The Current Status and Future of Layer 2 Sorters: The Evolution from Centralization to Decentralization

The main source of income for Layer2 comes from the Gas fees paid by users when they transact on the Rollup. After deducting the Gas fees required for Layer2 to submit data to Layer1, the remaining portion is basically pure profit. According to statistical data, the profit for OP Mainnet in the second half of 2023 is approximately $5.23 million, Arbitrum's profit for the entire year is $16.5 million, and zkSync Era's profit from March to December 2023 reached $22.24 million.

The enormous profits behind this are closely related to the unique sequencer operating on the Layer 2 network.

The sorter plays a key role in Layer 2, primarily responsible for receiving and executing user transactions, and then submitting the sorted and compressed batch to Layer 1. This is similar to a bus driver in a public transportation system, responsible for picking up passengers and optimizing space utilization inside the bus.

Currently, there are several common sorting algorithm implementations:

1. Centralized Sorter: Operated exclusively by the Layer2 team or a designated organization, with high efficiency and low cost.

2. Permissionless Sorter: Anyone can sort submitted transactions, but it may lead to resource waste.

3. Other decentralized solutions.

There are usually two standards for sorting: first-come, first-served or sorted by Gas fees. Most Layer 2 solutions adopt the first method, but in fact, the sorter can decide the sorting rules themselves.

To prevent malicious behavior by the sorter, different Layer 2 solutions have adopted different constraint measures. Optimistic Rollup uses fraud proofs, while ZK Rollup employs validity proofs.

Currently, mainstream Layer 2s such as OP Mainnet and Arbitrum One still use centralized sequencers. Although efficient, it has also raised some issues:

1. Weak censorship resistance: A single entity operation is easily influenced by regulation.

2. Low Activity: High risk of single point of failure.

3. Possible improper acquisition of MEV gains.

To address these issues, mainstream Layer 2 solutions have proposed decentralized sequencer schemes, including:

1. Geographic Decentralization: Deploy multiple sorters in different locations around the world.

2. Sorter Auction: Bidding for operating rights through smart contracts.

3. Leader election: randomly selected from the stakers.

4. Based Rollup: Directly led by Ethereum validators for sorting.

In addition, there is a solution for shared sorters, where multiple Layer 2s share a third-party sorter network, which can enhance atomic composability and prevent MEV extraction.

However, whether decentralized sequencers can perfectly solve existing problems is still up for discussion. Taking MEV as an example, after Ethereum's The Merge, block proposers have extracted a large amount of MEV, creating a huge market. If Rollup sequencers are completely open, a similar pattern may also emerge.

Although shared sorters can improve interoperability, if widely used, they may create new centralization issues. These require further thought and exploration.

The decentralization of blockchain is a long process. Sorters, as an important role in Rollup, have attracted much attention, and it is believed that suitable solutions will be found in the future.