Blockchain and Ethereum

Why is data replicated in distributed systems?

For reliability,
fault tolerance,
performance,
ow latency,
recovery.

What problem does message ordering cause?

Different nodes may process messages in different orders, leading to inconsistent state.

What is Total Order Broadcast?

m1 delivered before m2 on one node

m1 delivered before m2 on all nodes

What is State Machine Replication (SMR)?

Replicates state machine across servers
execute same inputs in same order
maintain identical state

Why are blockchains examples of SMR?

All nodes process the same transactions in the same order to reach the same state.

What is a blockchain?

A decentralized, distributed, append-only ledger shared across many nodes.

Why are blocks linked with hashes?

To ensure immutability and detect tampering.

What happens if a past block is modified?

All subsequent blocks become invalid.

What does the blockchain state represent?

Ownership of coins and all smart contract data.

What is a state transition?

Applying a transaction to the current state to produce a new state.

Why does a transaction need a nonce?

to prevent replay attacks and enforce ordering per account.

What is the purpose of consensus?

To agree on the next valid block in a decentralized system.

Why is Proof of Work inefficient?

It requires massive computational power and energy consumption.

What is staking in Proof of Stake?

Locking ETH as collateral to participate as a validator.

What is a slot in Ethereum PoS?

A 12-second time window where a block can be proposed.

What is an epoch?

A group of 32 slots

What causes slashing?

Dishonest or malicious validator behavior.

What is the Ethereum Virtual Machine?

The global computer that executes smart contracts and whose state everyone agrees on

Why does every node execute every transaction?

For trust and consensus, not performance.

What problem does gas solve?

Infinite loops and denial-of-service attacks.

What are smart contracts

programm running on blockchain
self executing
immutable
decentralized application: like voting or currencies

What is the difference between view and non-view functions?

iew functions read state (no gas externally), non-view functions modify state and cost gas.