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What is a Rollup?

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A Rollup is scratch paper for Ethereum. Rollup handles all the complex data processing, allowing each Ethereum block to accommodate multiple smart contract transactions.

原文标题：《 A Non-Technical Introduction to Rollups 》

原文作者： Benjamin Simon

Original source: Ethereum enthusiasts

原文编译：闵敏 & 阿剑

This article was published on June 1, 2021. 1.

introduce

Let's start with an analogy.

Imagine that we're back in our junior high school math class. Our "cruel" junior high math teacher gave us each a test paper with a list of 100 large division questions. Our task is to solve as many problems as possible. However, there is a catch: we can only write the final solution and each calculation step on an answer sheet. After a few minutes, we had a sense of foreboding, and the more we thought about it, the more ridiculous the task seemed. After a few questions, the answer sheet was full.

What does this nightmare story have to do with Rollup?

In this story, the answer sheet is the Ethereum block and the arithmetic is the smart contract transaction. Ethereum is very congested right now. There are too many transactions to include in each block. To make matters worse, the vast majority of transactions are computationally intensive, such as flash loans and transactions routed through aggregators. With the rise of Defi, simple transfers and payments are a thing of the past.

That's the crux of the matter. Just as a math teacher requires us to write down every calculation step on an answer sheet, Ethereum has to process and record every calculation of every transaction.

Until Rollup shows up.

Rollup moves computation down the chain and stores only the minimum amount of transaction data on the chain. In this basic sense, a Rollup is scratch paper for Ethereum. Rollup handles all the complex data processing, allowing each Ethereum block to hold multiple smart contract transactions.

This metaphor not only gives an intuition of the problem Rollup aims to solve (" network congestion due to computing overload "), but also gives an idea of what Rollup's solution needs to do (" move computing down the chain "). However, how Rollup actually works, how it is presented, and what makes it attractive is something that we need to further investigate.

What is a Rollup

Rollup is essentially a separate blockchain, but with some modifications. Like Ethereum, the Rollup protocol uses a "virtual machine" to execute smart contract code. Rollup's virtual machine runs independently of Ethereum's own virtual machine (EVM), but is managed by a smart contract on Ethereum. Therefore, this connection allows Rollup and Ethereum to communicate with each other. Rollup is responsible for executing the transaction and processing the data, while Ethereum is responsible for receiving and storing the results. 2.

Technically, the main difference between a Rollup chain and a traditional blockchain is the way blocks are produced.

Typically, a blockchain is maintained by a distributed network of participants (" miners "or" validators, "depending on the type of chain). These participants create blocks by consensus. In simple terms, participants vote to decide how to process a set of transactions (how to create the next block). Blocks with a majority of support are written to the blockchain forever.

In contrast, Rollup chains do not operate on majority rule. The party monitoring the state of the Rollup can send an "assertion" to Ethereum to explain how the transaction is being processed. Importantly, Ethereum will decide whether or not to accept this assertion, whether or not it is supported by a majority of the participants on Rollup. (3) & have spent This usually means that on Rollup, only one party is actually responsible for processing transactions and generating blocks. ④ (Translator: This is not absolute)

Is Rollup centralized

Rollup's ability to process transactions so efficiently is partly due to its centralized nature in terms of block production. However, it also raises the worrying question of how Rollup can ensure that blocks are produced correctly in the absence of majority consensus -- and what if the block creators are bad.

Since cryptocurrency users are used to consensus-based blockchains, this centralization can be confusing. Indeed, if this article ends here, we might mistake Rollup for just "a database replicated by a single participant," as Avalanche cofounder Kevin Seqniqi recently tweeted.

In fact, it soon becomes clear that the concerns about Rollup's centralized nature are more logically misguided than misplaced. In general, a centralized blockchain without a strong consensus mechanism is indeed vulnerable to disruption and malicious manipulation. However, in the case of Rollup solutions, the lack of decentralization does not affect their security and integrity. (5) & have spent To understand why, we need to take a deeper look at how Rollup works.

Importance of data availability

Think back to the math paper mentioned at the beginning of the article. Scratch paper is our life saver. As long as we write down the calculations on scratch paper, we just need to write down the problems and solutions on the answer sheet. Rollup is like scratch paper, "moving smart contract computing down the chain and storing the minimum amount of transaction data on the chain."

In fact, the latter part of the statement (storing transaction data on the chain) is critical to how Rollup operates. As far as ROLLUP is concerned, only computation (data processing) is moved down the chain. The input data (the "call data") for each transaction handled by Rollup is still stored on Ethereum.

Why is it important to keep transaction data on the chain? Take the math test paper for example, we finally handed over to the teacher on the answer sheet only math problems and solutions. That way, even if we didn't write down every calculation step, the teacher could check our work. Similarly, the availability of data on an eternal chain means that any computation on a Rollup can be repeated by the Ethereum base layer. There is an upper limit to the amount of computation Ethereum can perform in a single transaction).

In short, Rollup's on-chain data availability introduces a built-in review process. Ethereum can reconfirm the integrity of transactions processed on Rollup before permanently writing them into the ledger -- much like the U.S. Supreme Court's right to judicial review. 6.

Limited advantage

The Rollup's main advantage is its limitations. Rollup can only send transactions to the base layer and cannot force the base layer to accept them because Ethereum can override any Rollup transaction if necessary. Rollup transactions have no true finality due to the review process.

With the limitations of Rollup in mind, we return to the centralization issue mentioned above. Rollup's one-sided block creation does have the potential for evil. However, when this happens, Ethereum rejects the transaction after completing the review process and penalizes the blocker. All landowners

For Optimistic Rollup and ZK Rollup, the "review process" works in different ways (see details here). However, regardless of the Rollup, the review process is far more efficient than Ethereum processing the transaction itself (without relying on Rollup).

In summary, the Rollup system is based on "checks and balances" that guarantee Ethereum's sovereign chain status. The Ethereum consensus is the final arbiter of facts. Notice that the side chain is completely different. Side Chain doesn't have a review process like Rollup does, but instead has a separate consensus mechanism to process transactions. The side chain can give a transaction finality directly, whereas a transaction on Rollup can't get finality until Ethereum confirms it. As a result, side chains need more assumptions of trust because they cannot benefit from Ethereum's decentralized security. In fact, I think side chains are more like EVM-compliant Layer 1 than Rollup.

conclusion

Overall: Rollup moves computations down the chain, freeing up more space on the chain. On-chain data availability is important because it allows Ethereum to review the integrity of Rollup transactions. In turn, the review process can check the ROLLUP block, eliminating the need for a consensus mechanism.

Rollup could ultimately allow Ethereum to have its cake and eat it: freeing up multiple times as much space on the chain without breaking the network's decentralized security. In my opinion, at least, this is exactly what we would expect from an elegant extensibility solution.

Thanks to t11s and Hasu for your insightful comments and suggestions.

footnotes

① Note: I recently started a series of articles about Rollup on Deribit Insights. Part 1 explores the differences between ZK Rollup and Optimistic Rollup. Many readers have asked me to recommend a non-technical primer on Rollup, since my article requires some background knowledge. In fact, there are some general science articles that will help a lot. However, I haven't seen a comprehensive introduction to beginners without any technical jargon. My goal in writing this article is to fill that gap.

Incidentally, this connection is maintained on the chain through something called the "state root" (the state root tracks the internal state of the Rollup, including account balances, transactions, code, etc.). For more details, read Vitalik's introduction.

③ In fact, in Optimistic Rollup, the block maker can (correctly) handle the transaction even if the other network participants object. This is the 1/n honest participant assumption of Optimistic Rollup (see the section "Secuirty Considerations" in this article if you want to learn more about this).

④ Optimistic Rollup and ZK Rollup have different responsibilities for the block producer. If you want to learn more about these Considerations, please read the section called "Secuirty Considerations" in this article.

⑤ Haseeb Qureshi has a very thought-provoking article on decentralization. Qureshi's argument is that decentralization per se is not necessarily good, and centralization per se is not necessarily bad. Although Qureshi doesn't discuss Rollup in this article, his argument is highly relevant.

(6) In Marberly v. Madison in 1803, the Marshall Court ruled that the Supreme Court had the power to strike down congressional legislation it deemed unconstitutional, effectively giving the Court final review of all legislative matters.

⑦ In fact, ZK Rollup uses cryptography to prevent its block makers from doing evil things in transaction processing, because the zero-knowledge proof of the related transaction itself is invalid.