# Optimistic Rollups vs ZK-Rollups By [0xhubman.eth](https://paragraph.com/@0xhubman.eth) ยท 2024-11-19 --- In this post, we'll be exploring a foundational concept of the Ethereum ecosystem: the differences between Optimistic Rollups and ZK-Rollups. Rollups are a layer 2 (L2) scaling solution that has been embraced by the ETH heads to help scale the Ethereum network. Rollups aren't the only scaling solution in town but they are by far the most popular amongst developers and adopted by end users. Popular examples of Optimistic Rollups include [Optimism](https://optimism.io/), [Base](https://www.base.org/), and [Arbitrum](https://arbitrum.io/). Popular examples of ZK-Rollups include [zkSync](https://zksync.io/), [Starknet](https://www.starknet.io/), and [Polygon zkEVM](https://polygon.technology/polygon-zkevm). Before diving into the differences between Optimistic Rollups and ZK-Rollups, let's examine the similarities they share to help conceptualize them: * Both help increase the transaction throughput of Ethereum mainnet * Both have cheaper gas costs than Ethereum mainnet through the use of batching transaction when posting them back to Ethereum mainnet * Both keep transaction data on-chain (i.e., on Ethereum mainnet), as opposed to off-chain via a data availability solution or some other mechanism Now that we know how these two types of rollups are similar, let's dive into the main difference between them. The main difference hinges of how these rollups approach data validation. In other words, how rollups validate the transactions that took place on L2 and post them back to Ethereum mainnet. For data validation, Optimistic Rollups utilize Fraud Proofs while ZK-Rollups utilize Validity Proofs. ### Fraud Proofs * When batching transaction and posting them back to Ethereum mainnet, Optimistic Rollups assume all batches are valid (i.e., they are optimistic everyone followed the rules and are not malicious) and put the burden on Verifiers to prove something is invalid via a Fraud Proof * Every time a batch is submitted, there is a dispute period where any Verifier can submit a Fraud Proof to invalidate a batch. If a batch is proven to be invalid, whoever submitted the invalid batch (i.e., the sequencer) has their staked ETH slashed, or confiscated, and given to the Verifier who submitter the Fraud Proof as a reward * Verifiers must also deposit some ETH with their submitted Fraud Proofs to avoid malicious Verifiers spamming Fraud Proofs * Due to this dispute period, withdrawal times from Optimistic Rollups back to Ethereum mainnet can take anywhere between 1-2 weeks * Additionally, due to the architecture of this validation process, Optimistic Rollups are much easier to build on top of and port over existing smart contracts from Ethereum mainnet * Optimistic Rollups were first to market and have seen widespread adoption; however, there is a ceiling on how much throughput these solutions can achieve using the Optimistic method compared to ZK-Rollups. ### Validity Proofs * When batching transactions and posting them back to Ethereum mainnet, ZK-Rollups provide immediate proof that submitted batches are valid via Validity Proofs * Validity Proofs utilize ZK-SNARKS, a mathematically complex cryptographic proof that verifies all transactions in the submitted batch * Due to immediately providing proof that a batch is valid, withdrawal times from ZK-Rollups to Ethereum mainnet happen relatively quickly (typically under 24 hours) * Due to the complex architecture of this validation process, ZK-Rollups are much more difficult to build on top of and usually are preferred for single-use applications (such as payments or NFT minting) instead of general purpose. However, with the launches of zkSync Era and Polygon zkEVM, these specific L2s provide an EVM-compatible environment for developers to build on top of, while also benefiting from the security of Validity Proofs. * Vitalik himself has stated in the long-run, ZK-Rollups (specifically zkEVM implementations) are the winners in the Ethereum scaling wars. Below are a few additional resources that do a great job of breaking down these complex topics as well. * Video - [ZK Rollups vs Optimistic Rollups](https://www.youtube.com/watch?v=qV6YbuWhDXM&ab_channel=DeFiTeller) * Video - [ZK Rollups and Optimistic Rollups Competing to Scale Ethereum](https://www.youtube.com/watch?v=wTPkpjGzAh0&ab_channel=TheDefiant-DeFi%2CWeb3%26NFTInsights) * Video - [Optimistic vs ZK-Rollups: Which is Better](https://www.youtube.com/watch?v=7B-dbMuG6hI&ab_channel=CoinGecko) * [L2Beat](https://l2beat.com/scaling/tvl) Until next time! --- *Originally published on [0xhubman.eth](https://paragraph.com/@0xhubman.eth/optimistic-rollups-vs-zk-rollups)*