# OP Stack Tutorial

This tutorial provides practical instructions and code examples for developers intending to integrate EthStorage with an OP Stack testnet as a long-term DA solution.

## L1 Contracts

In the OP Stack, the `BatchInbox` is an L1 address to which the L2 batch submitter, or batcher, submits transaction batches in the form of L1 calldata or blobs. It is currently designed as a standard Externally Owned Account (EOA) address to reduce gas costs by avoiding the execution of any EVM code.

To enable EthStorage to store transaction batches as blobs, a smart contract must be deployed to accept blob transactions from the batch submitter to replace the EOA. Subsequently, the smart contract invokes the `put(key, blob)` function of the EthStorage storage contract.

<figure><img src="https://522424581-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FMYKgpUdEAwOLB9jkjh8m%2Fuploads%2Fgit-blob-628506bde5344842f39fe67c77407afbdb2cf50a%2Frollup-contract.jpg?alt=media" alt=""><figcaption><p>The BatchInbox Contract serves as the Application Contract in the rollup integration scenario</p></figcaption></figure>

An early version of the `BatchInbox` smart contract with EIP-4844 integration can be found [here](https://github.com/ethstorage/es-op-batchinbox/blob/main/src/BatchInbox.sol). The smart contract employs the blobhash of the blob as the key to put the blob in the EthStorage's key-value store, and covers the upfront storage payment using its ETH balance.

The EthStorage storage contract address on the Sepolia testnet can be found [here](https://docs.ethstorage.io/information#public-testnet-1-deployment).

## L2 Batch Submitter

Some essential changes need to be made for the OP Stack's batch submitter when the `BatchInbox` is a smart contract.

Firstly, it will check if the `BatchInbox` address is a smart contract or an EOA. It is important to estimate gas usage before submitting the transaction to the smart contract, rather than relying on intrinsic gas as in the EOA case. Furthermore, when interacting with a smart contract, it is crucial to implement the verification of transaction status and error handling to ensure data integrity.

For detailed implementation, refer to [this pull request](https://github.com/ethstorage/optimism/pull/22).

## Blob Retrieval

The Ecotone upgrade enables the OP Stack to use EIP-4844 blobs as DA source for L2 chain derivation. A blob can be retrieved from any one of several different sources configured. With a Beacon node location configured, an op-node is expected to retrieve blobs that within the retention window (\~18 days) from a local Beacon node or a third-party RPC service.

Another configurable option, `--l1.beacon-archiver`, enables op-nodes to fetch the expired blobs that normal Beacon nodes would "prune" or remove from [EthStorage blob archiver API](https://docs.ethstorage.io/rollup-guide/..#introduction-to-ethstorage-archiver-api).

The following example specifies a living EthStorage archiver API on the Sepolia testnet.

```
--l1.beacon-archiver https://archive.testnet.ethstorage.io:9635
```

Like the Beacon API, the blobs can be queried via the `/eth/v1/beacon/blobs/` endpoint, using the `versioned_hashes` filter to skip irrelevant blobs. The process operates by combining the archiver and the Beacon location into a resource pool for retrieving blobs. If the op-node fails to obtain blobs from the Beacon node, it will use the archiver endpoint as a fallback.

One detail worth noting is how the Beacon API's response to expired blobs affects the retrieval process. At least some of the Beacon clients (e.g., [Prysm](https://github.com/prysmaticlabs/prysm/blob/feb16ae4aaa41d9bcd066b54b779dcd38fc928d2/beacon-chain/rpc/lookup/blocker.go#L225)) return 200 status code and an empty list while [the OP Stack code](https://github.com/ethereum-optimism/optimism/blob/develop/op-service/sources/l1_beacon_client.go#L212) is expecting an error. [Addressing this issue](https://github.com/ethereum-optimism/optimism/pull/10269) is crucial for the proper functioning of the retrieval of historical blobs.


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