The “Why”s of Optimistic Rollup

What Is Optimistic Rollup?

I wrote the first minimal viable spec for what is now known as optimistic rollup back in June. This spec provided a concrete parametrized high-level spec to the more general ideas introduced in my earlier paper, Building Scalable Decentralized Payment Systems (co-authored with Mikerah). The goal was to enable for the first time in blockchain history a permissionless, trust-minimized, scalable side chain.

Not this one. (Source)

I’ll Do You One Better: Why Is Optimistic Rollup?

As it turns out, there are a number of subtle differences between optimistic rollup and previous scaling proposals, making the former the most promising short- to medium-term scaling solution, and the latter mostly relegated to the history books. This section covers the intuitions behind these critical differences.

Merged Consensus

One of the key distinguishing features of optimistic rollup is merged consensus, a consensus protocol that is verifiable on-chain (save for actual block validation, which is done implicitly through fraud proofs). But what is a decentralized consensus protocol?

  • A fork choice rule (how to choose between two otherwise valid chains)
  • A block validity function (state transition function)
  • A leader selection algorithm (who is permitted to attempt to progress the chain by extending its tip with a new block)
  • A Sybil resistance mechanism (Proof-of-Work, Proof-of-Stake, etc.)

Sustainable Scaling

Now that we know how and why optimistic rollup is permissionless through merged consensus, why is it sustainably scalable?

Non-Interactiveness FTW

Optimistic rollup places a huge emphasis on non-interactive fraud proofs. Why?

Demystifying Transaction Latency Claims

Optimistic rollup does not reduce transaction latency. Every side chain block needs to be committed to the main chain, so you don’t get blocktimes lower than the main chain. Barring the use of fully-collateralized channels, there is no known secure and trustless way of reducing this latency.

Optimistic rollup gives you instant, gas-free transactions!

On Data Availability Challenges

The original paper suggested (implicitly and explicitly) three ways of solving the data availability problem:

  1. Post all data on-chain all the time
  2. Use data availability challenges to only post the data when needed
  3. Use data availability proofs

Further Improvements

I’ve written a number of performance improvements that build upon the original minimal spec, summarized here. Note that the focus is mostly on UTXO-based payments and predicates rather than general smart contract execution.

Teams Building Optimistic Rollup

Since the writing of the first optimistic rollup minimal spec months ago, I was excited to discover that many developer teams have started building out implementations of optimistic rollup, with various twists and modifications. Here’s a non-exhaustive list (order, inclusion, or exclusion should not be taken as endorsement or critique) of what we’ve seen publicly so far:

Conclusions

The coming year will be a game-changer for Ethereum, as projects implementing optimistic rollup go live, massively increasing the throughput of Ethereum and driving further innovation on the scalable, sustainable data availability front.

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