Data Feed: Stream a data feed on-chain at regular intervals

Overview

A Data Feed utilizes a cron job, which is a time-based job scheduler in Unix-like operating systems, to automatically trigger the submission of transactions to a smart contract on a blockchain. This means that the program will execute the transaction submission at regular intervals without requiring any manual intervention.

CronJobTriggered Source:

1. Time-based job scheduler defined in the CronJobTriggered Source definition.

MakeHttpRequest Task:

1. Makes an API request to the coinbase domain in a secure manner

2. Extracts the ethPrice from the API response

3. Sends the price to the Transact To EVM Function task

TransactToEvmFunction task:

1. Sends back the ethPrice to the contract via function defined by the user in the next block

Midpoint

$ npm install -g midpoint-cli

$ midpoint init blank my-data-feed

1 . Cron Job Triggered

$ midpoint add-source cronJobTriggeredSource cron-job-source

We define the Cron Job Triggered source so that contract can make a midpoint request.

{
    "cronJobTriggeredSource": {
        "crontab": "* * * * *",
        "excludeLogs": [],
        "extracts": [
            {
                "name": "Midpoint_ID",
                "from": "Midpoint_ID"
            }
        ]
    }
}

2. Make Http Request

$ midpoint add-task makeHttpRequest get-eth-price

We define the Make HTTP Request Task to get the ethPrice from the coinbase endpoint.

{
    "makeHttpRequest": {
        "urlRaw": "https://api.coinbase.com/v2/prices/ETH-USD/spot",
        "method": "GET",
        "urlType": "raw",
        "extracts": [
            {
                "name": "ethPrice",
                "from": "body.data.amount"
            }
        ]
    }
}

3. Shift Decimal

$ midpoint add-task makeHttpRequest shift-eth-price

We define the Shift Decimal Task to return a usable price on-chain.

{
    "shiftDecimal": {
        "inputValue": "{{ethPrice}}",
        "decimalPlaces": "5",
        "outputName": "ethPriceScaled",
        "round": "false",
        "excludeLogs": []
    }
}

4. Transact To EVM Function

$ midpoint add-task transactToEvmFunctionTestnet post-eth-price-on-blockchain

We define the Transact to EVM Function Task to send the summary back to the contract.

{
    "transactToEvmFunctionTestnet": {
        "chainId": "80001",
        "contractAddress": "0xC05f817a17bC0c505E48AC9CC52b5cb556F7B296",
        "arguments": [
            {
                "name": "Midpoint_ID",
                "datatype": "uint64",
                "value": "{{Midpoint_ID}}"
            },
            {
                "name": "EthPrice",
                "datatype": "uint64",
                "value": "{{ethPriceScaled}}"
            }
        ],
        "functionName": "latestEthPrice"
    }
}

5. Define Path

We define our path file.

cron-job-source => get-eth-price
get-eth-price => shift-eth-price
shift-eth-price => post-eth-price-to-blockchain

6. Publish Midpoint

$ midpoint publish