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Rollup

Rollup

Overview

Rollup is a blockchain Layer 2 scaling solution. Its core idea is to "roll up" a large number of off-chain transactions, compress the transaction data, and publish it on-chain, thereby reducing the difficulty and cost of transaction verification. Through this approach, Rollups can significantly increase blockchain throughput while maintaining security.

An operator collects user transactions off-chain, batch-processes them, and executes a smart contract on-chain to submit the packaged transaction data to the contract. This is equivalent to executing a batch of off-chain transactions at once, but recording only a single transaction on-chain, achieving a significant scaling effect.

Two Main Types

ZK-Rollup (Zero-Knowledge Rollup)

ZK-Rollup uses zero-knowledge proof technology to verify transaction validity. Each batch of transactions uploaded to the chain is accompanied by a validity proof, typically using zero-knowledge proof technologies like zk-SNARK or zk-STARK. The smart contract verifies this proof to confirm that all transactions are valid, without needing to re-execute each transaction.

Data submitted on-chain includes: compressed transaction data, the state Merkle tree roots before and after transaction execution, and the zero-knowledge proof. Since the proof can be verified instantly, users can typically complete fund withdrawals within approximately 10 minutes.

Optimistic Rollup

Optimistic Rollup employs an "optimistic" assumption and a Fraud Proof mechanism. When each batch of transactions is submitted on-chain, the smart contract assumes by default that they are correct and does not require immediate verification. If someone discovers and successfully proves that a transaction is fraudulent, the smart contract rolls back the incorrect state.

The challenge period for this solution is typically 7 days, meaning users need to wait one week to withdraw funds from Layer 2 to Layer 1. However, within Layer 2 itself, transactions are confirmed instantly, providing a smooth user experience.

Core Advantages

  • High Throughput: By batch-processing transactions, TPS can reach thousands or even tens of thousands
  • Low Cost: Transaction fees can be reduced to 1-5% of the main chain's costs
  • Inherited Security: Data and final state are stored on the main chain, inheriting the main chain's security guarantees
  • EVM Compatibility: Most Rollup solutions are compatible with the Ethereum Virtual Machine, allowing developers to easily migrate applications

Market Landscape

According to L2Beat statistics, as of June 2022, Optimistic Rollups accounted for 74.3% of the total value locked in Ethereum Layer 2 networks, while zero-knowledge proof solutions accounted for 25.9%. As technology develops, ZK-Rollup's market share is gradually increasing.

Major projects include: Arbitrum, Optimism (Optimistic Rollup), zkSync, StarkNet, Polygon zkEVM (ZK-Rollup), and others.

Rollup has become the mainstream path for Ethereum scaling. The Ethereum roadmap explicitly positions Rollup as the core scaling strategy, further reducing Rollup costs through upgrades like Data Availability Sampling (DAS) and EIP-4844. In the future, interoperability and unified liquidity across multiple Rollup chains will also become important development directions.