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Json Api

The JsonApi attestation type enables data collection from an arbitrary Web2 source. You can learn more about it in the official specification repo.

We will now demonstrate how the FDC protocol can be used to collect the data of a given Star Wars API request. The request we will be making is https://swapi.dev/api/peaople/3/. The same procedure works for all public APIs.

In this guide, we will follow the steps outlined in the FDC overview. We will define a scripts/fdcExample/JsonApi.ts file that will encapsulate the whole process.

scripts/fdcExample/JsonApi.s.sol
import { run, web3 } from "hardhat";
import { TransferEventListenerInstance } from "../../typechain-types";
import {
  prepareAttestationRequestBase,
  submitAttestationRequest,
  retrieveDataAndProofBase,
} from "./Base";

const JsonApi = artifacts.require("TransferEventListener");

const { VERIFIER_URL_TESTNET, VERIFIER_API_KEY_TESTNET, COSTON2_DA_LAYER_URL } =
  process.env;

...

async function main() {
  const data = await prepareAttestationRequest(
    apiUrl,
    postprocessJq,
    abiSignature
  );
  console.log("Data:", data, "\n");

  const abiEncodedRequest = data.abiEncodedRequest;
  const roundId = await submitAttestationRequest(abiEncodedRequest);

  const proof = await retrieveDataAndProof(abiEncodedRequest, roundId);

  const characterList: StarWarsCharacterListInstance =
    await deployAndVerifyContract();

  await interactWithContract(characterList, proof);
}

main().then((data) => {
  process.exit(0);
});

The function names mostly mirror the steps described in the FDC guide.

Prepare request​

In this guide we will demonstrate how to prepare an attestation request through a verifier server. At the end of the section we will provide a breakdown of the abi encoded request; thus we will demonstrate how it can be constructed manually.

To use the verifier server, we send a request to its prepareRequest endpoint. A JSON request to the verifier follows the same structure for all attestation types, with field values varying between types.

Required Fields​

  • attestationType is the UTF8 hex string encoding of the attestation type name, zero-padded to 32 bytes.
  • sourceId is the UTF8 hex string encoding of the data source identifier name, zero-padded to 32 bytes.
  • requestBody is different for each attestation type.

In the case of JsonApi, requestBody is a JSON containing the fields:

  • url: url of the data source; as string
  • postprocessJq: JQ filter to postprocess the json data received from the URL; as string
  • abi_signature: ABI signature of the Solidity struct that will be used to decode the data; as string

Reference Documentation​

Example Values​

  • url: the above address https://swapi.dev/api/people/3/
  • postprocessJq: {name: .name, height: .height, mass: .mass, numberOfFilms: .films | length, uid: (.url | split(\\"/\\") | .[-2] | tonumber)}
  • abi_signature:
{\\"components\\": [
{\\"internalType\\": \\"string\\", \\"name\\": \\"name\\", \\"type\\": \\"string\\"},
{\\"internalType\\": \\"uint256\\", \\"name\\": \\"height\\", \\"type\\": \\"uint256\\"},
{\\"internalType\\": \\"uint256\\", \\"name\\": \\"mass\\", \\"type\\": \\"uint256\\"},
{\\"internalType\\": \\"uint256\\", \\"name\\": \\"numberOfFilms\\", \\"type\\": \\"uint256\\"},
{\\"internalType\\": \\"uint256\\", \\"name\\": \\"uid\\", \\"type\\": \\"uint256\\"}
],
\\"name\\": \\"task\\",\\"type\\": \\"tuple\\"}
scripts/fdcExample/JsonApi.ts
// Request data
const apiUrl = "https://swapi.dev/api/people/3/";
const postprocessJq = `{name: .name, height: .height, mass: .mass, numberOfFilms: .films | length, uid: (.url | split(\"/\") | .[-2] | tonumber)}`;
const abiSignature = `{\"components\": [{\"internalType\": \"string\", \"name\": \"name\", \"type\": \"string\"},{\"internalType\": \"uint256\", \"name\": \"height\", \"type\": \"uint256\"},{\"internalType\": \"uint256\", \"name\": \"mass\", \"type\": \"uint256\"},{\"internalType\": \"uint256\", \"name\": \"numberOfFilms\", \"type\": \"uint256\"},{\"internalType\": \"uint256\", \"name\": \"uid\", \"type\": \"uint256\"}],\"name\": \"task\",\"type\": \"tuple\"}`;

// Configuration constants
const attestationTypeBase = "IJsonApi";
const sourceIdBase = "WEB2";
const verifierUrlBase = JQ_VERIFIER_URL_TESTNET;

Encoding Functions​

To encode values into UTF8 hex:

  • toUtf8HexString: Converts a string to UTF8 hex.
  • toHex: Zero-right-pads the string to 32 bytes.

These functions are included in the Base library within the example repository, but they can also be defined locally in your contract or script.

scripts/fdcExample/Base.ts
function toHex(data: string) {
  var result = "";
  for (var i = 0; i < data.length; i++) {
    result += data.charCodeAt(i).toString(16);
  }
  return result.padEnd(64, "0");
}
scripts/fdcExample/Base.ts
function toUtf8HexString(data: string) {
  return "0x" + toHex(data);
}

Because of the console.log commands it will produce JSON strings that represent valid requests; we can then pass this to the interactive verifier to check the response.

The process of posting a request to a verifier server is identical for all attestation types. It differs only in values used. For that reason we define a base function that the prepareAttestationRequest function will call.

The prepareAttestationRequestBase function formulates a request for the verifier server, and posts it to the given URL.

scripts/fdcExample/Base.ts
async function prepareAttestationRequestBase(
  url: string,
  apiKey: string,
  attestationTypeBase: string,
  sourceIdBase: string,
  requestBody: any,
) {
  console.log("Url:", url, "\n");
  const attestationType = toUtf8HexString(attestationTypeBase);
  const sourceId = toUtf8HexString(sourceIdBase);

  const request = {
    attestationType: attestationType,
    sourceId: sourceId,
    requestBody: requestBody,
  };
  console.log("Prepared request:\n", request, "\n");

  const response = await fetch(url, {
    method: "POST",
    headers: {
      "X-API-KEY": apiKey,
      "Content-Type": "application/json",
    },
    body: JSON.stringify(request),
  });
  if (response.status != 200) {
    throw new Error(
      `Response status is not OK, status ${response.status} ${response.statusText}\n`,
    );
  }
  console.log("Response status is OK\n");

  return await response.json();
}

In the example repository, it is once again included within the Base library file.

We construct the URL by appending to the verifier address https://fdc-verifiers-testnet.flare.network/ the path verifier/btc/JsonApi/prepareRequest. If we were using another source, we would replace the string eth with sgb or flr accordingly (we would also have to replace testETH with testSGB or testFLR). Thus, the function that prepares the verifier request looks like:

scripts/fdcExample/JsonApi.ts
async function prepareAttestationRequest(
  apiUrl: string,
  postprocessJq: string,
  abiSignature: string,
) {
  const requestBody = {
    url: apiUrl,
    postprocessJq: postprocessJq,
    abi_signature: abiSignature,
  };

  const url = `${verifierUrlBase}JsonApi/prepareRequest`;
  const apiKey = JQ_VERIFIER_API_KEY_TESTNET!;

  return await prepareAttestationRequestBase(
    url,
    apiKey,
    attestationTypeBase,
    sourceIdBase,
    requestBody,
  );
}
Details

Understanding the abiEncodedRequest. If everything went right, the abiEncodedRequest should look something like this (minus the line breaks - we split it after the 0x symbol and then after every 64 characters (32 bytes), for the sake of clarity).

0x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Let's break it down line by line:

  • First line: toUtf8HexString("JsonApi")
  • Second line: toUtf8HexString("testETH")
  • Third line: message integrity code (MIC), a hash of the whole response salted with a string "Flare", ensures the integrity of the attestation
  • Remaining lines: ABI encoded JsonApi.RequestBody Solidity struct

What this demonstrates is that, with some effort, the abiEncodedRequest can be constructed manually.

Submit request to FDC​

This step transitions from off-chain request preparation to on-chain interaction with the FDC protocol. We will submit the validated request to the blockchain using deployed official Flare smart contracts. To streamline the process of accessing these, the Flare smart contracts periphery package is shipped with the ContractRegistry library.

The ContractRegistry library allows us to

We define a Helpers contract that will give us access to the following contracts:

  • FdcHub: for posting the request to
  • FdcRequestFeeConfigurations: calculates the fee of the request
  • FlareSystemsManager: for calculating the round ID
  • Relay: confirms the round has finalized
contracts/fdcExample/Helpers.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.25;

import {console} from "hardhat/console.sol";
import {ContractRegistry} from "@flarenetwork/flare-periphery-contracts/coston2/ContractRegistry.sol";
import {IFdcHub} from "@flarenetwork/flare-periphery-contracts/coston2/IFdcHub.sol";
import {IFdcRequestFeeConfigurations} from
    "@flarenetwork/flare-periphery-contracts/coston2/IFdcRequestFeeConfigurations.sol";
import {IFlareSystemsManager} from "@flarenetwork/flare-periphery-contracts/coston2/IFlareSystemsManager.sol";
import {IRelay} from "@flarenetwork/flare-periphery-contracts/coston2/IRelay.sol";

contract Helpers {
    function getFdcHub() public view returns (IFdcHub) {
        return ContractRegistry.getFdcHub();
    }

    function getFdcRequestFeeConfigurations() public view returns (IFdcRequestFeeConfigurations) {
        return ContractRegistry.getFdcRequestFeeConfigurations();
    }

    function getFlareSystemsManager() public view returns (IFlareSystemsManager) {
        return ContractRegistry.getFlareSystemsManager();
    }

    function getRelay() public view returns (IRelay) {
        return ContractRegistry.getRelay();
    }
}

We expose the Helpers contract through the following function.

scripts/fdcExample/Base.ts
async function getHelpers() {
  const helpers: HelpersInstance = await Helpers.new();
  return helpers;
}

To submit the attestation request, we first access the deployed FdcHub contract. We determine the fee for our attestation type, and then request the attestation of the FDC, paying the required fee. Lastly, we calculate the voting round Id from the transaction that carried the attestation request; we will need it to query the data and proof.

scripts/fdcExample/Base.ts
async function submitAttestationRequest(abiEncodedRequest: string) {
  const fdcHub = await getFdcHub();

  const requestFee = await getFdcRequestFee(abiEncodedRequest);

  const transaction = await fdcHub.requestAttestation(abiEncodedRequest, {
    value: requestFee,
  });
  console.log("Submitted request:", transaction.tx, "\n");

  const roundId = await calculateRoundId(transaction);
  console.log(
    `Check round progress at: https://${hre.network.name}-systems-explorer.flare.rocks/voting-epoch/${roundId}?tab=fdc\n`,
  );
  return roundId;
}
scripts/fdcExample/Base.ts
async function getFdcHub() {
  const helpers: HelpersInstance = await getHelpers();
  const fdcHubAddress: string = await helpers.getFdcHub();
  return await FdcHub.at(fdcHubAddress);
}

The request fee is obtained from the fdcRequestFeeConfigurations contract. We once again connect to the fdcRequestFeeConfigurations contract through the ContractRegistry library.

scripts/fdcExample/Base.ts
async function getFdcRequestFee(abiEncodedRequest: string) {
  const helpers: HelpersInstance = await getHelpers();
  const fdcRequestFeeConfigurationsAddress: string =
    await helpers.getFdcRequestFeeConfigurations();
  const fdcRequestFeeConfigurations: IFdcRequestFeeConfigurationsInstance =
    await FdcRequestFeeConfigurations.at(fdcRequestFeeConfigurationsAddress);
  return await fdcRequestFeeConfigurations.getRequestFee(abiEncodedRequest);
}

The round ID is calculate from the timestamp of the block, containing the transaction requesting attestation. We first subtract from the block timestamp the timestamp of the first voting epoch. Then, we divide the number by the duration of the voting epoch (90 seconds). Instead of hard-coding them, we retrieve these values from another official Flare contract, the flareSystemsManager.

scripts/fdcExample/Base.ts
async function calculateRoundId(transaction: any) {
  const blockNumber = transaction.receipt.blockNumber;
  const block = await ethers.provider.getBlock(blockNumber);
  const blockTimestamp = BigInt(block!.timestamp);

  const flareSystemsManager: IFlareSystemsManagerInstance =
    await getFlareSystemsManager();
  const firsVotingRoundStartTs = BigInt(
    await flareSystemsManager.firstVotingRoundStartTs(),
  );
  const votingEpochDurationSeconds = BigInt(
    await flareSystemsManager.votingEpochDurationSeconds(),
  );

  console.log("Block timestamp:", blockTimestamp, "\n");
  console.log("First voting round start ts:", firsVotingRoundStartTs, "\n");
  console.log(
    "Voting epoch duration seconds:",
    votingEpochDurationSeconds,
    "\n",
  );

  const roundId = Number(
    (blockTimestamp - firsVotingRoundStartTs) / votingEpochDurationSeconds,
  );
  console.log("Calculated round id:", roundId, "\n");
  console.log(
    "Received round id:",
    Number(await flareSystemsManager.getCurrentVotingEpochId()),
    "\n",
  );
  return roundId;
}

We obtain the flareSystemsManager contract through the ContractRegistry library and the previously defined Helpers contract as well.

scripts/fdcExample/Base.ts
async function getFlareSystemsManager() {
  const helpers: HelpersInstance = await getHelpers();
  const flareSystemsManagerAddress: string =
    await helpers.getFlareSystemsManager();
  return await FlareSystemsManager.at(flareSystemsManagerAddress);
}

Retrieve data and proof​

To retrieve the data and proof, we must first wait for the voting round in which the attestation request was submitted to finalize; this takes no more than 180 seconds, but is on average much less. After the round has been finalized, we post a request to a DA Layer provider.

We can check if the request was submitted successfully on the AttestationRequests page on the Flare Systems Explorer website. To check if the round has been finalized, go to Finalizations page.

If you want to learn more about how the FDC protocol works, check here.

Because the process includes waiting for the voting round to finalize, we prepare a sleep function. The function pauses the execution of the script for a given number of milliseconds.

scripts/fdcExample/Base.ts
function sleep(ms: number) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

The only difference between the retrieveDataAndProof functions of all six attestation types is the URL of the DA Layer server. For that reason, we will define as separate retrieveDataAndProofBase function that will handle most of the logic. The function waits for the round to finalize - rechecking every 10 seconds if necessary. Then, it prepares a proof request, and posts it to the DA Layer server. Because it might take a few seconds for the server to generate the proof, the function ensures that the response actually contains a sufficient response, and retries otherwise.

scripts/fdcExample/Base.ts
async function retrieveDataAndProofBase(
  url: string,
  abiEncodedRequest: string,
  roundId: number,
) {
  console.log("Waiting for the round to finalize...");
  // We check every 10 seconds if the round is finalized
  const relay: IRelayInstance = await getRelay();
  while (!(await relay.isFinalized(200, roundId))) {
    await sleep(10000);
  }
  console.log("Round finalized!\n");

  const request = {
    votingRoundId: roundId,
    requestBytes: abiEncodedRequest,
  };
  console.log("Prepared request:\n", request, "\n");

  await sleep(10000);
  var proof = await postRequestToDALayer(url, request, true);
  console.log("Waiting for the DA Layer to generate the proof...");
  while (proof.response_hex == undefined) {
    await sleep(5000);
    proof = await postRequestToDALayer(url, request, false);
  }
  console.log("Proof generated!\n");

  console.log("Proof:", proof, "\n");
  return proof;
}

We access the Flare's official Relay contract with a helper function.

scripts/fdcExample/Base.ts
async function getRelay() {
  const helpers: HelpersInstance = await getHelpers();
  const relayAddress: string = await helpers.getRelay();
  return await IRelay.at(relayAddress);
}

The following function posts a proof request to the DA Layer.

scripts/fdcExample/Base.ts
async function postRequestToDALayer(
  url: string,
  request: any,
  watchStatus: boolean = false,
) {
  const response = await fetch(url, {
    method: "POST",
    headers: {
      //   "X-API-KEY": "",
      "Content-Type": "application/json",
    },
    body: JSON.stringify(request),
  });
  if (watchStatus && response.status != 200) {
    throw new Error(
      `Response status is not OK, status ${response.status} ${response.statusText}\n`,
    );
  } else if (watchStatus) {
    console.log("Response status is OK\n");
  }
  return await response.json();
}

The main prepare the URL of the DA Layer's proof-by-request-raw endpoint. We contact this specific endpoint, because it return the abi encoded IJsonApi.Response struct, and is thus unambiguous.

scripts/fdcExample/JsonApi.ts
async function retrieveDataAndProof(
  abiEncodedRequest: string,
  roundId: number,
) {
  const url = `${COSTON2_DA_LAYER_URL}api/v1/fdc/proof-by-request-round-raw`;
  console.log("Url:", url, "\n");
  return await retrieveDataAndProofBase(url, abiEncodedRequest, roundId);
}

The response the DA Layer server returns has the following fields:

  • The field attestationType holds the UTF8 encoded hex string of the attestation type name, padded to 32 bytes. Thus, it should match the value of the attestationType parameter in the Prepare the request step. In our case, that value is 0x4164647265737356616c69646974790000000000000000000000000000000000.
  • The array proofs holds the Merkle proofs of our attestation request.
  • Lastly, responseHex is the ABI encoding of the chosen attestation type response struct. In this case, it is the IJsonApi.Response struct.

We can ascertain the form of the proof request, as well as examine the response in advance, trough the interactive documentation of the DA Layer server.

An example complete proof response and decoded IJsonApi.Response.

An example DA Layer response for a request using the data provided in this example is:

{
  response_hex: '0x
  0000000000000000000000000000000000000000000000000000000000000020
  494a736f6e417069000000000000000000000000000000000000000000000000
  5745423200000000000000000000000000000000000000000000000000000000
  00000000000000000000000000000000000000000000000000000000000e6c41
  0000000000000000000000000000000000000000000000000000000000000000
  00000000000000000000000000000000000000000000000000000000000000c0
  00000000000000000000000000000000000000000000000000000000000003a0
  0000000000000000000000000000000000000000000000000000000000000060
  00000000000000000000000000000000000000000000000000000000000000a0
  0000000000000000000000000000000000000000000000000000000000000140
  000000000000000000000000000000000000000000000000000000000000001f
  68747470733a2f2f73776170692e6465762f6170692f70656f706c652f332f00
  0000000000000000000000000000000000000000000000000000000000000078
  7b6e616d653a202e6e616d652c206865696768743a202e6865696768742c206d
  6173733a202e6d6173732c206e756d6265724f6646696c6d733a202e66696c6d
  73207c206c656e6774682c207569643a20282e75726c207c2073706c69742822
  2f2229207c202e5b2d325d207c20746f6e756d626572297d0000000000000000
  0000000000000000000000000000000000000000000000000000000000000173
  7b22636f6d706f6e656e7473223a205b7b22696e7465726e616c54797065223a
  2022737472696e67222c20226e616d65223a20226e616d65222c202274797065
  223a2022737472696e67227d2c7b22696e7465726e616c54797065223a202275
  696e74323536222c20226e616d65223a2022686569676874222c202274797065
  223a202275696e74323536227d2c7b22696e7465726e616c54797065223a2022
  75696e74323536222c20226e616d65223a20226d617373222c20227479706522
  3a202275696e74323536227d2c7b22696e7465726e616c54797065223a202275
  696e74323536222c20226e616d65223a20226e756d6265724f6646696c6d7322
  2c202274797065223a202275696e74323536227d2c7b22696e7465726e616c54
  797065223a202275696e74323536222c20226e616d65223a2022756964222c20
  2274797065223a202275696e74323536227d5d2c226e616d65223a2022746173
  6b222c2274797065223a20227475706c65227d00000000000000000000000000
  0000000000000000000000000000000000000000000000000000000000000020
  0000000000000000000000000000000000000000000000000000000000000100
  0000000000000000000000000000000000000000000000000000000000000020
  00000000000000000000000000000000000000000000000000000000000000a0
  0000000000000000000000000000000000000000000000000000000000000060
  0000000000000000000000000000000000000000000000000000000000000020
  0000000000000000000000000000000000000000000000000000000000000006
  0000000000000000000000000000000000000000000000000000000000000003
  0000000000000000000000000000000000000000000000000000000000000005
  52322d4432000000000000000000000000000000000000000000000000000000',
  attestation_type: '0x494a736f6e417069000000000000000000000000000000000000000000000000',
  proof: [
    '0xab2384609341b65b4686cf9accd981f8c7f58e47aa41bc49ec60f655c8d99840',
    '0xc30b0590a4ea59adc7aa5486a9ece81bdcc756ac4d22e09dbe171bf8b50e53b5',
    '0x4a5fc6d814dd3c52e199396de4d48f7c18274bbf72f9065c1e68306f4fd22c34'
  ]
}

The proof field is dependent on the round in which the attestation request was submitted; it contains proofs for all of the requests submitted in that round. In the case of a single attestation request it is an empty list [] (the proof is the merkle root itself).

The decoded IJsonApi.Response struct is:

[
  attestationType: '0x494a736f6e417069000000000000000000000000000000000000000000000000',
  sourceId: '0x5745423200000000000000000000000000000000000000000000000000000000',
  votingRound: '945217',
  lowestUsedTimestamp: '0',
  requestBody: [
    'https://swapi.dev/api/people/3/',
    '{name: .name, height: .height, mass: .mass, numberOfFilms: .films | length, uid: (.url | split("/") | .[-2] | tonumber)}',
    '{"components": [{"internalType": "string", "name": "name", "type": "string"},{"internalType": "uint256", "name": "height", "type": "uint256"},{"internalType": "uint256", "name": "mass", "type": "uint256"},{"internalType": "uint256", "name": "numberOfFilms", "type": "uint256"},{"internalType": "uint256", "name": "uid", "type": "uint256"}],"name": "task","type": "tuple"}',
    url: 'https://swapi.dev/api/people/3/',
    postprocessJq: '{name: .name, height: .height, mass: .mass, numberOfFilms: .films | length, uid: (.url | split("/") | .[-2] | tonumber)}',
    abi_signature: '{"components": [{"internalType": "string", "name": "name", "type": "string"},{"internalType": "uint256", "name": "height", "type": "uint256"},{"internalType": "uint256", "name": "mass", "type": "uint256"},{"internalType": "uint256", "name": "numberOfFilms", "type": "uint256"},{"internalType": "uint256", "name": "uid", "type": "uint256"}],"name": "task","type": "tuple"}'
  ],
  responseBody: [
    '0x000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000a00000000000000000000000000000000000000000000000000000000000000060000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000060000000000000000000000000000000000000000000000000000000000000003000000000000000000000000000000000000000000000000000000000000000552322d4432000000000000000000000000000000000000000000000000000000',
    abi_encoded_data: '0x000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000a00000000000000000000000000000000000000000000000000000000000000060000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000060000000000000000000000000000000000000000000000000000000000000003000000000000000000000000000000000000000000000000000000000000000552322d4432000000000000000000000000000000000000000000000000000000'
  ]
]

Use the data​

We will now define a simple contract, that will demonstrate how the data can be used onchain. The contract will receive character data from the Star Wars API, and store it in a StarWarsCharacter struct. It will do so only if the proof is valid.

src/fdcExample/JsonApi.sol
struct StarWarsCharacter {
    string name;
    uint256 numberOfMovies;
    uint256 apiUid;
    uint256 bmi;
}

We will also need a DataTransportObject struct, that will allow us to decode the data.

src/fdcExample/JsonApi.sol
struct DataTransportObject {
    string name;
    uint256 height;
    uint256 mass;
    uint256 numberOfMovies;
    uint256 apiUid;
}

The code of the contract is as follows.

src/fdcExample/JsonApi.sol

contract StarWarsCharacterList {
    mapping(uint256 => StarWarsCharacter) public characters;
    uint256[] public characterIds;

    function isJsonApiProofValid(
        IJsonApi.Proof calldata _proof
    ) private view returns (bool) {
        // Inline the check for now until we have an official contract deployed
        return
            ContractRegistry.auxiliaryGetIJsonApiVerification().verifyJsonApi(
                _proof
            );
    }

    function addCharacter(IJsonApi.Proof calldata data) public {
        require(isJsonApiProofValid(data), "Invalid proof");

        DataTransportObject memory dto = abi.decode(
            data.data.responseBody.abi_encoded_data,
            (DataTransportObject)
        );

        require(characters[dto.apiUid].apiUid == 0, "Character already exists");

        StarWarsCharacter memory character = StarWarsCharacter({
            name: dto.name,
            numberOfMovies: dto.numberOfMovies,
            apiUid: dto.apiUid,
            bmi: (dto.mass * 100 * 100) / (dto.height * dto.height)
        });

        characters[dto.apiUid] = character;
        characterIds.push(dto.apiUid);
    }

    function getAllCharacters()
        public
        view
        returns (StarWarsCharacter[] memory)
    {
        StarWarsCharacter[] memory result = new StarWarsCharacter[](
            characterIds.length
        );
        for (uint256 i = 0; i < characterIds.length; i++) {
            result[i] = characters[characterIds[i]];
        }
        return result;
    }
}

Verify proof​

FDC optimizes on-chain storage costs by implementing a hybrid data verification system. Instead of storing complete datasets on-chain, it stores only Merkle proofs, while maintaining the actual data through trusted off-chain providers. This approach significantly reduces gas costs while preserving data integrity.

When requested, data providers supply the original data along with its corresponding Merkle proof. The protocol verifies data authenticity by comparing the provided Merkle proof against the on-chain Merkle root. A successful match confirms the data's integrity and authenticity within the FDC system.

While data verification is optional if you trust your data provider, FDC ensures transparency by making verification possible at any time. This capability is crucial for maintaining system integrity and allowing users to independently verify data when needed, particularly in production environments.

FDC provides verification functionality through the FdcVerification contract. If we proof is valid, the function verifyEVMTransaction will return true, otherwise false.

We deploy and verify this contract with the deployAndVerifyContract function in the scripts/fdcExample/JsonApi.ts file.

scripts/fdcExample/JsonApi.ts
async function deployAndVerifyContract() {
  const args: any[] = [];
  const characterList: StarWarsCharacterListInstance =
    await StarWarsCharacterList.new(...args);
  try {
    await run("verify:verify", {
      address: characterList.address,
      constructorArguments: args,
    });
  } catch (e: any) {
    console.log(e);
  }
  console.log("StarWarsCharacterList deployed to", characterList.address, "\n");
  return characterList;
}

Interact with contract​

We define an additional function that allows us to interact with the just deployed contract. The interactWithContract function also takes the proof retrieved in the previous step as an argument. It abi decodes the response_hex value to an IJsonApi.Response struct. From that and the array of proofs, it constructs an IJsonApi.Proof object, on which it call the registerAddress function of the AddressRegistry contract deployed above. The contract verifies the address, and the script prints it to the console.

scripts/fdcExample/JsonApi.ts
async function interactWithContract(
  characterList: StarWarsCharacterListInstance,
  proof: any,
) {
  console.log("Proof hex:", proof.response_hex, "\n");

  // A piece of black magic that allows us to read the response type from an artifact
  const IJsonApiVerification = await artifacts.require("IJsonApiVerification");
  const responseType =
    IJsonApiVerification._json.abi[0].inputs[0].components[1];
  console.log("Response type:", responseType, "\n");

  const decodedResponse = web3.eth.abi.decodeParameter(
    responseType,
    proof.response_hex,
  );
  console.log("Decoded proof:", decodedResponse, "\n");
  const transaction = await characterList.addCharacter({
    merkleProof: proof.proof,
    data: decodedResponse,
  });
  console.log("Transaction:", transaction.tx, "\n");
  console.log(
    "Star Wars Characters:\n",
    await characterList.getAllCharacters(),
    "\n",
  );
}

We can run the whole script by calling the following console command.

yarn hardhat run scripts/fdcExample/JsonApi.ts