Why is data availability so important for Layer2? (Part 1)

The Significance of Data Availability for Layer2 (Part 1)

Ethereum Foundation researcher Dankrad Feist once tweeted that if data availability is not achieved without using Ethereum, it is not L2. According to his statement, many chains, such as Arbitrum Nova, Polygon, and Mantle, would be kicked out of the L2 team.

So, what exactly is data availability? What data availability problems does L2 face? Why is there so much controversy surrounding data availability in the L2 layer? This article will focus on these questions and try to uncover the mystery of data availability.

What is Data Availability

Simply put, data availability refers to the situation where block producers publish all transaction data of a block to the network so that validators can download it.

If a block producer publishes complete data and enables validators to download it, we say the data is available; if it withholds some data, preventing validators from downloading the complete data, we say the data is not available.

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The Difference Between Data Availability and Data Retrieval

Often, we easily confuse data availability with data retrieval, but the two are quite different.

• Data availability pertains to the stage when blocks are produced but not yet added to the blockchain through consensus. Therefore, data availability is not related to historical data but rather to whether newly published data can be accessed through consensus.
• Data retrieval, on the other hand, pertains to the stage when data has already been permanently stored in the blockchain through consensus, i.e., the ability to retrieve historical data. Nodes in Ethereum that store all historical data are called archive nodes.

Therefore, a co-founder of L2BEAT once stated in a lengthy tweet that full nodes are not obliged to provide us with historical data and the reason we can obtain it is only because full nodes are kind enough to do so.

He also suggested replacing the term “data availability” with “data publishing” since the former can be misleading, a view that was also endorsed by the founder of Celestia.

Data Availability Issues in L2

Although the concept of data availability originates from Ethereum, currently, we are primarily concerned with data availability at the L2 level.

In L2, the sequencer acts as the block producer, and they need to publish sufficient transaction data for validators to check the validity of transactions. (To learn more about sequencers, please read our past article “Research Report | The Principles, Status, and Future of Sequencers“.)

However, in this process, two problems arise: ensuring the secure execution of the validation mechanism and reducing the cost of data publishing. The following will provide specific explanations.

Issues with Securing Validation Mechanisms

We know that OP Rollup verifies the validity of transactions using fraud proofs, while ZK Rollup uses validity proofs.

• For OP Rollup: if the sequencer does not publish complete data for tracing blocks, challengers in fraud proofs will be unable to launch valid challenges.
• For ZK Rollup: although validity proofs do not require data availability, ZK Rollup as a whole still requires data availability. Without traceable block data, users will not know their balances and may lose assets.

To ensure secure validation, current L2 sequencers generally publish L2 state data and transaction data on the more secure Ethereum network, relying on Ethereum for settlement and data availability.

Therefore, the data availability layer is actually where L2 publishes transaction data. Currently, mainstream L2 solutions use Ethereum as the data availability layer.

The Cost Issue of Publishing Data

Current L2 solutions simply have data availability and settlement occur on Ethereum, which provides sufficient security but also incurs significant costs. This is the second issue faced by L2, which is how to reduce the cost of publishing data.

The total gas paid by users to L2 mainly consists of the gas used for executing transactions on L2 and the gas used for L2 to submit data to L1. The former cost is minimal, while the latter is the major cost for users. Among these, the transaction data published for data availability constitutes a significant portion of the data submitted by L2 to L1, while the proof data verifying the validity of transactions only accounts for a small part.

Therefore, in order to make L2 more cost-effective, the cost of publishing data must be reduced. How can this be achieved? There are mainly two methods:

• Reduce the cost of publishing data on L1. For example, the upcoming EIP-4844 upgrade on Ethereum. If you are interested in the EIP-4844 upgrade, you can read the previous article “Web3 Introduction: Understanding the Great Advantages of Layer2: EIP-4844” in the Insight Weekly;
• Imitate Rollup, separating transaction execution from L1, and also separating data availability from L1 to reduce costs, which means not using Ethereum as the data availability layer.

Controversy Surrounding L2 and the Data Availability Layer

To discuss the controversy surrounding L2 and the data availability layer, we need to start with modular blockchains. Modular blockchains decouple the core functionalities of the entire blockchain, creating relatively independent parts, and expand the performance of a single blockchain through combinations of various dedicated networks.

Although there is still some controversy regarding the layering of modular blockchains, the generally accepted division of modular blockchains is into four layers: Execution Layer, Settlement Layer, Consensus Layer, and Data Availability Layer. The functions of each module are shown in the following diagram:

Modular blockchains are similar to Lego bricks, allowing for customization and the use of the best building blocks to create a solid model, thus solving the “impossible triangle” problem of blockchain.

However, current Layer 2 solutions still rely on Ethereum for functionality other than the execution layer. Due to cost considerations, many Layer 2 solutions are preparing to separate the data availability layer from Ethereum, treating Ethereum only as a settlement and consensus layer.

Interestingly, Ethereum seems reluctant to let Layer 2 solutions obtain data availability from elsewhere. Dankrad Feist, a researcher at the Ethereum Foundation, once tweeted that a Rollup without Ethereum as the data availability layer is not a true Rollup, and therefore not Layer 2.

Moreover, L2BEAT’s latest definition of Layer 2 states that any scaling solution that does not publish data on Layer 1 is not considered Layer 2 because using off-chain data availability solutions cannot guarantee that operators will provide the published data.

Of course, there is still no definitive answer to what constitutes Layer 2. Both Ethereum Foundation members and L2BEAT insist that the data availability layer should remain on Ethereum, seemingly for security reasons. But is there a concern about destabilizing Ethereum’s position?

Ethereum’s vision is to become a supercomputer platform. Later, in order to improve network performance, Rollup was developed, and many ecosystems moved to the more cost-effective Layer 2 solutions. However, because Ethereum provides security, its position has not been greatly affected. But if Layer 2 were to remove the data availability layer involving data publishing from Ethereum, it would fundamentally weaken the reliance on Ethereum’s security and gradually drift away from Ethereum, posing a threat to its position.

Nevertheless, the development of projects related to data availability layer continues to thrive. In the next article about data availability, the author will provide a detailed introduction to the main data availability solutions and related projects in the market, so stay tuned.

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