OptimisticRollup (ORU) has gained wide recognition, with companies such as FuelLabs, OffchainLabs, Optimism, and Hubble building the OptimisticRollup infrastructure.

Recommended reading: "How is the Ethereum Layer 2 ecosystem organized?"

Original title: "Introduction to OptimisticRollup: A Simple Explanation"

Written by: John Adler

Translated and proofread by: Min Min and Ajian

OptimisticRollup is currently the most promising scalability solution for Ethereum and all blockchains. But what is OptimisticRollup (ORU)? Why are Ethereum developers and scalability researchers so excited about it?

In this article, we will introduce the basics of ORU in a simple and easy-to-understand manner. If you want to dive deeper into the technical details, please refer to this article (Chinese translation).

Features

ORU has many desirable features that are unparalleled among all blockchain scalability technologies. A well-designed ORU system will have the following characteristics:

Trustless: Unlike traditional sidechain technologies, ORU is trustless (or more professionally, it has minimal trust requirements). You can withdraw your funds from the Rollup at any time without relying on the majority of block producers on ORU to be honest.Permissionless: Unlike Plasma, ORU is permissionless. Anyone can become a block producer on ORU because all block data on the Rollup is published on Ethereum and can be obtained from Ethereum. The selection of the next leader is an implementation issue, not a fundamental limitation.Non-custodial: As mentioned above, due to the trustlessness and permissionlessness of ORU, you can withdraw your funds at any time and no one can stop you. Therefore, ORU is non-custodial.Expressive: Unlike ZKRollup, ORU (both theoretically and practically) has high expressiveness. It can handle both Bitcoin-like UTXO payments and mature EVM-compatible execution.Open participation: Unlike payment channels, ORU supports smart contracts and is open to everyone, just like Uniswap.Capital efficiency: Unlike payment channels, ORU does not require users to lock up funds in advance.Resistant to on-chain congestion: Unlike payment channels and Plasma, ORU can withstand on-chain congestion because its fraud proofs are at the block level, not like the closure mechanism of payment channels or the exit mechanism of Plasma.No new cryptography required: Unlike ZKRollup, ORU does not require any new cryptography.Fast (non-instant) finality: Unlike ZKRollup, ORU does not require generating proofs, so ORU blocks can be immediately published to Ethereum. Once these blocks are published on Ethereum, they can obtain finality similar to Ethereum because valid ORU blocks cannot be rolled back.

A Brief History of OptimisticRollup

One of the earliest initiatives to enhance Bitcoin scalability was sidechains. Sidechains are blockchains that run alongside the parent chain but have different characteristics, such as shorter block times, larger block sizes, and stronger expressive power for smart contracts. However, traditional sidechains have a fatal flaw: if the majority of miners/validators on a sidechain are dishonest, user funds can be stolen.

Over the years, many technologies have been trying to enhance the security of sidechains to ensure that user funds are not stolen even when the majority of participants are dishonest (this is called trust-minimized two-way pegging). Earlier examples include merged mining and shadow chains, followed by Plasma and ZKRollup. Interestingly, prior to the emergence of ORU, a similar solution was delayed state execution under the sharding mechanism (we will talk about this soon!).

The culmination of this research is what we now know as OptimisticRollup. In June 2019, the article "The Minimal Viable Merge Consensus" first outlined this technology. Since then, the Ethereum community has been strongly supporting ORU as a scalability solution for Ethereum-style smart contract execution, without having to wait for Serenity Phase 2.

How OptimisticRollup Works

Visualization of OptimisticRollup

As a trust-minimized two-way pegged sidechain (in other words, even if every validator on the sidechain is dishonest, funds cannot be stolen), ORU is exceptionally simple in its operation (of course, my article on the "principle" explains it in more detail).

The aggregator collects transactions from the Rollup, packages them into Rollup blocks, and sends the Rollup block along with a