ecosystem and provide examples of their usage in smart contracts.
  • How can you implement Upgradable Contracts using the proxy pattern in Solidity?
  • What are the potential risks and challenges in creating smart contracts that interact with multiple other contracts in the blockchain ecosystem?

Practical Tasks (Mandatory):

  1. Implement a Crowdfunding contract that allows users to fund a project with ERC20 tokens using OpenZeppelin's ERC20 library. Ensure that the contract validates whether the project has reached its funding target before funds can be withdrawn.

    • Features: contribute(uint256 _amount), withdraw(), and getFundingStatus().
    • Unit Testing: Write comprehensive tests for all functionalities, including contribution limits and withdrawal conditions.
    • Documentation: Include detailed comments explaining each feature's logic and security considerations.

  2. Design a Decentralized Autonomous Organization (DAO) system that allows token holders to submit proposals and vote on them using OpenZeppelin's governance libraries.

    • Features: Proposals can include changes to rules or token distribution. Each vote is weighted by token ownership, requiring quorum for approval.
    • Unit Testing: Ensure all DAO functionalities work as intended through rigorous testing.
    • Documentation: Provide insights into how governance works within your DAO design.

  3. Create an upgradable contract system using OpenZeppelin's proxy pattern that allows for future upgrades without losing state or data.

    • Implement functions for upgrading logic while maintaining user data integrity.
    • Unit Testing: Write tests to confirm that upgrades do not affect existing data or functionality.
    • Documentation: Explain the proxy pattern's advantages for upgradability in your documentation.

Submission Instructions

  • Junior: Submit your smart contract in .sol file format via email or GitHub repository along with unit tests and documentation.
  • Middle: Include a brief description of your approach for both mandatory tasks, unit tests, and documentation explaining your design choices.
  • Senior: Provide comprehensive documentation on your contract design, including security considerations, gas optimization strategies, unit test coverage, and explanations for each feature implemented.

We expect practical tasks to be completed within 2 to 4 hours. Please also provide explanations for your design choices during implementation.

Good luck! We look forward to reviewing your work and discussing your solutions further.