Blockchain networks need to process a large number of transactions while maintaining security, decentralization, and low costs. However, popular Layer 1 blockchains can face congestion, limited transaction throughput, and high fees when network activity increases.

Rollups are one of the most important technologies designed to address this scalability challenge.

Rollups process transactions outside the main blockchain execution environment and then publish transaction data or cryptographic information back to the underlying blockchain. By moving much of the computational workload away from the Layer 1 network, rollups can significantly increase transaction capacity and reduce costs.

There are two major types of rollups:

• Optimistic Rollups
• Zero-Knowledge (ZK) Rollups

Although both approaches aim to improve blockchain scalability, they use fundamentally different methods to verify transactions.

In this guide, we explain what rollups are, how they work, the difference between Optimistic and ZK-Rollups, their advantages and disadvantages, and how they contribute to the future of blockchain scalability.

What Are Rollups in Blockchain?

A rollup is a Layer 2 scaling solution that executes transactions away from the main blockchain while using the underlying Layer 1 blockchain for important security and data availability functions.

Instead of recording every transaction individually on the main blockchain, a rollup can process many transactions together and submit a compressed representation or relevant data back to Layer 1.

This approach can reduce the amount of work that the main blockchain needs to perform.

For example, imagine 1,000 users making transactions.

A traditional Layer 1 system may need to process and record each transaction directly. A rollup can execute those transactions on Layer 2, organize them into a batch, and submit the necessary information to Layer 1.

This allows the blockchain to support a much larger number of transactions.

How Do Rollups Work?

Although Optimistic and ZK-Rollups use different verification mechanisms, the general process looks like this:

1. Users submit transactions to the Layer 2 network.
2. The rollup processes the transactions.
3. Multiple transactions are grouped into a batch.
4. The rollup publishes relevant transaction data to the Layer 1 blockchain.
5. The Layer 1 network provides settlement and security according to the rollup’s design.

The key idea is simple:

More transactions are processed off the main execution layer, while the underlying blockchain remains responsible for verifying or securing the resulting state.

Why Are Rollups Important?

Blockchain scalability has traditionally involved a difficult trade-off between decentralization, security, and performance.

Rollups attempt to improve scalability without requiring users to move to an entirely separate blockchain with a completely independent security model.

Their potential benefits include:

• Higher transaction throughput
• Lower transaction fees
• Reduced congestion on Layer 1
• Faster application interactions
• Greater scalability for decentralized applications
• Continued connection to Layer 1 security

Rollups are particularly important for applications that generate large numbers of transactions, including decentralized finance, blockchain gaming, marketplaces, and other Web3 applications.

What Is an Optimistic Rollup?

An Optimistic Rollup assumes that submitted transactions and state updates are valid unless someone challenges them.

This is where the term “optimistic” comes from.

Instead of requiring every transaction batch to be mathematically proven correct before it is accepted, the system initially treats the batch as valid.

A challenge period allows participants to dispute an invalid state transition.

How Optimistic Rollups Work

A simplified Optimistic Rollup process looks like this:

1. Users send transactions to the Layer 2 network.
2. The transactions are executed and grouped into batches.
3. A transaction batch and state update are submitted to Layer 1.
4. The system assumes the submission is correct.
5. A dispute window allows someone to challenge an invalid result.
6. If a valid challenge is made, the incorrect state transition can be rejected or corrected according to the protocol’s rules.

Optimistic Rollups therefore rely heavily on fraud proofs or fault proofs to detect invalid state transitions.

Advantages of Optimistic Rollups

Strong Compatibility With Existing Applications

Optimistic Rollups can provide relatively strong compatibility with Ethereum’s smart contract environment, making it easier for developers to migrate existing applications.

Lower Computational Requirements for Verification

Unlike ZK-Rollups, Optimistic Rollups generally do not require a validity proof for every batch before it is accepted.

Mature Ecosystem

Optimistic Rollup technology has become an important part of the Ethereum Layer 2 ecosystem, with several major networks using this approach.

Disadvantages of Optimistic Rollups

Withdrawal Delays

Moving assets from an Optimistic Rollup back to Layer 1 can involve a waiting period because the system needs to provide time for potential challenges.

Third-party bridges and liquidity providers can sometimes offer faster exits, but these introduce different trust and liquidity considerations.

Reliance on Fraud or Fault Proof Systems

The security model depends on the ability of the system and its participants to detect and challenge invalid state transitions.

Dispute Complexity

The mechanisms used to resolve fraudulent or disputed transactions can be technically complex.

What Is a ZK-Rollup?

A ZK-Rollup uses cryptographic validity proofs to demonstrate that a batch of transactions has been processed correctly.

“ZK” stands for Zero-Knowledge, referring to a family of cryptographic techniques that can prove certain statements without revealing all the information used to establish the proof.

In the context of rollups, the key concept is the validity proof.

Rather than simply assuming a transaction batch is correct and waiting for challenges, the rollup generates a proof that allows the Layer 1 blockchain to verify that the state transition follows the protocol’s rules.

How ZK-Rollups Work

A simplified ZK-Rollup process looks like this:

1. Users submit transactions to Layer 2.
2. The rollup executes the transactions.
3. Transactions are grouped into a batch.
4. A cryptographic validity proof is generated.
5. The proof and required data are submitted to Layer 1.
6. The Layer 1 blockchain verifies the proof.
7. If the proof is valid, the state update can be accepted.

This model can provide strong and efficient verification of large numbers of transactions.

Advantages of ZK-Rollups

Fast Finality

Because validity is established through a cryptographic proof rather than a challenge period, ZK-Rollups can potentially provide faster finality once the relevant proof is accepted.

Strong Cryptographic Verification

The Layer 1 blockchain can verify the validity proof instead of independently executing every Layer 2 transaction.

Potentially Efficient Scaling

A single proof can represent the correct execution of many transactions, allowing the main blockchain to verify a compact cryptographic object rather than redoing all computation.

Disadvantages of ZK-Rollups

Technical Complexity

Generating validity proofs can require sophisticated cryptography and significant computational resources.

Development Challenges

Building applications for ZK environments can involve additional technical considerations, particularly when existing smart contracts or applications need to be adapted.

Proof Generation Costs

Generating proofs can be computationally expensive, although improvements in hardware, algorithms, and proof systems continue to reduce these costs.

Optimistic Rollups vs. ZK-Rollups

The fundamental difference is how transaction correctness is established.

Neither approach is universally better.

The best solution depends on the application’s requirements, development environment, security assumptions, costs, and desired user experience.

Optimistic Rollups vs. ZK-Rollups: Which Is Better?

There is no single answer.

Optimistic Rollups can be attractive when compatibility with existing Ethereum applications and development tools is a major priority.

ZK-Rollups can be attractive when fast verification, cryptographic validity proofs, and long-term scalability are more important.

The distinction is also becoming less rigid as Layer 2 technology evolves. Developers are continuously improving proof systems, virtual machines, interoperability, and developer tooling.

Rollups and Ethereum

Ethereum has become the primary ecosystem for rollup-based scaling.

Instead of attempting to process every transaction directly on Ethereum’s main execution layer, Layer 2 networks can execute large numbers of transactions and use Ethereum for settlement and data availability.

Ethereum’s broader scaling roadmap increasingly emphasizes rollups and improving the amount and cost of data that Layer 2 networks can publish.

Technologies such as EIP-4844, also known as proto-danksharding, introduced blob transactions designed to make data publication more efficient for rollups.

What Is the Difference Between Rollups and Sidechains?

Rollups and sidechains are not the same thing.

A rollup generally relies on the underlying Layer 1 blockchain for important security and settlement functions.

A sidechain, on the other hand, is a separate blockchain with its own consensus and security assumptions.

This distinction is important because transaction fees and performance are not the only factors to consider when evaluating a scaling solution.

The security model matters as well.

Rollups vs. Layer 1 Blockchains

A Layer 1 blockchain processes transactions directly as part of its base protocol.

A rollup operates as a Layer 2 system and moves much of the transaction execution away from Layer 1.

This creates a division of responsibilities:

Layer 1:

• Settlement
• Consensus
• Security
• Data availability

Layer 2 Rollup:

• Transaction execution
• Transaction batching
• State updates
• Proof or dispute mechanisms

The exact architecture differs between rollups, but this division allows the Layer 2 network to handle a much larger volume of activity.

Are Rollups Secure?

Rollups are designed to inherit important security properties from their underlying Layer 1, but the exact security model depends on how each rollup is implemented.

Users should consider factors such as:

• The proof system
• Data availability
• Upgrade mechanisms
• Smart contract security
• Sequencer design
• Bridge architecture
• Decentralization of network operators
• Withdrawal mechanisms

Therefore, simply being called a “rollup” does not guarantee identical security characteristics across different networks.

What Is a Rollup Sequencer?

A sequencer is responsible for ordering and processing transactions on many Layer 2 networks.

The sequencer can receive user transactions, determine their ordering, execute them, and organize them into batches.

The role of sequencers is important because centralization at the sequencing layer can introduce additional risks or limitations.

Many rollup projects are therefore researching or implementing approaches to make sequencing more decentralized and resilient.

The Future of Rollups

Rollups are expected to remain an important component of blockchain scalability.

Future developments are likely to focus on:

• More efficient zero-knowledge proofs
• Faster proof generation
• Lower transaction fees
• Better interoperability
• Decentralized sequencers
• Improved developer tools
• Greater smart contract compatibility
• Better data availability
• More efficient cross-Layer 2 communication

As these technologies mature, users may interact with Layer 2 networks without even realizing that their transactions are being processed through a rollup.

Frequently Asked Questions About Rollups

What are blockchain rollups?

Blockchain rollups are Layer 2 scaling solutions that process transactions outside the main blockchain execution environment and submit relevant data or cryptographic proofs to the underlying Layer 1 blockchain.

What is an Optimistic Rollup?

An Optimistic Rollup assumes submitted state updates are valid unless they are successfully challenged during a dispute period.

What is a ZK-Rollup?

A ZK-Rollup uses cryptographic validity proofs to demonstrate that a batch of transactions has been processed according to the protocol’s rules.

Which is faster, Optimistic or ZK-Rollups?

The answer depends on the specific implementation. ZK-Rollups can offer faster finality because they do not rely on a long fraud-proof challenge period, but proof generation can require significant computation.

Are ZK-Rollups more secure than Optimistic Rollups?

Not necessarily in every practical situation. They use different security models and implementation choices. Security depends on the underlying proof system, smart contracts, data availability, upgrade mechanisms, and other protocol components.

Are rollups cheaper than Layer 1 transactions?

Rollups can significantly reduce transaction costs by batching many transactions and distributing Layer 1 costs across them. Actual fees vary by network, transaction type, and current demand.

Do rollups replace Layer 1 blockchains?

No. Rollups are designed to work alongside Layer 1 networks. The Layer 1 blockchain can provide settlement, consensus, and other security functions while the rollup handles transaction execution.

Conclusion

Rollups are one of the most important technologies for blockchain scalability. By moving transaction execution to Layer 2 and batching many transactions together, rollups can increase throughput and reduce costs while maintaining a connection to the underlying blockchain.

The two primary approaches—Optimistic Rollups and ZK-Rollups—solve the verification problem in different ways.

Optimistic Rollups rely on the assumption that transactions are valid unless challenged, while ZK-Rollups use cryptographic validity proofs to demonstrate that transactions have been processed correctly.

As proof technology, Layer 2 infrastructure, and data availability continue to improve, rollups will likely play an increasingly important role in making blockchain networks faster, cheaper, and capable of supporting millions of users and a broader range of decentralized applications.