# Fee Delegation Example

## Table of Contents <a href="#table-of-contents" id="table-of-contents"></a>

* [1. Introduction](#1-introduction)
* [2. How fee delegation works](#2-how-fee-delegation-works)
  * 2.1 Transaction signing by the sender
  * 2.2 Transaction signing by the fee payer
* [3. Simple server and client for fee delegation](#3-simple-server-and-client-for-fee-delegation)
  * 3.1 Sender's client
  * 3.2 Fee payer's server
* [4. Run example](#4-run-example)
  * 4.1 Run `feepayer_server.js`
  * 4.2 Run `sender_client.js`
  * 4.3 Check `feepayer_server.js`
  * 4.4 Klaytn scope

## 1. Introduction <a href="#id-1-introduction" id="id-1-introduction"></a>

This tutorial helps you to write a simple server-client example using caver-js SDK to illustrate how fee delegated value transfer transaction works in Klaytn. This tutorial and the example code is using the Baobab testnet.

## 2. How fee delegation works <a href="#id-2-how-fee-delegation-works" id="id-2-how-fee-delegation-works"></a>

Let's skim through how fee delegation works.

### 2.1 Transaction signing by the sender <a href="#id-2-1-transaction-signing-by-the-sender" id="id-2-1-transaction-signing-by-the-sender"></a>

`Sender` always should sign the transaction before sending a transaction.

To sign a transaction, use [signTransaction](/content/dapp/sdk/caver-js/v1.4.1/api-references/caver.klay.accounts.md#signtransaction) which signs a transaction with given private key.

```
// using the event emitter
const senderAddress = "SENDER_ADDRESS";
const senderPrivateKey = "SENDER_PRIVATEKEY";
const toAddress = "TO_ADDRESS";

const { rawTransaction: senderRawTransaction } = await caver.klay.accounts.signTransaction({
  type: 'FEE_DELEGATED_VALUE_TRANSFER',
  from: senderAddress,
  to: toAddress,
  gas: '300000',
  value: caver.utils.toPeb('1', 'KLAY'),
}, senderPrivateKey)
```

If there are no errors, then `senderRawTransaction` will have a signed transaction which is signed by the `senderPrivateKey`.

Now, you need to send the `senderRawTransaction` to the fee payer. There will be various ways to implement this. In this tutorial, we will provide you a simple server-client code as an example of sending a `senderRawTransaction` to the fee payer.

### 2.2 Transaction signing by the fee payer <a href="#id-2-2-transaction-signing-by-the-fee-payer" id="id-2-2-transaction-signing-by-the-fee-payer"></a>

When `fee payer` receives the `senderRawTransaction`, `fee payer` signs the `senderRawTransaction` again with their private key and sends the transaction to Klaytn. The below code snippet illustrates the process. `klay.sendTransaction` method signs the transaction with the given account's private key before sending the transaction. Before running the code, please replace `"FEEPAYER_ADDRESS"` and `"PRIVATE_KEY"` with the actual values.

Note that when the `fee payer` submits the transaction to Klaytn on behalf of the `sender`, the `senderRawTransaction` type must be a `FEE_DELEATED` type of transaction. In the below example, [sendTransaction(FEE\_DELEGATED\_VALUE\_TRANSFER)](/content/dapp/sdk/caver-js/v1.4.1/api-references/caver.klay/transaction/sendtx_value_transfer.md#sendtransaction-fee_delegated_value_transfer) method is invoked, because the original `senderRawTransaction` generated by the sender was [TxTypeFeeDelegatedValueTransfer](/content/klaytn/design/transactions/fee-delegation.md#txtypefeedelegatedvaluetransfer).

```
const feePayerAddress = "FEEPAYER_ADDRESS";
const feePayerPrivateKey = "PRIVATE_KEY"

caver.klay.accounts.wallet.add(feePayerPrivateKey, feePayerAddress);

caver.klay.sendTransaction({
  senderRawTransaction: senderRawTransaction,
  feePayer: feePayerAddress,
})
.on('transactionHash', function(hash){
    ...
})
.on('receipt', function(receipt){
    ...
})
.on('error', console.error); // If an out-of-gas error, the second parameter is the receipt.
```

## 3. Simple server and client for fee delegation <a href="#id-3-simple-server-and-client-for-fee-delegation" id="id-3-simple-server-and-client-for-fee-delegation"></a>

Let's write a simple server and client using above fee delegation code.

### 3.1 Environment setup <a href="#id-3-1-environment-setup" id="id-3-1-environment-setup"></a>

We will use `npm` and [caver-js](/content/dapp/sdk/caver-js/v1.4.1/getting-started_1.4.1.md) to setup environment for this example as below.

```
$ mkdir example
$ cd example
$ npm init
$ npm install caver-js@latest
```

### 3.1 Sender's client <a href="#id-3-1-sender-s-client" id="id-3-1-sender-s-client"></a>

First, we are going to write a `sender_client.js` as below.

In the example, please replace `"SENDER_ADDRESS"`, `"SENDER_PRIVATEKEY"` and `"TO_ADDRESS"` with the actual values.

```javascript
var net = require('net');
var client = new net.Socket();

const Caver = require('caver-js');
const caver = new Caver('https://public-en-baobab.klaytn.net');
const senderAddress = "SENDER_ADDRESS";
const senderPrivateKey = "SENDER_PRIVATEKEY";
const toAddress = "TO_ADDRESS";

sendFeeDelegateTx = async() => {
    // sign transaction with private key of sender
    const { rawTransaction: senderRawTransaction } = await caver.klay.accounts.signTransaction({
      type: 'FEE_DELEGATED_VALUE_TRANSFER',
      from: senderAddress,
      to: toAddress,
      gas: '300000',
      value: caver.utils.toPeb('0.00001', 'KLAY'),
    }, senderPrivateKey)

    // send signed raw transaction to fee payer's server
    client.connect(1337, '127.0.0.1', function() {
            console.log('Connected to fee delegated service');
    });
    client.write(senderRawTransaction);

    client.on('data', function(data) {
            console.log('Received data from server: ' + data);
    });

    client.on('close', function() {
            console.log('Connection closed');
    });
}

sendFeeDelegateTx();
```

The above code signs a fee delegated value transfer transaction with `senderPrivateKey` and sends the signed `senderRawTransaction` to the fee payer's server which is running on port `1337` on `127.0.0.1`, i.e. localhost.

### 3.2 Fee payer's server <a href="#id-3-2-fee-payer-s-server" id="id-3-2-fee-payer-s-server"></a>

Now let's write the fee payer's server, `feepayer_server.js`, which signs received `senderRawTransaction` with `feePayerPrivateKey` and sends it to Baobab testnet.

In the below example, please replace `"FEEPAYER_ADDRESS"` and `"FEEPAYER_PRIVATEKEY"` with actual values.

```javascript
const Caver = require('caver-js');
const caver = new Caver('https://public-en-baobab.klaytn.net');
const feePayerAddress = "FEEPAYER_ADDRESS";
const feePayerPrivateKey = "FEEPAYER_PRIVATEKEY";

// add fee payer account
caver.klay.accounts.wallet.add(feePayerPrivateKey, feePayerAddress);

var net = require('net');


feePayerSign = (senderRawTransaction, socket) => {
    // fee payer
    caver.klay.sendTransaction({
      senderRawTransaction: senderRawTransaction,
      feePayer: feePayerAddress,
    })
    .on('transactionHash', function(hash){
        console.log('transactionHash', hash);
    })
    .on('receipt', function(receipt){
        console.log('receipt', receipt);
        socket.write('Tx hash is '+ receipt.transactionHash);
        socket.write('Sender Tx hash is '+ receipt.senderTxHash);
    })
    .on('error', console.error); // If an out-of-gas error, the second parameter is the receipt.
}

var server = net.createServer(function(socket) {
       console.log('Client is connected ...');
    socket.write('This is fee delegating service');
    socket.write('Fee payer is ' + feePayerAddress);
        socket.on('data', function(data) {
            console.log('Received data from client:', data.toString());
            feePayerSign(data.toString(), socket);
        });
});

server.listen(1337, '127.0.0.1');
console.log('Fee delegate service started ...');
```

The server listens on port `1337`.

When there is incoming `data`, it signs the `data` with `feePayerPrivateKey` and sends it to the Klaytn blockchain. It assumes that the `data` is `senderRawTransaction` from the `sender_client.js`.

## 4. Run example <a href="#id-4-run-example" id="id-4-run-example"></a>

Prepare two terminals, one for `sender_client.js` and another for `feepayer_server.js`.

### 4.1 Run `feepayer_server.js` <a href="#id-4-1-run-feepayer_server-js" id="id-4-1-run-feepayer_server-js"></a>

Below the command will start the fee payer's server.

```
$ node feepayer_server.js
Fee delegate service started ...
```

The server starts and is now listening on port 1337.

### 4.2 Run `sender_client.js` <a href="#id-4-2-run-sender_client-js" id="id-4-2-run-sender_client-js"></a>

Let's run `sender_client.js` to send a fee delegated transaction.

```
$ node sender_client.js
Signed a fee delegated value transfer transaction.
Sending a signed transaction to fee delegated service.
Connected to fee delegated service
Received data from server: This is fee delegating service
Received data from server: Fee payer is 0x2645BA5Be42FfEe907ca8e9d88f6Ee6dAd8c1410
Received data from server: Tx hash is 0xd99086aa8188255d4ee885d9f1933b6cc062085c1196731ba599b2fb8f2dbbd7
Received data from server: Sender Tx hash is 0xe1f630547f287177a0e92198b1c67212b24fc1ad5a1f0b1f94fd6f980281fdba
```

It will sign a transaction with the `sender` private key and send the signed transaction to the fee delegated service (i.e., fee payer's server). Then it will receive the response from the fee delegate service including the `Fee payer` address, `Tx hash`, and `Sender Tx hash`. `Tx hash` is hash of a transaction submitted to the Klaytn network, while `Sender Tx hash` is hash of a transaction without the fee payer's address and signature. For more details, please take a look at [SenderTxHash](/content/klaytn/design/transactions.md#sendertxhash).

### 4.3 Check `feepayer_server.js` <a href="#id-4-3-check-feepayer_server-js" id="id-4-3-check-feepayer_server-js"></a>

On the server's console, you will see below outputs. It prints the transaction receipt from the Klaytn.

```
$ node feepayer_server.js
Fee delegate service started ...
Client is connected ...
Received data from client: 0x09f88b3a8505d21dba00830493e094fc83add44939ef818ce62dacea23697fa17257838609184e72a000940ecc24157e38b1997aace56f32ccb381b16e1710f847f8458207f5a0e636e67d01acc1f368db5e60290721e9059b13b0bf74af6d46391cc48bd31a81a0135118878be87f808e064f64fa4f13d6dc5bd9888b154ecd17f02980063b9e4280c4c3018080
transactionHash { messageHash:
   '0xa4cd7d479d19251a1981086431eff5514c5edf61731a6e5271b2a137a156f7e7',
  v: '0x07f6',
  r:
   '0xdb9c3a5b75c20f15cba9bc28eebcaff58701f57d4a226e7e3cb2bc4544b3c96c',
  s:
   '0x7646e929aa6467fa1c849f3abf37054df4cb08e6ee160f6375517ae2609f4b11',
  rawTransaction:
   '0x09f8e33a8505d21dba00830493e094fc83add44939ef818ce62dacea23697fa17257838609184e72a000940ecc24157e38b1997aace56f32ccb381b16e1710f847f8458207f5a0e636e67d01acc1f368db5e60290721e9059b13b0bf74af6d46391cc48bd31a81a0135118878be87f808e064f64fa4f13d6dc5bd9888b154ecd17f02980063b9e42942645ba5be42ffee907ca8e9d88f6ee6dad8c1410f847f8458207f6a0db9c3a5b75c20f15cba9bc28eebcaff58701f57d4a226e7e3cb2bc4544b3c96ca07646e929aa6467fa1c849f3abf37054df4cb08e6ee160f6375517ae2609f4b11',
  txHash:
   '0xd99086aa8188255d4ee885d9f1933b6cc062085c1196731ba599b2fb8f2dbbd7' }
transactionHash 0xd99086aa8188255d4ee885d9f1933b6cc062085c1196731ba599b2fb8f2dbbd7
receipt { blockHash:
   '0x1c61f03d2f0eba86c0f58ee7d1be8e8e425f47e9c46433474bd11c5a3f0528b2',
  blockNumber: 3175653,
  contractAddress: null,
  feePayer: '0x2645ba5be42ffee907ca8e9d88f6ee6dad8c1410',
  feePayerSignatures:
   [ { V: '0x7f6',
       R:
        '0xdb9c3a5b75c20f15cba9bc28eebcaff58701f57d4a226e7e3cb2bc4544b3c96c',
       S:
        '0x7646e929aa6467fa1c849f3abf37054df4cb08e6ee160f6375517ae2609f4b11' } ],
  from: '0x0ecc24157e38b1997aace56f32ccb381b16e1710',
  gas: '0x493e0',
  gasPrice: '0x5d21dba00',
  gasUsed: 31000,
  logs: [],
  logsBloom:
   '0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000',
  nonce: '0x3a',
  senderTxHash:
   '0xe1f630547f287177a0e92198b1c67212b24fc1ad5a1f0b1f94fd6f980281fdba',
  signatures:
   [ { V: '0x7f5',
       R:
        '0xe636e67d01acc1f368db5e60290721e9059b13b0bf74af6d46391cc48bd31a81',
       S:
        '0x135118878be87f808e064f64fa4f13d6dc5bd9888b154ecd17f02980063b9e42' } ],
  status: true,
  to: '0xfc83add44939ef818ce62dacea23697fa1725783',
  transactionHash:
   '0xd99086aa8188255d4ee885d9f1933b6cc062085c1196731ba599b2fb8f2dbbd7',
  transactionIndex: 0,
  type: 'TxTypeFeeDelegatedValueTransfer',
  typeInt: 9,
  value: '0x9184e72a000' }
```

### 4.4 Klaytn scope <a href="#id-4-4-klaytn-scope" id="id-4-4-klaytn-scope"></a>

You can also find the above transaction on the [Klaytn scope](https://baobab.scope.klaytn.com).

It shows that the transaction is `TxTypeFeeDelegatedValueTransfer` and `Fee payer` is `0x2645ba5be42ffee907ca8e9d88f6ee6dad8c1410` or `feepayerAddress` that you entered, while `From` is a different address which should be the `senderAddress` in above example.

![Fee delegated Tx](/files/OlGinS2lpdxN56OcPCMg)


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