Sovereign Rollups are innovative blockchain scaling solutions that rely on a modular architecture to increase performance while preserving considerable security. They function as independent modular blockchains, only relying on another chain for consensus and data availability. As a result, Sovereign Rollups can be custom-built to suit the precise needs of the applications they host and the people that use them.

This article will explore the motivations that lead to the rise of Sovereign Rollups in other blockchains, and describe how they can be implemented on top of Nomos.

From Rollups to Sovereign Rollups

Rollups were originally designed as scaling solutions for blockchains such as Ethereum, moving transactions onto Layer 2 chains to free up bandwidth on the Layer 1. Updates to the rollup state are posted to a bridge contract on the L1, which uses some validation mechanism (such as fraud or validity proofs) to determine the canonical state. The idea behind rollups was to use the trust-minimised bridge contract as a way to ensure that assets on the rollup share the same security properties as those on the L1 chain while benefitting from the rollup’s scalability.

However, as noted by @luis_0x, the assumptions that motivated the proliferation of rollups do not match the way people actually use them. Because trust-minimised bridging takes time to finalise an asset transfer, so-called “non-canonical” bridges cropped up to support faster bridging, albeit with greater trust assumptions. Users flocked to these bridges, with the past week’s data showing 14,500 users relying on Optimism’s LayerZero alone, as compared to just 243 users making use of its canonical bridge. Between third-party bridged tokens and native assets issued on the rollup, most of the value on rollups today is stored in assets not secured by the L1 at all.

The advent of modular blockchains allowed people to rethink the way blockchain functionality can be implemented. Rather than having one blockchain protocol handle all of its functionality (potentially with a rollup to shoulder some of the burden of transaction execution), modular blockchains split that functionality into several “modules”. These are:

• Execution: Processing transactions and updating the blockchain state.
• Verification: Verifying the correctness of state updates. This module is optional.
• Consensus: Reaching agreement on transaction ordering.
• Data Availability: Ensuring that transaction data can be retrieved if desired.

Each of these modules can potentially be implemented independently in a more optimal way, acting together to deliver blockchain functionality.

Sovereign Rollups are one example of a technology made possible by this paradigm shift, in which there is no longer a need for the rollup state to be verified on an L1. As a result, Sovereign Rollups do not require a bridge contract to act as a single source of truth about their state, using their own validators to determine the correct state. In other words, Sovereign Rollups function as independent modular blockchains, only relying on another chain for consensus and data availability. State validity and execution, by contrast, are solely determined by the Sovereign Rollup. This independence from the base chain allows Sovereign Rollups to evolve and implement changes autonomously, without having to modify any smart contract. In this model, canonical bridges and the performance delays associated with them are no longer necessary, optimising the architecture for rollups’ predominant use cases.

Use Cases

Sovereign Rollups have complete freedom to define their own state, only relying on another chain to ensure consensus and the availability of rollup data. This design allows applications built on Sovereign Rollups to have low fees and maximise scalability. As a result, Sovereign Rollups can implement almost anything: ranging from applications to virtual machines that are home to many different applications. They are best suited for applications that require high performance and do not need strong interoperability. This could include gaming applications whose state changes quickly but is largely isolated from other blockchain applications, DeFi platforms focusing on high-frequency transactions of internally-issued assets, and a great variety of web applications not often implemented on-chain due to performance requirements.

Sovereign Rollups on Nomos

Nomos is a blockchain infrastructure designed for network states and other decentralised applications that require high levels of privacy, decentralisation, and resilience. It is built primarily to support Zones - blockchain execution environments that adhere to common specifications. Despite the advantages of Nomos Zones, their adherence to these specifications allows for less freedom in their implementation, and applications built on them are constrained by ZK performance limitations. Because of this, Nomos also supports the creation of Sovereign Rollups on top of its architecture.

The idea behind Sovereign Rollups is to make applications with the freedom to define an execution environment to suit their needs, with the Nomos Network providing economic security (the cost of obtaining enough stake to compromise consensus) and data availability guarantees that the rollup alone could not provide. Sovereign Rollups have complete freedom on how to validate their state, and therefore do not need to rely on a smart contract on the L1 as with Ethereum L2s. This could be done in any number of ways - including publishing ZK validity proofs to be verified by its validators, providing a challenge window for fraud proofs, or by requiring rollup nodes to re-execute the state transition function. If a Sovereign Rollup uses validity proofs, these are not posted on-chain, but are distributed off-chain to rollup validators and light nodes in a manner defined by that rollup. The flexibility defining Sovereign Rollups allows each one to select the privacy, decentralisation, and scalability properties that work best for its use case.

From the point of view of Bedrock (the Nomos base chain), Sovereign Rollup data is completely obscure, as Bedrock does not attempt to validate or even interpret data posted to it from Sovereign Rollups. All Bedrock does is provide operations via the Mantle, allowing rollups to temporarily post data in the form of a blob sent to the data availability (NomosDA) service. This blob is distributed among DA nodes, allowing all Nomos light clients to verify its availability. The blob commitments are written on-chain, ensuring that the rollup state enjoys the benefits of Nomos’ consensus protocol. Bedrock also allows rollups to inscribe data if they so desire, storing it permanently on all nodes. In this design, full Nomos nodes track and process every operation sent by Sovereign Rollups, while light nodes interested in only their rollup will only track the blobs and inscriptions targeting that rollup.

Because data availability is so inexpensive to use, Sovereign Rollups are highly scalable and support a great variety of applications. One possible implementation of a Sovereign Rollup that uses validity proofs is illustrated in Figure 1.

This example rollup’s sequencer processes transactions from clients and sends the batched state data to the data availability service, while simultaneously creating ZK proofs for its state updates and distributing them off-chain. Light nodes can then verify the proofs received from the sequencer and check the availability of the data via data availability sampling.

Sovereign Rollup Interoperability

Despite lacking the interoperability of Zones, Sovereign Rollups still have the ability to communicate with each other. This form of trustless communication only happens asynchronously via message-passing, unlike the atomic cross-Zone transactions available between Zones. NomosDA is used as the message buffer for communication between rollups.

Sovereign Rollup communication is crucial for enabling dependency relationships between applications located on different rollups. An application “A” on one rollup may implement a function that is only executed if an application “B” on another rollup is in a certain state. To check that these conditions are met, clients of “A” need to retrieve and verify the state of application “B”. However, because the dependency is only one-way, clients of “B” don't need to download data from application “A”. This is a consequence of the independence of Sovereign Rollups, which means that applications on one rollup cannot force applications on another rollup to depend on them. Bedrock includes a specific mechanism to encode these dependencies to provide a faster communication channel between rollups that read the state of others.

Conclusion

In conclusion, Sovereign Rollups represent a paradigm shift in blockchain architecture - one that emphasizes modularity and flexibility. By decoupling execution and verification from base chain security, Sovereign Rollups offer developers the freedom to build highly customized environments that serve their specific application needs, without sacrificing much security or scalability. Nomos provides robust consensus and data availability layers that can be used by Sovereign Rollups, without imposing constraints on the rollups themselves. As a result, Sovereign Rollups on Nomos can support a wide range of decentralized applications, while still having a level of interoperability through asynchronous communication. This architecture paves the way for a more diverse, scalable, and efficient blockchain future.

Sources and Further Reading

• Rollups as Sovereign Chains
• "Sovereign" Rollup L1s vs "Settled" L2s
• Rollups Are L1s (& L2s)