The role of a sequencer in zkSync is crucial for the operation and efficiency of the zk-rollup system. The sequencer is responsible for several key functions that ensure the smooth processing and finalization of transactions on the zkSync network.

Key Responsibilities of a Sequencer in zkSync

1. Transaction Ordering and Batching:

   • The sequencer collects transactions from users, orders them, and batches them into blocks. This ordered sequence of transactions is essential for maintaining the integrity and consistency of the blockchain state[1][5][17].

2. State Transition Execution:

   • Once transactions are batched, the sequencer executes these transactions to update the state of the zkSync network. This involves applying the state transition function to the ordered transactions, resulting in a new state[1][5][12].

3. Zero-Knowledge Proof Generation:

   • After executing the transactions, the sequencer generates zero-knowledge proofs (ZKPs) that attest to the correctness of the state transitions. These proofs are then submitted to the Layer 1 (L1) blockchain (e.g., Ethereum) for verification[1][5][12].

4. Data Availability and Finality:

   • The sequencer ensures that the transaction data and state updates are available and can be verified by the L1 blockchain. This involves posting the necessary data and proofs to the L1, which provides finality and security to the transactions processed on zkSync[1][5][12].

5. Handling Reorganizations:

   • In the event of a reorganization (reorg) on the L1 blockchain, the sequencer must be able to handle rollbacks and reapply transactions to maintain consistency. This ensures that the zkSync network remains in sync with the L1 blockchain[12].

Types of Sequencers

1. Centralized Sequencers:

   • Initially, zkSync uses a centralized sequencer to manage transactions. This approach provides low latency and high efficiency but comes with risks such as a single point of failure and potential censorship[10][19].

2. Decentralized Sequencers:

   • zkSync plans to transition to a decentralized sequencer model, where multiple nodes participate in the sequencing process. This enhances security, censorship resistance, and decentralization, reducing the risks associated with a single centralized entity[3][8][9][19].

3. Shared Sequencers:

   • Shared sequencers are a form of decentralized sequencer that can serve multiple rollup chains simultaneously. This model promotes interoperability and reduces the cost and complexity of deploying dedicated sequencers for each rollup chain[3][8][9].

Sequencer Components in zkSync

1. State Keeper:

   • The State Keeper component acts as the sequencer within the zkSync node. It retrieves transactions from the queue, orders them, and seals the corresponding blocks or batches based on the data obtained from the queue[12].

2. Reorg Detector:

   • This component tracks L1 batches that have not yet been finalized and handles rollbacks in case of reorgs, ensuring the zkSync network remains consistent with the L1 blockchain[12].

3. API and Fetcher:

   • The API component allows interaction with the zkSync node, while the Fetcher component maintains synchronization between the zkSync node and the main node, ensuring that the latest state and transaction data are available[12].

Conclusion

The sequencer in zkSync plays a pivotal role in ensuring the efficient and secure operation of the zk-rollup system. By ordering and batching transactions, executing state transitions, generating zero-knowledge proofs, and ensuring data availability and finality, the sequencer maintains the integrity and performance of the zkSync network. The transition from centralized to decentralized and shared sequencers aims to enhance the network's security, decentralization, and scalability[1][5][12][19].

Citations:

[1] https://docs.espressosys.com/sequencer/integrating-a-rollup/integrating-a-zk-rollup/zk-rollup-architecture

[2] https://github.com/matter-labs/zksync/blob/master/docs/architecture.md

[3] https://www.zeeve.io/blog/how-shared-sequencers-could-ensure-better-decentralization-in-l2-rollups/

[4] https://ethresear.ch/t/zkps-two-step-submission-algorithm-an-implementation-of-decentralized-provers/15504

[5] https://chain.link/education-hub/zero-knowledge-rollup

[6] https://docs.lite.zksync.io/dev/

[7] https://www.immunebytes.com/blog/zksync-a-secret-sauce-leveraging-blockchain-technology/

[8] https://blog.kroma.network/decentralized-sequencers-d2a4aeaf1084?gi=a28d5f865b36

[9] https://www.zeeve.io/blog/why-would-a-layer2-decentralize-its-sequencer/

[10] https://blog.matter-labs.io/introduction-to-hyperchains-fdb33414ead7

[11] https://docs.espressosys.com/sequencer/integrating-a-rollup/integrating-a-zk-rollup/using-the-espresso-sequencer

[12] https://docs.zksync.io/infra/component-breakdown.html

[13] https://delendum.xyz/writings/2022-11-27-decentralization-of-zk-rollups.html

[14] https://zksync.io/hyperscalability

[15] https://www.zeeve.io/blog/zksync-era-guide-everything-you-need-to-know/

[16] https://github.com/skalenetwork/levitation_decentralized_zk_sequencer

[17] https://limechain.tech/blog/what-are-rollup-sequencers/

[18] https://hackmd.io/%40yezhang/SkmyXzWMY

[19] https://docs.zksync.io/zk-stack/concepts/zk-chains.html

[20] https://blog.quarkslab.com/zksync-transaction-workflow.html