Modular Data Availability in OP Stack

Original Text: Javed Khan | celestia blog Translation: GWEI Research

Introduction

Since its release last year, OP Stack has gained tremendous traction among developers. It has been embraced by new rollup developers and modular infrastructure providers such as Caldera and Conduit, allowing developers to quickly launch their own rollups.

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As announced last year, modularity is a fundamental aspect of OP Stack’s vision:

Optimism’s rapid-approaching Bedrock upgrade will modularize OP Stack’s execution layer and proof system, enabling compatibility with future fraud and efficacy proofs.

Inspired by this, Celestia Labs has been focused on further pushing the modularity of OP Stack. As such, we are excited to announce the beta release of OP Stack’s Modular Data Availability (DA) interface, the first OP Stack Mod focused on developer feedback. The interface allows developers to define their DA layer and inherit security from any blockchain they like, whether it be Ethereum, Celestia, or Bitcoin.

Developers can now experiment with using Celestia for DA and “settle” their OP Stack version on Ethereum starting today. Caldera will soon be releasing Taro Testnet, which will allow developers and users to trial Modular DA on the first public testnet for OP Stack.

The Data Availability layer is the foundation of the rollup architecture, ensuring the availability of data necessary to independently verify a rollup chain. Below, we will explore the basics of data availability in the OP stack and how we’re modularizing it by defining a well-designed DA interface for publishing and retrieving data from L1.

Data Availability in the OP Stack: Today

How does OP Stack handle data availability today? For our purposes, we’ll dive into two fundamental components, the Rollup Node and Batcher, as outlined below.

To get a broader sense of how the rest of OP Stack works behind the scenes, check out the Optimism docs.

Rollup Node

The Rollup Node is the component responsible for deriving the correct L2 chain from an L1 block (and its associated receipts). The rollup node retrieves an L1 block, filters data transactions (typically in the form of transaction `calldata`), and derives the correct L2 chain from that data.

Batcher – The Batch Submitter

The Batcher, also known as the Batch Submitter, is the entity that submits L2 Sequencer data to L1 for the validators to use. Rollup nodes and batchers both work in a loop, which allows the batcher’s newly submitted L2 block data to be retrieved by the rollup node from L1 and used to derive the next L2 block.

Each transaction submitted by the batcher program contains calldata, which is L2 Sequencer data, divided into bytes called frames, and is the lowest abstraction level of data in Optimism.

Modular DA Interface for OP Stack

When creating a modular DA interface for OP Stack, our goal was simple: to enable rollup developers to designate any blockchain as their data availability layer, whether it be Ethereum, Celestia, or even Bitcoin. Without such an interface, each integration of a new DA layer could require developers to implement and maintain a separate branch of OP Stack.

OP Stack already includes abstractions for specifying the L1Chain and L2Chain in the codebase, allowing us to model a new blockchain-agnostic interface for the data availability chain, which we call DAChain.

Using the interface defined below, developers can implement DAChain to read from and write to any underlying blockchain or even a centralized backend like S3.

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Writing Phase
Below is an overview of the Celestia implementation of the interface that integrates with the batch processing program:
SimpleTxManager.send is the function responsible for creating and sending actual transactions, which is modified to call WriteFrame to write frames to Celestia and return a reference. The reference is then submitted as calldata to the batch processing inbox address in place of the usual frame data.
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Reading Phase
Below is an overview of the Celestia implementation of the interface that integrates with the rollup node:
DataFromEVMTransactions is the function responsible for returning frame data from a list of transactions. It is modified to actually retrieve the frame using the frame reference retrieved from calldata in the batch processing inbox and append it to the returned data.
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 Please note that the call to NamesBlockingcedData returns a byte slice array of all blobs submitted at the given BlockHeight, so we only return the TxIndex we are interested in.
Integrating Celestia as a DA Layer

Shows a chart of the OP stack architecture compared to the Celestia+OP stack integration.
By making some small modifications to the Rollup node and batcher program, we can make OP Stack use Celestia for DA.
This means that all the data needed to derive an L2 chain can be provided as local blob data on Celestia instead of being published to Ethereum, although a small fixed-size frame reference is still published to Ethereum as part of the batcher's calldata. The frame reference is used to look up the corresponding frame on Celestia using a celestia-node light node.
How does it work?
Writing Phase
As mentioned above, the batcher submits L2 sequencer data as bytes called frames to the batcher inbox contract address on Ethereum L1.
We retain the batcher and calldata transactions to ensure frame ordering, but we replace the frame in calldata with a fixed-size frame reference. What is a reference frame? It is a reference to a Celestia data transaction that has successfully included the frame data as part of Celestia.
We accomplish this by embedding a celestia-node light node in the batcher service. Whenever there is a new batch waiting to be submitted, we first submit the data transaction to Celestia using the light node and then submit only the frame reference in the batcher calldata.
Reading Phase
In the reading phase, we do the opposite, i.e., we use the frame reference in the batcher transaction calldata to resolve it and retrieve the corresponding actual frame data from Celestia. Again, we embed a celestia-node light node in the rollup node to query its transaction.
When deriving an L2 chain, the rollup node now transparently reads the data from the light node and can continue building new blocks. The light node only downloads data submitted by the rollup, not the entire chain like Ethereum.
Outlook
Fraud proofs are a key part of the Optimism after Bedrock roadmap, and we hope to explore upgrading our OP Stack x Celestia integration to use fraud proofs on the Ethereum mainnet.
To do this, we can leverage the Quantum Gravity Bridge (QGB), which relays cross-chain DA proofs to Ethereum, to enable on-chain verification that aggregated data is available on Celestia so that aggregated data can be used in fraud proofs. This will allow OP Stack Rollup to directly leverage the DA guarantees provided by Celestia.

                  
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