What are Ethereum Layer 2 solutions? Please explain L2 scaling technologies such as Rollups and State Channels - 面试题

Ethereum Layer 2 (L2) solutions are scaling solutions built on top of the Ethereum mainnet, aiming to increase transaction throughput, reduce transaction costs, while maintaining Ethereum's security. Here's a comprehensive analysis of Layer 2:

Basic Concepts of Layer 2

Layer 2 refers to second-layer networks built on top of the Ethereum mainnet (Layer 1). By moving some computation and storage off-chain, L2 achieves higher performance and lower costs. L2 ultimately submits transaction data to L1, inheriting L1's security.

Main Types of Layer 2

1. Rollup Solutions

Rollups are currently the most mainstream L2 solutions, executing transactions off-chain and publishing compressed transaction data to L1.

Optimistic Rollups

Assume transactions are valid and only re-execute when someone raises a challenge.

Representative Projects:

Arbitrum: Uses fraud proof mechanism
Optimism: Simplified fraud proof system

How It Works:

Sequencer collects and executes transactions
Publishes transaction data to L1
Validators submit state roots
Challenge period (about 7 days) allows fraud proof submission
State becomes final after challenge period

Advantages:

High generality, supports EVM
Low Gas costs (about 1/10 of mainnet)
Mature ecosystem, complete tooling

Disadvantages:

Withdrawal delays due to challenge period
Complex fraud proof mechanism

Zero-Knowledge Rollups (ZK-Rollups)

Use Zero-Knowledge Proofs (ZKP) to verify transaction validity.

Representative Projects:

zkSync: Developed by Matter Labs
StarkNet: Developed by StarkWare, uses STARK proofs
Polygon zkEVM: Polygon's ZK-Rollup solution

How It Works:

Sequencer collects and executes transactions
Generates zero-knowledge proofs
Publishes transaction data and proofs to L1
L1 verifies proofs, fast finalization

Advantages:

Fast withdrawal speed (no challenge period needed)
Security based on mathematical proofs
High data compression ratio

Disadvantages:

Limited generality (some solutions not fully EVM compatible)
Complex proof generation

2. State Channels

Participants conduct multiple transactions off-chain, only interacting with L1 when opening and closing channels.

Representative Projects:

Raiden Network: Ethereum payment channels
Connext: Cross-chain payment network

How It Works:

Participants lock funds on L1
Conduct multiple transactions off-chain
Update channel state
Submit final state to L1 when closing channel

Advantages:

Instant transaction confirmation
Extremely low Gas costs
High privacy

Disadvantages:

Requires participants to be online
Not suitable for complex applications
Locked funds

3. Sidechains

Independent blockchains connected to Ethereum mainnet through bridges.

Representative Projects:

Polygon: Most successful sidechain solution
Avalanche C-Chain: EVM-compatible sidechain

How It Works:

Independent consensus mechanism and validators
Communicates with L1 through two-way bridges
Periodically submits state snapshots to L1

Advantages:

High throughput
Low transaction costs
Complete independence

Disadvantages:

Security depends on own validators
Does not inherit L1's security

4. Plasma

Execute transactions on child chains and periodically submit Merkle roots to L1.

Advantages:

High scalability
Low costs

Disadvantages:

Data availability issues
Complex exit mechanism
Currently less used

Layer 2 Comparison

Feature	Optimistic Rollup	ZK-Rollup	State Channel	Sidechain
Security	High (inherits L1)	High (inherits L1)	High (inherits L1)	Medium (independent validators)
Finalization Time	~7 days	Minutes	Instant	Minutes
Gas Cost	Low	Lowest	Lowest	Low
Generality	High	Medium	Low	High
Withdrawal Speed	Slow	Fast	Fast	Fast

Technical Advantages of Layer 2

1. Scalability

Transaction throughput increased 100-1000x
Supports large-scale applications

2. Cost Reduction

Gas fees reduced by 90-99%
Makes small transactions feasible

3. User Experience

Faster confirmation times
Lower latency

4. Compatibility

Most L2s support EVM
Can directly port DApps

Layer 2 Development Practices

1. Choose Appropriate L2 Solution

javascript
// Configure L2 networks using Hardhat
module.exports = {
  networks: {
    arbitrum: {
      url: "https://arb1.arbitrum.io/rpc",
      chainId: 42161,
      accounts: [privateKey]
    },
    optimism: {
      url: "https://mainnet.optimism.io",
      chainId: 10,
      accounts: [privateKey]
    }
  }
};

2. Bridge Assets

javascript
// Use official bridge contract
const bridgeContract = new ethers.Contract(
  bridgeAddress,
  bridgeABI,
  signer
);

// Deposit to L2
await bridgeContract.deposit(
  tokenAddress,
  amount,
  l2Recipient
);

3. Deploy Smart Contracts

javascript
// Deploy contracts on L2
const MyContract = await ethers.getContractFactory("MyContract");
const contract = await MyContract.deploy();
await contract.deployed();
console.log("Contract deployed to:", contract.address);

Future Development of Layer 2

1. Technical Evolution

Improved EVM compatibility for ZK-Rollups
Hybrid Rollup solutions
Cross-L2 interoperability

2. Ecosystem

More DApps migrating to L2
Native L2 applications
Improved developer tools

3. Standardization

L2 bridging standards
Cross-chain communication protocols
Unified user experience

Common Questions

Q: Is Layer 2 secure?

A: Most L2 solutions inherit L1's security, especially Rollups. Sidechains' security depends on their own validators.

Q: How to choose an L2 solution?

A: Choose based on application needs:

Need fast finalization: Choose ZK-Rollup
Need full EVM compatibility: Choose Optimistic Rollup
Need high-frequency small payments: Choose State Channel
Need independent ecosystem: Choose Sidechain

Q: Are funds safe on L2?

A: Funds on L2 are protected by smart contracts. As long as the L2 protocol itself has no vulnerabilities, funds are secure.

Layer 2 is a key technology for Ethereum scaling and is driving the widespread adoption and popularization of blockchain technology.