Rollups : Behind The Scenes

Understanding Rollup Architecture

Rollups are pivotal in scaling blockchain networks by managing transactions off-chain while still ensuring they are secure and verifiable on the main blockchain.

Transaction Flow

• Submission: Users send transactions to the rollup system. These transactions are typically user interactions with smart contracts or transfers between addresses.

• Batching: Instead of processing each transaction individually on the blockchain, rollups batch multiple transactions together. This reduces the amount of data that needs to be written to the blockchain, significantly lowering costs.

• Off-Chain Processing: The batched transactions are executed off-chain in the rollup’s environment. This execution is where the actual state changes of these transactions occur.

• Proof Generation: Depending on the type of rollup:

  • ZK-Rollups generate zero-knowledge proofs to prove the validity of the batch without revealing the transaction details.

  • Optimistic Rollups assume transactions are valid and only compute proofs if a transaction is challenged.

• On-Chain Submission: The batch, along with either the validity proof (ZK) or a commitment to the state (Optimistic), is submitted to the blockchain.

Here's a breakdown of the architecture with references to each layer involved:

Data Availability Layer:

• Purpose: Ensures that all transaction data is accessible so that anyone can reconstruct the state if necessary. This is crucial for security and transparency.

• Implementation:

  • ZK-Rollups: The full transaction data might not be published, but enough data for proof verification is included. The rest can be kept off-chain but available via external storage solutions or through the rollup operator.

  • Optimistic Rollups: Typically, all transaction data is published on the blockchain to enable fraud proofs within a challenge period.

• Example: In Ethereum's context, data can be stored in the calldata of transactions, which is cheaper than storing it in form of transactions.

Execution Layer

• Functionality: This layer handles the actual computation of transactions. It runs the rollup's version of the Virtual Machine (VM) that processes transactions in the same way as the main chain's VM would, but off-chain.

• Environment:

  • For ZK-Rollups, this involves complex computations for generating proofs which need to be verified on-chain.

  • Optimistic Rollups execute transactions similarly but delay state updates until after the fraud-proof window closes.

• Security: While execution happens off-chain, the integrity of this layer is maintained through cryptographic techniques or time-based fraud challenges.

Settlement Layer:

• Role: This is the underlying blockchain where the transactions are ultimately settled. It provides finality to the transactions processed by the rollup.

• Interactions:

  • State Updates: For both types of rollups, the state of the blockchain is updated based on the rollup's transactions. For ZK-Rollups, this is immediate upon proof verification. For Optimistic Rollups, it's after the challenge period if no fraud is proven.

  • Dispute Resolution: In case of disputes in Optimistic Rollups, the settlement layer is used to run the contested transaction to validate or invalidate the claim.

• Example: Ethereum serves as the settlement layer for many rollups like Arbitrum or zkSync, ensuring that the final state is secure and immutable on the Ethereum blockchain.

Conclusion

Rollup architecture effectively decouples the execution of transactions from their settlement, allowing for significant scalability improvements. By understanding these layers:

• Transaction Flow highlights efficiency in transaction processing.

• Data Availability ensures transparency and auditability.

• Execution Layer manages off-chain computation for speed.

• Settlement Layer guarantees on-chain security and finality.

This architecture not only scales blockchain networks but also keeps the decentralized ethos intact by maintaining security and verifiability through the underlying blockchain.