Fusion Rollups

What Are Fusion Rollups?

Fusion rollups are a blockchain scalability solution that combines the best of other L2 approaches such as Appchains and Shared Rollups. They let applications decide which part of their state to share with other applications while maintaining their own blockspace.

What Are Shared Rollups?

Shared rollups are a type of layer 2 scaling solution for blockchains. They are characterized by their communal nature, where transactions and state applications are combined together. This leads to more efficient composability in applications using the same roll-up solution but limits the gains on computational scalability. They also present challenges in customizability and resource allocation.

What Are Application Specific?

Application-specific rollups, also known as appchains, are a specialized form of layer 2 solutions designed for specific applications or use cases on a blockchain. Unlike shared rollups that handle a variety of transactions, application-specific rollups are tailored to the needs of a particular application, allowing for greater optimization and efficiency in processing transactions. They offer a more isolated environment, which can be both an advantage and a limitation, depending on the application's requirements.

So fusion rollups become the third option. They represent a novel design in the blockchain architecture, combining application-specific rollups with a shared state. This approach seeks to balance the advantages of isolation in appchains with the communal resource management found in shared rollups. The idea is to allow applications to enjoy both the autonomy of owning their block space and the benefits of sharing a state layer with other applications.
At their core, fusion rollups operate by allowing different applications to maintain their distinct chains (app chains) while enabling them to share certain aspects of their state. In this model, applications can observe and interact with each other. 
For instance, a decentralized exchange (DEX) application, named "A" in our example, holds price information for various tokens. Another application, "B," creates index products based on these tokens. Application "B" tracks and replicates the state of application "A," thus always staying updated with the latest price information. A similar application, "C," could also follow this process, sharing the state of "A" with both "B" and "C."
This setup highlights a key feature of fusion rollups: while inter-application communication (as would exist between "B" and "C") might still face typical layer 2 composability challenges, the reverse interaction (from "A" to "B" and "C") is instantaneous and cost-free.
A striking aspect of fusion rollups is their customization capability. Applications can decide what proportion of their block space is dedicated to the shared state, allowing for a tailored approach that fits their specific needs and objectives.

Advantages

Enhanced Scalability: Compared to shared rollups, they offer much more scalability.
Customization and Flexibility: Applications have the liberty to customize their involvement in the shared state, providing a balance between autonomy and communal resource sharing.
Improved Interoperability: Fusion rollups facilitate a new level of interoperability among different applications, fostering a more integrated and cooperative blockchain ecosystem.
Cost-Effectiveness: The design inherently reduces transaction costs for certain interactions.

Disadvantages

Complexity in Implementation: The fusion rollup architecture is inherently complex, potentially posing challenges in its implementation and adoption.
One-way Interoperability: The gains on composability and interoperability only work in one direction.
In conclusion, fusion rollups offer an intriguing solution to the challenges of scalability, efficiency and interoperability. By fusing the advantages of appchains and shared rollups, they present a middle ground that promises both autonomy and communal benefit. As with any innovative technology, some challenges and drawbacks need to be addressed. This concept, still in its nascent stages, warrants close attention as it evolves and potentially transforms the landscape of blockchain applications.