Original author: Stephanie Dunbar, Messari Analyst
Original translation: Luffy, Foresight News
In order to address the limitations of traditional monolithic architectures, including slow innovation speed, scalability limitations, and lack of development flexibility at the application layer, modular blockchains have emerged, dividing the blockchain into different interchangeable components.
Splitting the components allows for customized optimizations at every layer of the blockchain technology stack, with specialized providers emerging in each functional component domain.
The currently prominent modular systems include the Ethereum ecosystem and the upcoming Celestia.
Rollup is the most secure form of modular blockchain.
Rollups have flourished, ranging from general-purpose execution environments (such as OP Mainnet and zkSync Era) to application-specific Rollups hosting individual applications.
Most Ethereum-centric Rollups are general-purpose L2, while an important narrative in the Celestia ecosystem is application specificity.
Rollups can be stacked to scale and customize for specific application use cases, while also being part of a composable Rollup bridging ecosystem.
Developers can join the ecosystem they identify with, choosing the execution environment, ordering scheme, validation system, consensus, and DA layer that best suits their needs, and even mix and match between different ecosystems.
The success of any Rollup ecosystem or application will depend on the competitive advantages brought by the chosen configuration.
Execution Layer
This layer is where new transactions are processed. It retrieves the current state of the blockchain, applies these new transactions, and calculates the resulting state. The function that controls the rules for changing the state is called the State Transition Function (STF).
Most projects built on the execution layer are general-purpose Rollups centered around Ethereum, such as Scroll, Taiko, and Linea.
Their goal is to maintain compatibility with the EVM, provide users with a familiar crypto experience, and offer developers reusable tools.
Alt-VM (Alternative Virtual Machine) is optimized for specific use cases, such as the Fuel Network for parallel transaction processing and the Aztec Network for privacy applications.
Arbitrum and Fluent introduce traditional programming languages using Wasm for smart contracts, while Cartesi allows Rollups to run on Linux.
Settlement Layer
The settlement layer is an optional layer in the modular stack. A shared settlement layer is used for validation and dispute resolution in various Rollups and can serve as a liquidity hub to bridge gaps between different Rollups.
Ordering
Transactions are submitted to the network by users, and the sequencer accepts these transactions, determines their order (in most cases), and publishes transaction data to the consensus and DA layer components.
Currently, all major Rollups use centralized sequencers. Decentralizing the sequencer enhances activity and resistance to censorship. Shared sequencer networks between multiple Rollups, such as Espresso and Astria, provide composability advantages close to atomicity.
Verification Layer
This layer ensures the correctness of execution and state transitions. There are currently 2 main systems under development:
Optimistic: Fraud-proofing to prevent fraudulent activities
ZK: Validity proof to cryptographically confirm the correctness of all transactions
Arbitrum and Optimism are leaders in optimistic rollup, proving the validity of the market through outsourced proofs by @RiscZero and @nil_foundation. Outsourced proofs can bring interoperability advantages, such as aggregated proofs for cross-chain bridges.
Consensus and DA (Data Availability)
At the consensus layer, nodes reach agreement on the final order of transactions, providing a unified view of Rollup history.
The DA layer further ensures that all necessary data is available to reconstruct the Rollup state. The DA layer acts as an immutable bulletin board where transaction data and proofs are published. Without DA, Rollup cannot guarantee liveness. Using the transaction data provided by the DA layer, anyone can continue calculating the next block from where the previous person left off.
The DA layer is also the ultimate determinant of Rollup throughput.
Ethereum is developing a dedicated fee market and "blob" space for Rollup, and projects like Celestia and Avail are focusing on lightweight implementations with specialized DA layers.
DACs that separate consensus and DA, like EigenDA, can provide low-cost, predictable fees and the ability to allocate reserved DA bandwidth.
Although these functions can be separated, only by running together can they provide complete security protection for the blockchain network.
The most critical aspect of consensus and DA layer is the trust minimization and composability advantage they bring.
The cost of changing consensus and DA layer would be high as Rollup would lose the composability with other Rollups it previously shared.
