Why is data availability so important for Layer 2?

Dankrad Feist, a researcher at the Ethereum Foundation, once said in a tweet that it is not L2 without using Ethereum for data availability. If you follow his arguments, many chains will be kicked out of L2 teams, such as Arbitrum Nova, Polygon, and Mantle.

So, what exactly is data availability, what kind of data availability problems does L2 face, and why is there so much controversy about the data availability layer, L2? This article will focus on these issues and try to demystify data availability.

What is data availability

In simple terms, data availability is when a block producer publishes all transaction data for a block to the network so that validators can download it.

If a block producer publishes full data and makes it available for validators to download, we say the data is available, and if it withholds some data that prevents validators from downloading the full data, we say the data is unavailable.

The difference between data availability and data retrievability

It's often easy to confuse data availability with data retrievability, but they're very different.

• Data availability is concerned with the stage at which a block is produced but not yet added to the blockchain by consensus, so data availability is not related to historical data, but to whether newly published data can pass consensus.
• Data retrievability refers to the stage after the data has been agreed and stored forever on the blockchain, i.e., the ability to retrieve historical data. Nodes that store all historical data in Ethereum are known as archive nodes.

Therefore, the co-founder of L2 BEAT once said in a long tweet that the full node is not obligated to provide us with historical data, and the reason why we can get it is only because the full node is kind enough.

! [Why is data availability so important for Layer 2?] (https://piccdn.0daily.com/202311/09074833/7h60wrwtzjdi2eb6.png!webp)

At the same time, he also said that the term Data Availability could be misleading and should be replaced with Data Publishing, a statement echoed by the founder of Celestia.

! [Why is data availability so important for Layer 2?] (https://piccdn.0daily.com/202311/09074833/zr6p2w9fkci2k99c.png!webp)

Data availability issues in L2

While the concept of data availability is derived from Ethereum, we are currently focusing on data availability at the L2 level.

In L2, the sequencer is the block producer, and they publish enough transaction data for validators to check if the transaction is valid. (For more information about Sequencer, please read the previous article "Research Report: The Principle, Present and Future of Sequencer")

However, there are two problems in this process, one is to ensure that the verification mechanism is carried out securely, and the other is to reduce the cost of publishing data. More on this below.

The issue of ensuring that the authentication mechanism is carried out safely

We know that OP Rollup uses fraud proofs to verify the validity of transactions, while ZK Rollups use validity proofs.

• For OP Rollups: If the Sequencer does not publish the complete data that can retrace the block, the challenger in the fraud proof will not be able to launch a valid challenge;
• For ZK Rollups: While validity proofs themselves do not require data availability, ZK Rollups as a whole still require data availability, and if there is no data that can retrace the block, then users will not be able to know their balances and may lose their assets.

In order to make the verification safe, the current L2 sequencer generally publishes the state data and transaction data of L2 on Ethereum, which has strong security, and relies on Ethereum for settlement and data availability.

Therefore, the data availability layer is actually where L2 publishes transaction data, and the current mainstream L2 uses Ethereum as the data availability layer.

Reduce the cost of publishing data

Today's L2 simply takes data availability and settlement on Ethereum, which has sufficient security, but also bears a huge cost. This is the second problem that L2 faces, which is how to reduce the cost of publishing data.

The total gas paid by users to L2 is mainly composed of the gas incurred by L2 executing transactions and the gas generated by L2 submitting data to L1, the former fee is negligible, and the latter is the majority of user fees, in which the transaction data released to ensure the availability of data accounts for the main part of L2's submission of data to L1, and the proof data verifying the validity of the transaction only accounts for a small part.

! [Why is data availability so important for Layer 2?] (https://piccdn.0daily.com/202311/09074833/s6p2cq474gaexvyf.png!webp)

Therefore, in order to make L2 cheaper overall, the cost of publishing data must be reduced. So, how do you reduce costs? There are two main ways:

• Reduce the cost of publishing data on L1, such as Ethereum's upcoming EIP-4844 upgrade, if you are interested in EIP-4844 upgrade, you can read the previous article "Web3 Science|Easy to Understand the Benefits of Layer 2: EIP-4844";
• Following the Rollup, transaction execution is detached from L1, and data availability can also be detached from L1 to reduce costs, i.e., Ethereum is not used as a data availability layer.

L2 disputes over the data availability layer

To talk about the L2 controversy over the data availability layer, we have to start with the modular blockchain. Modular blockchain is to decouple the core functions of the overall blockchain to form relatively independent parts, and expand the performance of a single blockchain through the combination of various private networks.

Although there is still some controversy about the layering of modular blockchains, it is generally accepted that modular blockchains are divided into four layers, namely the execution layer (ution), settlement layer (Settlement), consensus layer (Consensus) and data availability layer (Data Availability). The functions of each module are shown in the figure below

! [Why is data availability so important for Layer 2?] (https://piccdn.0daily.com/202311/09074833/8p6b0p83t6jet69i.png!webp)

Modular blockchains are similar to Lego bricks, which can be customized to build a good model with the best building blocks, alleviating the problem of the "impossible triangle" of the blockchain.

However, in addition to separating the execution layer from Ethereum, the other three layers of L2 still perform functions on Ethereum. However, due to cost considerations, many L2s are also preparing to separate the data availability layer from Ethereum, and use Ethereum only as a settlement layer and consensus layer.

Interestingly, Ethereum doesn't seem to want L2 to get data availability elsewhere, with Dankrad Feist, a researcher at the Ethereum Foundation, saying in a tweet that not using Ethereum as a data availability layer is not a rollup, and therefore not an L2.

! [Why is data availability so important for Layer 2?] (https://piccdn.0daily.com/202311/09074833/ep4ijrcx30s7v6hn.png!webp)

At the same time, L2 BEAT's latest definition of L2 also points out that the scaling scheme that does not publish data at L1 is not L2, because using an off-chain data availability solution cannot guarantee that the operator will provide the published data.

! [Why is data availability so important for Layer 2?] (https://piccdn.0daily.com/202311/09074833/aw0vm52c43kpqf44.png!webp)

Of course, there is no conclusive conclusion on what L2 is, and the above Ethereum Foundation members and L2 BEAT insist that L2 should keep the data availability layer on Ethereum for security reasons, but are there any concerns about Ethereum's status being shaken?

Ethereum's vision was to become a supercomputer platform, and later in order to improve network performance, it had to develop rollups and make many ecosystems run on cheaper L2 development, but because the security is provided by Ethereum, it has not had much impact on Ethereum's position. However, if L2 also strips Ethereum of the data availability layer involved in data publishing, it essentially weakens the dependence on Ethereum's security and gradually moves away from Ethereum, which poses a threat to Ethereum's position.

However, no matter what, it still can't stop the development of projects related to the data availability layer. In the next article on data availability, the author will introduce the main data availability solutions and specific related projects on the market in detail, so stay tuned.