Interacting with Ethereum using JSON-RPC

You can completely avoid using web3.js and interact with your Ethereum node using JSON-RPC. It is a stateless, lightweight RPC protocol. Etherum JSON-RPC exposes various APIs that can be used within the same process, over sockets, over HTTP, or in various message passing environments.

These APIs are commonly used to interact with blockchain where JavaScript is not supported. In this recipe, you will learn to communicate with Ethereum using various JSON-RPC commands.

Getting ready

You need to have a fully functioning Ethereum node to test these APIs. By default, both geth and Parity expose port 8545 for JSON-RPC. Some examples are given in JavaScript, but you can use any programming language that supports HTTPS requests.

Make sure that you are starting your geth node with enough --rpcapi permissions:

$ geth --rpc --rpcapi "web3,eth,personal"

We will focus on a few important APIs here and you can always find the complete list for reference at github.com/ethereum/wiki/wiki/JSON-RPC.

How to do it...

We will now learn how to use JSON-RPC to interact with Ethereum:

1. Build a basic JSON-RPC body with four parameters. The parameters are version, method, parameters, and ID:

{
    "jsonrpc":"2.0",
    "method":"", // method identifier
    "params":[], // list of parameters
    "id":0 // id
}

2. Use the net_peerCount method to return the number of peers connected to a node:

curl -X POST 
    --data '{"jsonrpc":"2.0","method":"net_peerCount","params":[],"id":74}'
    http://localhost:8545

3. You will get a response like this for all the requests you make. Some requests will return a hex value instead of a decimal/string. You may have to convert it to the required format manually:

{
  "id":74,
  "jsonrpc": "2.0",
  "result": "0x2" // 2
}

4. Create the same request in Node.js to get the peer count:

var request = require('request');

request({
     url: 'http://localhost:8545',
     method: 'POST',
     body: '{"jsonrpc":"2.0","method":"net_peerCount","params":[],"id":74}'
}, function(error, response, body) {
     if (!error && response.statusCode == 200) {
         console.log(body);
     }
});

5. Use this in Python:

import requests
data = '{"jsonrpc":"2.0","method":"net_peerCount","params":[],"id":74}'
response = requests.post('http://localhost:8545/', data=data)

6. And use this in Go:

body := strings.NewReader(`{"jsonrpc":"2.0","method":"net_peerCount","params":[],"id":74}`) 

req, err := http.NewRequest("POST", "http://localhost:8545", body) 
if err != nil {   
    // handle err 
} 

req.Header.Set("Content-Type", "application/x-www-form-urlencoded") 
resp, err := http.DefaultClient.Do(req) 
if err != nil {   
    // handle err 
}
 
defer resp.Body.Close()

7. Use any programming language that can make HTTP requests for interacting through JSON-RPC.
   Let's look into a few more important examples.
8. To get the current Ether base of the connected node, use eth_coinbase:

// Request
curl -X POST 
    --data '{"jsonrpc":"2.0","method":"eth_coinbase","params":[],"id":64}'
    http://localhost;8545

// Response
{
  "id":64,
  "jsonrpc": "2.0",
  "result": "0x824e470cCac64CC5fa4Abe953e64FA360EA11366" 
}

9. To view the list of accounts in the node, use eth_accounts:

// Request
curl -X POST 
    --data '{"jsonrpc":"2.0","method":"eth_accounts","params":[],"id":1}'
    http://localhost:8545

// Response
{
  "id":1,
  "jsonrpc": "2.0",
  "result": ["0x824e470cCac64CC5fa4Abe953e64FA360EA11366"]
}

10. To get the current block height, use eth_blockNumber:

// Request
curl -X POST 
    --data '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":83}'
    http://localhost:8545

// Response
{
  "id":83,
  "jsonrpc": "2.0",
  "result": "0x53D390" // 5493648
}

11. To get the balance of an account in the network, use eth_getBalance along with parameters:

// Request
curl -X POST 
    --data '{"jsonrpc":"2.0",
             "method":"eth_getBalance",
             "params":["0x824e470cCac64CC5fa4Abe953e64FA360EA11366", "latest"],
             "id":1
            }'
    http://localhost:8545

// Response
{
  "id":1,
  "jsonrpc": "2.0",
  "result": "0x5AF3107A4000" // 100000000000000
}

12. To send a transaction from an account, use eth_sendTransaction:

// Request
curl -X POST 
    --data '{
              "jsonrpc":"2.0",
              "method":"eth_sendTransaction",
              "params": <Trnsaction_Object>,
              "id":1
            }'
    http://localhost:8545

// Response
{
  "id":1,
  "jsonrpc": "2.0",
  "result": "0xf456c56efe41db20f32853ccc4cbea3d2ab011b2c11082150f29c36212345dbd"
}

13. To get the transaction receipt, use eth_getTransactionReceipt:

// Request
curl -X POST 
    --data '{
                "jsonrpc":"2.0",
                "method":"eth_getTransactionReceipt",
                "params":["<TransactionHash>"],
                "id":1
            }'
    http://localhost:8545

// Result
{ "id":1, "jsonrpc":"2.0", "result": <Receipt> }