What is a rollup?

CrabChain.org Team | Updated 13 October 2026

Rollups have become one of the main architectures used to expand the capacity of blockchain networks without completely giving up the security properties of a main chain. Their emergence responds to one of the structural problems that accompany the expansion of decentralized networks. As the number of users, transactions, and applications increases, the infrastructure must process more operations without costs and confirmation times rising sharply. A rollup proposes a solution based on executing transactions outside the main chain and subsequently transferring to it the information necessary to verify the results.

The idea

The fundamental idea is to separate transaction processing from final settlement. A blockchain such as Ethereum can act as a reference layer responsible for providing security, data availability, and dispute resolution, while an external infrastructure processes large volumes of operations. Instead of recording every individual transaction directly on the main chain, a rollup groups numerous operations and publishes a compact representation of them. This is the origin of the name, which relates to the idea of grouping or rolling many transactions into a single structure.

This architecture can considerably reduce the amount of data and number of operations that have to be processed directly on the main layer. The objective is not simply to create another faster blockchain, but to take advantage of the execution capacity of a secondary system and combine it with the guarantees of a reference network. The result is a multilayer infrastructure in which each component performs a specific function. The main chain retains a central role in security and settlement, while the rollup handles much of the execution work.

How it works

To understand how it works, it is necessary to distinguish among execution, data, and verification. When a user performs an operation within a rollup, it may be processed by nodes or sequencers managing the secondary network. Transactions are ordered, executed, and grouped into batches. The system then publishes certain data to the main chain and provides information that makes it possible to demonstrate that the rollup's new state is consistent with the operations performed. In this way, the main blockchain does not need to repeat the entire calculation for every transaction in order to maintain a verifiable reference to the system.

One of the most important elements is the sequencer. In many current implementations, this function is performed relatively centrally by one entity or a small group of operators. The sequencer receives transactions, decides their order, and produces the batches that are subsequently published. This concentration can improve performance and simplify infrastructure, but it also introduces questions related to censorship, uptime, and decentralization. The development of rollups includes efforts specifically aimed at reducing these dependencies and progressively distributing certain functions among a broader set of participants.

Optimistic rollups

There are two major families of rollups, commonly known as optimistic rollups and zero-knowledge rollups. Although both pursue similar goals, they use different mechanisms to guarantee the correctness of the network state. Optimistic rollups start from the premise that published batches are valid unless someone proves otherwise. When a new state is recorded, there is a period during which a participant can submit a fraud proof if an incorrect transition is detected.

This model allows processing to be relatively efficient because the main network does not need to immediately verify every operation using a complex cryptographic proof. Security depends, among other factors, on there being an effective possibility of challenging a fraudulent transition. One practical consequence is that certain withdrawals of funds to the main chain can require extended waiting periods when fraud-proof mechanisms are involved. The exact operation depends on each protocol and its security design.

Zero-knowledge rollups

Zero-knowledge rollups take a different approach. Instead of initially assuming that transitions are correct and allowing them to be challenged afterward, they generate cryptographic proofs that demonstrate that a set of transactions was executed correctly. These proofs, based on zero-knowledge proof techniques, can be verified by the main chain without requiring it to reproduce the entire computation. The concept of zero knowledge has a specific meaning here. The proof makes it possible to demonstrate the validity of a result without necessarily revealing all the information used to construct it.

The potential of this technology has driven the development of increasingly sophisticated systems capable of compressing large volumes of activity into relatively small proofs. However, that efficiency also introduces substantial cryptographic and engineering complexity. Generating the proofs can require significant computational resources, especially when large workloads are processed. The development of specialized hardware, algorithms, and architectures has therefore become an important part of the rollup ecosystem.

Data and security

An essential characteristic of these networks is that security does not depend solely on the code that processes transactions. It is also necessary to determine where the data that allow the state to be reconstructed and activity to be verified are stored. Data availability is fundamental because a validity proof can guarantee that a transition is correct, but users need access to the information required to understand and reconstruct the corresponding state. For this reason, data-availability design is one of the central components of any rollup-based scalability architecture.

The relationship between a rollup and its main blockchain also differentiates it from a conventional sidechain. Although both solutions process operations outside the main chain, their security models are not necessarily equivalent. A sidechain usually has its own consensus system and an independent set of validators. A rollup, by contrast, is designed to rely directly on the main chain for fundamental aspects of its security and settlement. This distinction explains why it is not appropriate to treat the two concepts as simple variants of the same technology.

Challenges

The growth of rollups has also been accompanied by a conceptual transformation in blockchain design. Rather than trying to have a single chain process all operations directly, modern design tends toward modular architectures in which different layers perform different tasks. Execution can be moved to specialized environments, while the main layer retains functions such as settlement, consensus, and data availability. This approach makes it possible to build systems with greater capacity without requiring every operation to run on the same infrastructure.

The potential benefits are significant, but they do not eliminate scalability problems. A rollup can reduce transaction costs by grouping many operations, although the final price depends on factors such as main-chain fees, the size of the data published, the compression mechanism, and the network's activity level. Similarly, a rollup architecture does not automatically guarantee full decentralization. Sequencer distribution, contract upgrade mechanisms, censorship possibilities, and administrative functions remain aspects that must be analyzed separately.

Interoperability is another challenge. If multiple rollups operate on the same blockchain, users may encounter an ecosystem fragmented across different environments. Moving assets between them requires bridges, interoperability mechanisms, or systems capable of communicating states from different networks. This can create additional complexity and, in some cases, new risk surfaces. The long-term objective is for this multiplicity of layers to become almost transparent to the end user.

From a technological perspective, rollups represent an important evolution in the way blockchain scalability is conceived. They do not necessarily seek to replace the main chain, but to extend its capabilities through an architecture in which much of the execution takes place outside it. In terms a specialized analyst might use, the fundamental change is to turn a general-purpose blockchain into a security and settlement layer on top of which numerous specialized execution infrastructures can operate.

A rollup is a system that groups and executes transactions outside the main chain and uses that chain as a reference to secure, publish, or verify the results. Its importance lies in the possibility of increasing processing capacity without completely abandoning the security infrastructure of an established blockchain. Optimistic rollups and zero-knowledge rollups represent two different approaches to that goal, with their own advantages, costs, and trade-offs. Their evolution will be decisive for the future architecture of decentralized networks, especially as these networks move from primarily managing digital-asset transfers to supporting financial applications, markets, video games, digital services, and other forms of large-scale economic activity.

Sources

• Optimistic rollups — ethereum.org
• Zero-knowledge rollups — ethereum.org