How to Implement Cross-Rollup Sequencing for Lower Gas Fees

Understand shared sequencing mechanics

Cross-rollup sequencing changes who decides the order of transactions. In a traditional setup, each rollup relies on its own sequencer to accept transactions, determine their order, and provide fast confirmations before posting data to the underlying chain. This often means a single operator or a tightly controlled service holds significant power over the flow of assets.

Shared sequencing moves this task from individual rollup-specific sequencers to a decentralized network. Instead of routing transactions through separate, siloed paths, a shared sequencer accepts transactions from multiple rollups simultaneously. This approach defragments the Layer-2 ecosystem by allowing different rollups to share the same ordering layer.

This mechanism differs significantly from traditional cross-chain bridging. While cross-chain bridges typically move assets between entirely different base networks (like Ethereum to Solana), shared sequencing operates between rollups that may or may not share the same consensus protocol. The goal is not just to move value, but to align the sequence of events across chains to reduce latency and eliminate the need for complex, multi-step bridging processes.

By centralizing the ordering layer, shared sequencing reduces the friction that usually drives up gas fees. Users no longer need to wait for separate finality proofs on different chains before interacting with the next step in a workflow. The result is a smoother experience where transactions are ordered consistently across rollups, lowering the computational overhead required to verify cross-rollup interactions.

Select a compatible sequencing protocol

Choosing the right shared sequencing infrastructure depends on your rollup’s security model and latency requirements. Shared sequencers act as the traffic controller, accepting transactions, deciding their order, and providing fast confirmations before data is posted to the underlying data availability layer. In many deployed systems, the sequencer is still a single operator, but moving to a shared model allows rollups to outsource this responsibility to specialized providers.

When evaluating options, you must balance decentralization, cost, and speed. Espresso Systems and Celestia offer distinct approaches to shared sequencing. Espresso provides a dedicated sequencer network that prioritizes low latency and high throughput, making it suitable for applications requiring immediate finality. Celestia, primarily a data availability layer, offers sequencing as a service via Interchain Security, leveraging existing validator sets to secure the sequencing process.

The table below compares the core metrics of these major shared sequencing providers. Use this data to align the protocol with your rollup’s specific needs, whether that is minimizing gas fees or maximizing cross-rollup composability.

For most developers, the decision comes down to whether you need a purpose-built sequencer or a generalized data availability solution that also handles ordering. If your application relies heavily on cross-rollup interactions, a dedicated sequencer like Espresso may offer the deterministic ordering required for atomic execution. If your primary concern is reducing data costs while maintaining security, Celestia’s model provides a robust alternative.

Configure atomic transaction execution

To implement cross-rollup sequencing, you must configure a shared validity sequencer that synchronizes transaction ordering across multiple Layer 2 networks. This setup ensures that transactions on different rollups are treated as a single atomic unit, meaning they either all execute successfully or all fail together. Without this synchronization, cross-chain operations risk partial execution, where one rollup processes the transaction while the other leaves it pending or reverts it.

The core mechanism relies on a shared sequencing layer that acts as the single source of truth for transaction ordering. Instead of each rollup maintaining independent orderings, the sequencer bundles transactions from both networks and submits them to their respective execution environments simultaneously. This approach aligns with the Shared Validity Sequencing (SVS) paradigm, which proposes a trigger-action model where smart contracts on one rollup can remotely invoke methods on another with guaranteed consistency.

1. Deploy the shared sequencer infrastructure

Begin by deploying the sequencer software that supports multi-rollup coordination. This infrastructure must be capable of receiving transaction payloads from multiple rollup clients and maintaining a global order book. The sequencer should be configured to handle the specific gas token and block production intervals of each connected rollup. Ensure the sequencer has low-latency connections to all target rollup nodes to minimize the time window between execution events.

2. Define the atomic transaction schema

Create a standardized transaction format that includes references to both source and destination rollups. This schema must contain the execution payload for each rollup, along with a shared nonce or sequence ID that links the two operations. The transaction should also include a validity proof or signature that confirms the user intends to execute both parts of the transaction simultaneously. This schema ensures that the sequencer can validate the atomicity requirements before proposing the block.

3. Implement synchronous execution logic

Configure the rollup execution environments to accept and process the atomic transaction bundle. The logic must enforce that the state changes on both rollups are committed in the same block or epoch. If the execution on one rollup fails due to a revert or out-of-gas error, the sequencer must trigger a rollback on the other rollup to maintain consistency. This synchronous execution requires tight coupling between the rollup state machines and the sequencer's validation layer.

4. Set up failure handling and reversion

Implement robust failure handling to manage scenarios where atomic execution cannot be completed. If the sequencer detects a conflict or a validation error, it should broadcast a reversion signal to all connected rollups. This signal must trigger a state rollback to the pre-transaction block, ensuring that no partial state changes remain. Users should be refunded their gas fees on both rollups to prevent loss of funds due to execution failures.

5. Validate end-to-end consistency

Test the configuration by submitting cross-rollup transactions and verifying that they are processed atomically. Use monitoring tools to track the sequencer's performance and the rollups' state transitions. Ensure that the shared sequencer can handle high throughput without introducing significant latency. Validate that the atomicity guarantees hold under various network conditions, including high congestion and node failures.

Mitigate cross-rollup MEV risks

Shared sequencing introduces new MEV vectors; always implement private mempool features to protect user trades.

When transactions from different rollups share a single sequencer, the attack surface for Maximal Extractable Value (MEV) expands significantly. In a fragmented environment, front-runners are limited to a single chain’s liquidity. In a shared sequencing model, bots can observe pending transactions across multiple rollups simultaneously, identifying arbitrage opportunities that span bridges and cross-chain protocols.

To mitigate sandwich attacks and front-running in this environment, you must decouple transaction submission from public visibility. Implement a private mempool or a relay-based submission system. This ensures that your users’ trades are encrypted or hidden from the public mempool until they are included in a block. Without this layer of privacy, shared sequencers become a central point of observation for MEV bots.

Prioritize relays that offer transaction encryption or use confidential computing environments. This prevents the sequencer operator—and any external observers—from seeing the transaction details before inclusion. By obscuring the intent of the trade, you remove the primary data point attackers need to execute profitable sandwich attacks. Always verify that your chosen sequencer infrastructure supports these privacy primitives natively, rather than relying on post-hoc fixes.

Verify finality and gas savings

After deploying cross-rollup sequencing, you need to confirm the system is actually delivering on its promises. The goal is to prove that shared sequencing reduces costs and maintains acceptable finality times compared to traditional bridging methods.

1. Measure gas cost reduction

Compare the gas fees for identical transactions across your rollups. Traditional bridging often requires separate transactions on each chain, adding overhead. With shared sequencing, you should see a consolidated fee structure. Use block explorers to track the gas used for a standard swap or transfer on your integrated rollups versus the same action using a standard bridge like LayerZero or Wormhole.

2. Check finality times

Finality is the point where a transaction is irreversible. Shared sequencers can sometimes introduce slight delays if the shared network is congested. Monitor the time from transaction submission to final confirmation. Ensure this time remains within your acceptable threshold, typically under a few minutes for most DeFi applications.

3. Validate transaction ordering

Cross-rollup composability relies on correct transaction ordering. Verify that dependent transactions across different rollups execute in the intended sequence. Use a testnet environment to simulate complex multi-step operations. If transactions arrive out of order, your sequencing logic needs adjustment.

4. Audit security and decentralization

Shared sequencers introduce a new trust assumption. Ensure the shared sequencer network is sufficiently decentralized to prevent single points of failure. Review the consensus mechanism and operator set. This step is critical for maintaining the security guarantees of your rollups.

Frequently asked questions about sequencing

What role does a sequencer play in a rollup?

A sequencer accepts transactions, decides their order, and gives users fast confirmations before the data is posted to the main chain. In many deployed systems, this sequencer is still a single operator or a tightly controlled service. Shared sequencing distributes this role to reduce bottlenecks and centralization risks.

What is a Layer-2 rollup?

Rollups are a Layer-2 scaling solution designed to reduce congestion on Ethereum’s network. They process transactions off-chain and post compressed data to the main chain, resulting in more processed transactions and lower fees while maintaining Ethereum’s high security levels.

What is a cross-chain protocol?

Cross-chain refers to the interoperability between different blockchain networks, enabling them to communicate, transfer data, and exchange assets. Cross-chain solutions aim to address blockchain fragmentation, where assets and applications are often confined to their native networks. Cross-rollup bridging functions similarly, except the two rollups may or may not share the same consensus protocol [1].

What are the different types of blockchain rollups?

There are two primary types of rollup: Optimistic rollups and Zero-knowledge (ZK) rollups. Optimistic rollups assume transactions are valid unless proven otherwise, while ZK rollups use cryptographic proofs to verify validity immediately. Both types benefit from cross-rollup sequencing to share liquidity and reduce costs.

[1] https://hackmd.io/@EspressoSystems/SharedSequencing

Work through to Cross-Rollup Sequencing