Zero-knowledge (ZK) Rollups have become a vital solution for blockchain scalability. They offer high transaction throughput and reduced costs without compromising the security of the underlying layer-1 blockchain.

However, ensuring data availability is one of the most pressing challenges in implementing and optimizing ZK Rollups. Data availability is essential to maintaining the rollup's integrity and usability, as it ensures that the state data necessary for verifying transactions remains accessible to all network participants.

This guide will explore the challenges and innovations in ensuring data availability within ZK Rollups, focusing on proven strategies, real-world implementations, and technologies that address this critical concern.

The Critical Role of Data Availability in ZK Rollups

In ZK Rollups, data availability refers to the requirement that all necessary data—specifically the data required to reconstruct the rollup state from the published data—be available on-chain so that anyone can verify the correctness of the rollup's state transitions.

Unlike other layer-2 solutions, ZK Rollups do not execute transactions on the layer-1 blockchain; instead, they post succinct cryptographic proofs (known as validity proofs or ZK proofs) that attest to the correctness of a large batch of transactions.

The actual transaction data is often compressed and published on-chain as calldata, stored in Ethereum’s history logs. This method significantly reduces the amount of data that needs to be stored on-chain, helping to lower costs and improve scalability. However, ensuring that this calldata remains available and verifiable poses new challenges.

The Importance of Data Availability

• State Reconstruction: The most immediate consequence of data unavailability is users' inability to reconstruct the rollup's state. Without access to the necessary data, users cannot verify the current state, leading to a severe erosion of trust in the rollup.

• Security Guarantees: ZK Rollups fundamentally depend on the availability of state data to prevent malicious actors from manipulating or obscuring their actual state. If the state data is withheld or inaccessible, the rollup's security is compromised, making it vulnerable to attacks.

• Economic Incentives: Proper data availability is also crucial for maintaining the financial incentives underpinning ZK Rollups' operation. Validators and other participants rely on access to data to perform their roles, such as verifying state transitions, and are, in turn, economically rewarded for their contributions.

• Verification of State Transitions: For ZK Rollups to function securely, validators and users must be able to independently verify the state transitions proposed by the rollup. This process ensures that the rollup correctly processes transactions and maintains an accurate state.

• Trustless Withdrawals: One key feature of ZK Rollups is the ability for users to withdraw their assets back to the layer-1 blockchain without relying on the rollup operator. To do this, users must generate cryptographic proofs that require access to the rollup's state data.

Challenges in Data Availability for ZK Rollups

Data Withholding Attacks

A critical issue in ZK Rollups is the possibility of data withholding attacks. Here, a malicious actor could generate a rollup block but withhold the data others require to validate it.

If validators cannot access this data, they cannot confirm the validity of the state transitions within the rollup. This attack threatens the entire system's security and undermines trust in the rollup's integrity.

Off-Chain Data Storage

ZK Rollups need reliable off-chain data storage to ensure data availability, but traditional centralized methods pose risks due to single points of failure. Decentralized options like IPFS or Arweave offer better security, though they present challenges in maintaining data persistence across distributed networks.

For instance, IPFS uses content-addressable storage, retrieving data by its hash, which enhances integrity but requires strong incentives to keep the data hosted. In ZK Rollups, ensuring critical transaction data remains accessible over time requires well-designed incentives in decentralized storage networks.

Latency and Bandwidth Constraints

Latency and bandwidth are critical challenges for off-chain data availability in ZK Rollups. Retrieving data from decentralized networks can slow down performance, mainly when data is spread across various sources with different access speeds.

As ZK Rollups scale and handle more transactions, the volume of data to be off-loaded and retrieved grows, increasing the strain on bandwidth and further impacting how quickly data can be processed.

Proof Generation and Verification

Generating and verifying ZK proofs are computationally demanding tasks. While they reduce on-chain data by proving the correctness of state transitions without revealing the underlying data, they require access to all relevant information for proof generation.

Optimizing this process involves balancing minimal data storage with the need for all necessary data to generate valid proofs. Despite advancements in zk-SNARKs and zk-STARKs improving proof efficiency, they still depend heavily on the availability of the underlying data.

Data availability in ZK Rollups.

Data Availability Solutions in ZK Rollups

ZK Rollups face unique challenges in ensuring data availability due to their reliance on off-chain computations and the need to securely publish the resulting state data on-chain.

The following are intricate and specific solutions that ZK Rollup implementations use or explore to overcome these data availability challenges. These solutions are unique to ZK Rollups or have been adapted specifically for them.

Data Availability Committees (DACs)

DACs are trusted entities that ensure the data required for reconstructing a rollup’s state is always accessible. They store and verify the data before it’s accepted into the rollup’s state, signing a certificate of data availability before the state transition is finalized on-chain.

An example of a ZK Rollup solution that uses DAC is StarkEx from StarkWare. The platform uses DACs to secure data, ensuring redundancy and resilience against losses. DACs help reduce on-chain costs by offloading data storage, lowering gas fees, and enhancing security by providing data that can be reconstructed even if some members fail.

However, relying on a small group of trusted entities introduces centralization risks, potentially leading to vulnerabilities if the committee fails or colludes.

Hybrid (On-Chain/Off-Chain) Models

Hybrid models in ZK Rollups balance cost, efficiency, and security by combining on-chain data commitments with trusted off-chain mechanisms like Data Availability Committees (DACs). StarkEx uses a mix of on-chain commitments and DACs to maintain data integrity while lowering costs. zkSync employs on-chain Merkle trees alongside off-chain data proofs, offering flexible data storage.

These models are cost-effective, reduce gas fees by minimizing on-chain storage, and are scalable, efficiently handling larger transaction volumes. However, they also face centralization risks due to reliance on DACs, and their implementation is complex, requiring careful coordination between on-chain and off-chain systems.

On-Chain Data Posting (Full Data Availability)

Posting all data on-chain in ZK Rollups ensures maximum availability by publishing state data directly on the layer-1 blockchain, offering the highest levels of transparency and security. Loopring and Immutable X use this approach, guaranteeing data is always accessible and verifiable.

However, this method has significant downsides, primarily the high cost of storing large volumes of data on-chain, especially on networks like Ethereum, where gas fees can be substantial. As rollups scale, these costs become increasingly prohibitive, raising scalability and long-term feasibility challenges.

Calldata Optimization

Some ZK Rollups optimize data availability using Ethereum’s calldata to store transaction data. Calldata offers a more cost-effective on-chain storage option since it’s cheaper than storing data directly in Ethereum. While this method still involves posting data on-chain, it does so at a reduced cost.

For example ,dYdX, built on StarkEx, uses calldata to store transaction data, significantly cutting costs while ensuring data remains accessible for state reconstruction. However, this approach can lead to data redundancy, as duplicating data on-chain can create inefficiencies as the system scales.

Ethereum Calldata functionality.

Data Compression Techniques

Some ZK Rollups use advanced data compression techniques to reduce the amount of data posted on-chain, optimizing costs and data availability. These techniques help minimize gas costs while ensuring necessary data remains accessible for verification and state reconstruction by compressing transaction data or state updates before posting.

For instance, Polygon zkEVM and StarkNet compress data to reduce on-chain data requirements, lowering gas fees while maintaining data integrity. However, decompression adds complexity to the verification process, potentially increasing the time and computational resources needed for state verification.

Exploring Innovative Data Availability Solutions for ZK Rollups

While existing methods like on-chain data posting and DACs have served well, they come with cost, complexity, and scalability trade-offs. Several innovative solutions are being explored to address these challenges that could significantly enhance data availability for ZK Rollups.

Proof of Data Availability (PoDA)

Proof of Data Availability (PoDA) is an innovative solution that ensures the availability of essential data for ZK Rollups without requiring full on-chain storage.

PoDA allows validators to confirm data accessibility across a distributed network by generating cryptographic proofs, addressing challenges like high gas fees and data withholding.

Current Developments and Impact: Syscoin’s implementation of PoDA on its Tanenbaum testnet demonstrates its practicality in real-world applications, reducing overhead and enhancing security in ZK Rollups.

Proto-Danksharding

Proto-Danksharding, introduced via EIP-4844, enhances data availability that can be leveraged by ZK Rollups. It uses data blobs—large data chunks stored off-chain but committed on-chain in compressed form—to reduce the cost of data storage.

Central to this is the use of KZG commitments, which ensure the integrity of the data blobs without needing full data access. This allows ZK Rollups to efficiently verify data without full access, maintaining security and scalability.

Data blobs are stored temporarily, preventing blockchain bloat and supporting higher transaction volumes, which can make ZK Rollups more cost-effective and capable of handling higher transaction volumes.

Specialized Data Availability Layers for ZK Rollups

Specialized Data Availability Layers are explicitly designed to handle the data availability needs of ZK Rollups. These layers are separate from the main rollup and provide a dedicated network or service for storing and ensuring data availability. Celestia and Polygon Avail are two prominent examples of specialized data availability layers that can be utilized for ZK rollup solutions.

A view showing ZK Rollup using a specialized data Availability Layer. Source: AvailBlog

Celestia - Data Availability Sampling (DAS)

Celestia is a modular blockchain network offering a dedicated data availability layer compatible with various rollups, including ZK Rollups. A key innovation is Data Availability Sampling (DAS), which allows nodes to verify data availability efficiently by randomly sampling data portions rather than storing or verifying the entire dataset. If enough samples are accessible and correct, it is statistically likely that the full dataset is available.

Additionally, Namespaced Merkle Trees (NMTs) are used to organize data by namespaces, facilitating more efficient sampling and verification, even for large datasets.

Celestia’s DAS and NMTs reduce the overhead of data availability, enabling ZK Rollups to scale without overloading the main blockchain. These technologies also lower costs by requiring only partial data storage and verification, making ZK Rollups more economically viable.

Polygon Avail - Pluggable Data Availability Layers

Polygon Avail is a specialized data availability layer that works with various rollups, including ZK Rollups. It provides a pluggable data availability solution that developers can tailor to their specific needs, allowing them to choose or switch between different data availability layers depending on the rollup’s security and cost requirements. By offloading data availability functions to Avail, ZK Rollups can significantly reduce on-chain storage costs and lower transaction fees.

Polygon Avail enhances ZK Rollups by offering flexibility to customize data strategies based on cost, security, or performance needs. It also supports higher throughput and efficient resource management, enabling ZK Rollups to scale effectively without compromising data availability.

GoldRush’s Role in Enhancing Data Availability in ZK Rollups

GoldRush (formerly Covalent) supports enhancing data availability for ZK Rollups by providing the essential infrastructure for efficient data access and verification.

While it doesn't directly address the core data availability challenges of ZK Rollups—such as ensuring that all necessary transaction and state data is always accessible—GoldRush complements these systems by ensuring that once data is made available, it can be efficiently retrieved and verified. Other ways it supports ZK Rollup’s data availability are:

1. Data Access and Retrieval: GoldRush’s API allows developers to access and query blockchain data efficiently, supporting the verification processes needed in ZK Rollups. This ensures that data, whether stored on-chain or off-chain, can be quickly retrieved and utilized for state transitions.

2. Integration with Decentralized Storage: GoldRush integrates seamlessly with decentralized storage solutions like IPFS and Arweave, helping to ensure that off-chain data remains accessible and verifiable, which is crucial for maintaining the integrity of ZK Rollups.

3. Transparency and Monitoring: Through real-time monitoring and analytics, GoldRush enhances transparency in ZK Rollup operations. It allows developers to track and verify that necessary data is available and correctly used, thus preventing potential issues like data withholding.

Integrating GoldRush into the ZK Rollup Infrastructure

GoldRush is a critical data access and verification layer in the ZK Rollup stack. While ZK Rollups focus on scaling and ensuring data availability, GoldRush complements these efforts by providing reliable access to blockchain data. It enables developers and operators to interact with, analyze, and verify data that supports rollup state transitions. The following points describe GoldRush's position in the stack:

1. Post-Data Availability: After ensuring data is available through on-chain or off-chain methods, GoldRush facilitates data retrieval and verification, bridging raw data storage and applications needing to process this data.

2. API Integration: GoldRush’s API integrates at various points in the rollup infrastructure, from validation nodes verifying state transitions to user-facing applications displaying transaction history and rollup states.

Conclusion

In this guide, we've delved deep into the intricacies of data availability in ZK Rollups, covering both current implementations and innovative solutions on the horizon. From understanding the critical role of data availability, exploring advanced techniques like PoDA and Proto-Danksharding, and examining the impact of tools like GoldRush, we've highlighted how the ecosystem is evolving to meet the demands of scalability, efficiency, and security.

As ZK Rollups mature, these developments will be crucial in driving their adoption and ensuring they can effectively scale Ethereum while maintaining the network's core principles. The future of Ethereum's scalability rests on these advancements, promising a more robust and decentralized blockchain ecosystem.