###### Abstract  
Layer 2 solutions have emerged in recent years as a valuable alternative to increase the throughput and scalability of blockchain-based architectures. The three primary types of Layer 2 solutions are state channels, sidechains, and rollups. The rollups are particularly promising, allowing significant improvements in transaction throughput, security, and efficiency, and have been adopted by many real-world projects, such as Polygon and Optimistic. However, the adoption of Layer 2 solutions has led to other challenges, such as the data availability problem, where transaction data processed off-chain must be posted back on the main chain. This is crucial to prevent data withholding attacks and ensure all participants can independently verify the blockchain state. This paper provides a comprehensive survey of existing rollup-based Layer 2 solutions with a focus on the data availability problem and discusses the major advantages and disadvantages of them. Finally, an analysis of open challenges and future research directions is provided. blockchain; layer 2; rollups; data availability +
Footnote †: journal: _Future Internet_  
to Proof-of-Stake (PoS) is an L1 approach aimed at increasing throughput by changing the consensus mechanism. However, an L1 solution involves changing the core design of the blockchain, which tends to raise new issues and leads to forking in the blockchain network. Therefore, the limitations of L1 solutions encourage a new research direction for scaling blockchain transaction throughput, known as L2 protocols. These protocols are developed on top of L1; therefore, they inherit the security and decentralization of the underlying base blockchain. The main idea behind L2 protocols is to process transactions outside of the main blockchain while relying on the base blockchain for security, decentralization, and data availability. For instance, ZK-rollups are a prominent L2 solution where transactions are executed off-chain, and the summary of these transactions, along with cryptographic proofs, are periodically posted to L1. This technique inherits the security and decentralization of L1 while significantly improving scalability. In this way, L2 solutions can increase blockchain scalability and performance without altering the core blockchain design, playing a pivotal role in the evolution of blockchain technology [6]. Several L2 alternatives are available in the literature, and they can be categorized into the following main groups: state channels, sidechains, plasma, rollups, and validiums. Among the various L2 solutions, rollups are considered particularly promising because of their ability to process transactions off-chain, thereby improving scalability, while inheriting security and decentralization of the underlying L1 by posting transaction data and proof of state changes [7]. There are two main categories of rollups: _zk-rollups_ and _optimistic rollups_, which differ from each other in terms of transaction execution, verification, and processing time. Although rollups are a promising solution for ensuring scalability, they introduce new challenges. In particular, _data availability_ is a challenge in which transaction data need to be accessible to any participant. Rollups post the proofs of off-chain computations to L1 to verify that the computations were performed correctly. However, these proofs are not sufficient for independent verification of state transition. The actual transaction data are required, allowing any participant to reconstruct and verify the state independently without relying solely on the rollup operator. This also prevents potential attacks, such as data withholding. The rollup operator could withhold critical transaction data, preventing users from verifying the correctness of the rollup state. This could lead to situations where users cannot challenge incorrect state transitions or recover funds, making the rollup vulnerable to fraud and censorship.