A rollup, in its essence, functions as a series of blocks. However, what sets it apart from traditional blockchain structures is its unique operational mechanism. In a typical blockchain, operators, or full nodes, perform the task of not only executing transactions but also achieving consensus on the sequence and authenticity of these transactions. In contrast, a rollup reassigns this responsibility. It relies on an external proof verification system to vouch for the validity of transactions. Consequently, the foundational blockchain takes on the role of arriving at a consensus on the definitive sequence of transactions, ensuring their accessibility, and thereby safeguarding the overall integrity of the rollup. This transparent and open system guarantees that any participant can access this data and accurately determine the rollup's current status.
Leveraging the Power of Underlying Networks
The genius behind the rollup design is its ability to tap into the security mechanisms of an established network. By dispatching condensed transaction data to this network, a rollup not only harnesses its security provisions but also amplifies its offchain transaction potential. This design choice is advantageous for both investors and end-users, especially in terms of economic feasibility. Rollups present an attractive proposition of manageable fees, primarily by sidestepping the frequent bottlenecks experienced in primary L1 executions. Moreover, as the volume of transactions incorporated into a rollup block escalates, the financial burden associated with transferring data to the principal network disperses. Thus, each transaction becomes incrementally cost-effective.
The Role of Sequencers
Sequencers in an ORU operate analogously to a meticulous clerk. Instead of documenting each transaction as it occurs, they collate numerous off-chain events, creating a summarized version. This summary, termed the "state root," is then uploaded to the primary blockchain, known as L1. By functioning in this manner, sequencers alleviate computational burdens from the main chain.
Sequencers' objectives are transparent: present information accurately. Successful execution of this task results in rewards, establishing a direct correlation between honest behavior and financial incentives. But what happens if a sequencer errs or attempts deceit? This is where the system's ingenious check-and-balance comes into play, introducing the role of verifiers.
Currently, the process of block production on optimistic rollups is commonly overseen by a singular sequencer, often maintained by the originating team. This means ORUs exhibit a centralized nature, although strides are being made to transition to a more decentralized structure. For instance, with "Base," the sequencer is operated singularly by the Base team.
Drawing parallels to block building for Layer 1, the role of the rollup operator is undergoing a transition, becoming more specialized (and centralized) since its initial instantiation. As the economic landscape becomes more expansive, specialization emerges as a natural progression. This results in more resilient systems, thanks to the segregation of duties. However, this expanded specialization also means that the design space has ballooned, necessitating a novel navigational approach.
Balancing Efficiency with Security
Navigating these costs, single rollups encounter critical decisions, balancing security aspirations with efficiency goals. A manifestation of this is the potential inclination towards a less secure, yet more cost-effective data availability layer.
Historically, data publishing costs have been the dominant financial drain. Yet, the horizon seems promising. With the imminent activation of EIP-4844 on Ethereum and the subsequent introduction of full Danksharding, a significant reduction in these costs is anticipated. This will likely empower rollups with the fiscal efficiency requisite for scalability and diversification into novel use cases.
In the grander scheme, the quest for cost efficiency, especially concerning data, seems destined to be fulfilled through offchain innovations. Aggregation emerges as the beacon of hope, promising economies of scale. For instance:
Optimistic rollups could exploit shared sequencing services. An especially intriguing proposition is the potential for shared batch posting. This would expedite batch compression, particularly benefitting smaller entities by providing a blend of reduced costs and enhanced security via accelerated data posting.
Zk rollups have the enticing possibility of shared provers. These provers can conglomerate numerous SNARKs into an overarching proof prior to L1 posting. This model, especially with its recursive aggregation potential, can drastically optimize the L1 data market's utilization. However, this comes at the expense of heightened offchain computation.
Sequencer Case Study: Polygon zkEVM
As previously stated, sequencers collect L2 transactions, batch them, and then propose the batch as a valid L2 transaction to the PoE smart contract. One important tenant of Polygon is that sequencers remain permissionless. This tenant allows anyone to join and contribute to the network for maximum decentralization. Anyone running the zkNode, the Hermez 2.0 software, can be a sequencer.
Sequencers must pay L1 gas fees and deposit an additional MATIC fee to propose a batch on L1. This collateralization prevents spam and incentivizes sequencers only to suggest valid batches and transactions. The batch cost will vary based on the network load and is calculated automatically by the protocol's smart contract. Sequencers get paid in MATIC transaction fees by a user.
These batches will post to the L1 as calldata like a typical rollup. Any new permissionless node will reconstruct the data and ultimately synchronize its state. Once mined, these L1 transactions determine which L2 transactions get executed and in what order. This generates a deterministic new state, which network nodes can compute as a virtual future state. The new state will be established when the ZKP is created and mined in L1. This is the second component of the protocol.
Introducing Verifiers: The Gatekeepers of Integrity
Drawing a parallel from the financial world, if sequencers are akin to clerks, verifiers function as auditors. Their primary responsibility within the Layer 2 (L2) system is to ensure the integrity of state commitments submitted by sequencers. Each state root presented by the sequencer is given a seven-day "challenge period." During this window, verifiers, upon detecting inconsistencies or inaccuracies, can lodge a formal challenge.
Challenging inaccuracies isn't just a task—it's an economic opportunity. Verifiers receive financial rewards for successfully identifying erroneous state roots. Yet, the system safeguards against potential misuse. If a verifier falsely challenges a legitimate state root, penalties are imposed. This economic structure fortifies the system against unwarranted interruptions and ill-intentioned actions.
However, there's an essential distinction to understand. A successful challenge doesn't modify the network or transaction sequence. It merely excises the contentious commitment, which is eventually superseded by an alternate proposal. This method fortifies the commitment's integrity without amending the chain's real state.
At the moment, optimistic rollups like Base and Optimism don't have operational fraud proof systems. This absence means users place trust in sequencers to present authentic state commitments to L1. Notably, even without fraud proof systems, there's a mandatory seven-day challenge period when extracting assets. This waiting period is in place primarily to acclimate users to the inherent delay and to provide a temporal cushion against potential system exploits.
