Knowledge base
7 mins
What Are Optimistic Rollups?

Enter optimistic rollups – a Layer 2 scaling solution designed to alleviate bottlenecks by processing transactions off of the main Ethereum chain (Layer 1) and only settling final states on-chain


Ethereum's scalability issue, characterized by slow transaction speeds and high fees, has been a persistent hurdle. Enter optimistic rollups – a Layer 2 scaling solution designed to alleviate these bottlenecks by processing transactions off of the main Ethereum chain (Layer 1) and only settling final states on-chain. This innovation not only promises to boost Ethereum's transaction throughput but also to significantly reduce transaction fees, making Ethereum more accessible and efficient for users and developers alike.

How Optimistic Rollups Work

Delving deeper into the workings of optimistic rollups reveals a blend of ingenuity and technical sophistication aimed at solving Ethereum's scalability and transaction speed challenges. At their core, optimistic rollups are a Layer 2 scaling solution. They handle transactions outside of the main Ethereum blockchain (Layer 1), but in a way that transactions can be verified and settled on the main chain. This process not only increases the number of transactions that can be processed per second but also significantly reduces transaction fees by aggregating multiple transactions into a single transaction on the Ethereum blockchain.

Transaction Batching and State Roots

Optimistic rollups work by aggregating or "batching" numerous transactions off-chain and then submitting them to the Ethereum network as a single transaction. This aggregation takes place within a smart contract on the Ethereum blockchain, known as the rollup contract. The essential components of this process are the state roots and Merkle roots, which serve as cryptographic proofs of the state of transactions within the Rollup.

**State Roots: **These are cryptographic hashes that represent the state of the rollup at a given time. Each batch of transactions submitted to the Ethereum mainnet includes a pre-state root (representing the state before the batch of transactions) and a post-state root (representing the state after the transactions). This allows the Ethereum network to verify the integrity of the transactions processed by the rollup.

Merkle Roots: The Merkle root is a single hash that summarizes all the transactions in a batch. It enables anyone to verify the inclusion of a specific transaction in the batch by providing a Merkle proof without having to reveal the entire set of transactions. This feature is critical for maintaining privacy and efficiency in transaction processing.

Sequencers and Data Availability

A sequencer is a node within the optimistic rollup network responsible for aggregating transactions, generating state roots, and submitting them to the Ethereum blockchain. The sequencer plays a crucial role in ensuring data availability. It does so by posting the data associated with each batch of transactions (e.g., calldata) on-chain, allowing other nodes to reconstruct the state of the rollup if needed. This mechanism ensures the security and integrity of the rollup, even if the sequencer behaves maliciously or becomes unavailable.

Fraud Proofs and Dispute Resolution

One might wonder: if transactions are processed off-chain, how can we ensure their validity? This is where the concept of fraud proofs and dispute resolution comes into play.

The dispute resolution mechanism in optimistic rollups is a critical component designed to ensure the integrity and security of transactions processed off the Ethereum mainnet. This mechanism allows any participant in the network to challenge the validity of transactions within a specific period after they have been posted to the Ethereum blockchain. Here's a detailed breakdown of how it works:

1. The Challenge Window

After a batch of transactions has been submitted to the Ethereum mainnet by the sequencer, there's a predefined challenge period or window. This period is crucial as it provides time for network participants to review the batched transactions for any discrepancies or fraudulent activities. The typical duration of this challenge window can vary but is often set to a week or more to allow ample time for thorough review.

2. Submitting a Fraud Proof

If, during the challenge period, a participant (referred to as a "verifier") detects a fraudulent transaction within the batch, they can submit a fraud proof to the Ethereum mainnet. This fraud proof is essentially evidence that a particular transaction within the batch is invalid. The proof requires demonstrating the incorrectness of the transaction without revealing private data related to other transactions in the batch. This is often achieved through cryptographic proofs that leverage the unique properties of Merkle trees and state roots.

3. The Dispute Resolution Process

Upon submission of a fraud proof, the optimistic rollup's smart contract on the Ethereum blockchain initiates a dispute resolution process. This process involves:

Verification of the Fraud Proof: The smart contract examines the submitted fraud proof to determine its validity. This may involve recomputing the state transition or executing the disputed transaction in a sandboxed environment within the smart contract to verify if it leads to the same state transition as claimed by the sequencer.

Bisection and Iteration: In more complex disputes, the process may involve a bisection method. In this method, the disputed batch is split into smaller parts, and the challenger is asked to specify which part contains the fraud. This iterative process narrows down the disputed transaction(s) to a manageable size, making it easier to verify the fraud claim.

Resolution: If the fraud proof is found to be valid, the fraudulent transaction is nullified, and the rollup is reverted to its correct state before the fraudulent transaction. The sequencer who submitted the fraudulent batch may be penalized, often by forfeiting a bond or deposit. If the fraud proof is invalid, the transactions are finalized, and the challenger may face a penalty for the false accusation.

4. Finality and Security

This dispute resolution mechanism ensures that all transactions within an optimistic rollup achieve finality only after the challenge window has closed without any successful fraud proofs. It underscores the security of optimistic rollups by ensuring that only valid transactions are finalized and incorporated into the Ethereum blockchain. Moreover, it reinforces the principle that the security of the network does not rely solely on the honesty of the sequencer but is upheld by the vigilance of the entire network and the ability to challenge and verify transactions.

In essence, the dispute resolution mechanism in optimistic rollups is designed to leverage the collective security and verification capabilities of the Ethereum network, ensuring that despite operating off-chain for scalability, the integrity and trustlessness of the blockchain are maintained.

L1/L2 Interoperability and Asset Movement

A critical feature of optimistic rollups is their seamless integration with Ethereum's mainnet, facilitating smooth asset movement and interoperability between Layer 1 and Layer 2. Users can deposit assets into a bridge contract on Ethereum, which are then represented on the optimistic rollup, allowing for fast, cheap transactions. Exiting the rollup involves a waiting period, which is necessary for fraud detection (dispute resolution), but liquidity providers can offer solutions in the form of immediate liquidity, enhancing user experience.

Advantages of Optimistic Rollups

The advantages of optimistic rollups are multifold. They drastically reduce transaction fees and increase throughput, making Ethereum more scalable and efficient. Their compatibility with the Ethereum Virtual Machine (EVM) ensures that existing dApps can easily migrate to optimistic rollups, leveraging the enhanced performance without significant changes to their codebase. Moreover, the optimistic execution model fosters a secure and trustworthy environment, which is crucial for the growth and adoption of decentralized technologies.

Challenges and Limitations

Despite their potential, optimistic rollups are not without challenges. The dispute resolution period, necessary for fraud detection, introduces a delay in withdrawing funds from Layer 2 to Layer 1, potentially affecting liquidity and user experience. Furthermore, the reliance on sequencers raises concerns about centralization and data availability. However, ongoing improvements and innovations within the ecosystem are addressing these issues, pushing optimistic rollups closer to their ideal state of efficiency, security, and user-friendliness.


Optimistic rollups represent a significant leap forward in Ethereum's quest for scalability. By efficiently processing transactions off-chain and settling final states on the mainnet, they offer a promising solution to the network's scalability challenges. Despite some hurdles, the continuous development and growing adoption of optimistic rollups underscore their potential to transform Ethereum into a more scalable, efficient, and user-friendly platform. As the technology matures, it will undoubtedly play a crucial role in the evolution of the blockchain and cryptocurrency landscape, ushering in a new era of decentralized applications and services.