Blockchain Smart Contract Fundamentals Questions and Answers

Question 1

A developer deploys a smart contract to the Ethereum mainnet. After deployment, they discover a logical bug in one of the functions. What is the primary consequence of blockchain's immutability in this scenario?

Accepted Answer

The contract's code is permanently unchangeable, and the bug cannot be directly fixed in the deployed version.

Answer

Immutability is a core feature of blockchains, meaning that once data (like a smart contract) is written, it cannot be altered or deleted. This ensures security and trust but also means that any bugs in a deployed contract are permanent in that specific instance. Mitigation requires deploying a new, corrected contract and migrating state, or having implemented an upgradeability pattern (like a proxy) beforehand.

Question 2

What is the primary function of the Ethereum Virtual Machine (EVM)?

Accepted Answer

To provide a sandboxed, deterministic execution environment for smart contracts.

Answer

The EVM is the computation engine of the Ethereum network. It acts as a decentralized, sandboxed virtual machine that executes smart contract bytecode. This isolated environment ensures that smart contracts run exactly as written on all nodes in the network without interfering with the host system, which is essential for maintaining consensus.

Question 3

A smart contract is designed to trigger an insurance payout for a flight delay. To function correctly, it needs to know the official landing time of a specific flight. How does a smart contract on a deterministic blockchain typically obtain this type of real-world information?

Accepted Answer

Through a trusted external data feed service known as a blockchain oracle.

Answer

Blockchains and smart contracts cannot directly access off-chain data because it would break the deterministic nature required for consensus. A blockchain oracle is a necessary third-party service that acts as a bridge, fetching and verifying real-world data (like flight times or asset prices) and relaying it to the smart contract in a secure and reliable way.

Question 4

A smart contract has a `withdraw` function where the code first sends Ether to the user's address and then updates the user's balance in the contract's storage. Which classic smart contract vulnerability does this specific order of operations create?

Accepted Answer

Reentrancy Attack

Answer

This sequence creates a reentrancy vulnerability. A malicious contract can exploit this by implementing a fallback function that recursively calls the `withdraw` function *after* receiving the Ether but *before* the original call updates the balance. This allows the attacker to drain the contract's funds. The secure pattern is Checks-Effects-Interactions: update the balance first, then send the Ether.

Question 5

Which of the following best describes the role of "gas" in the context of the Ethereum blockchain?

Accepted Answer

A unit of measurement for the computational effort required to execute operations.

Answer

Gas is the unit that measures the amount of computational work required to perform operations, such as transactions or smart contract executions, on the Ethereum network. Each operation has a fixed gas cost. The total transaction fee (paid in ETH) is the amount of gas used multiplied by the gas price (in Gwei), which prevents network spam and compensates validators for their computational resources.

Blockchain Practice Test

Blockchain Practice Test

Blockchain Smart Contract Fundamentals Questions and Answers