# Fee Delegation Example
## Table of Contents
* [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
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
Let's skim though how fee delegation works.
### 2.1 Transaction signing by the sender
`Sender` always should sign the transaction before sending a transaction.
To sign a transaction, use [signTransaction](https://klaytn.gitbook.io/docs/sdk/caver-js/v1.4.1/api-references/caver.klay.accounts#signtransaction) which signs a transaction with given private key.
```
// using the event emitter
const senderPrivateKey = "PRIVATE_KEY"
const { rawTransaction: senderRawTransaction } = await caver.klay.accounts.signTransaction({
type: 'FEE_DELEGATED_VALUE_TRANSFER',
from: '0x90B3E9A3770481345A7F17f22f16D020Bccfd33e',
to: '0x75c3098Be5E4B63FBAc05838DaAEE378dD48098d',
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
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)](https://klaytn.gitbook.io/docs/sdk/caver-js/v1.4.1/api-references/caver.klay/transaction/sendtx_value_transfer#sendtransaction-fee_delegated_value_transfer) method is invoked, because the original `senderRawTransaction` generated by the sender was [TxTypeFeeDelegatedValueTransfer](https://klaytn.gitbook.io/docs/klaytn/design/transactions/fee-delegation#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
Let's write a simple server and client using above fee delegation code.
### 3.1 Environment setup
We will use `npm` and [caver-js](https://klaytn.gitbook.io/docs/bapp/sdk/caver-js/v1.4.1/getting-started_1.4.1) to setup environment for this example as below.
```
$ mkdir example
$ cd example
$ npm init
$ npm install caver-js@latest
```
### 3.1 Sender's client
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://your.en.url:8651');
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 `senderRawTranscation` 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
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://your.en.url:8651');
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
Prepare two terminals, one for `sender_client.js` and another for `feepayer_server.js`.
### 4.1 Run `feepayer_server.js`
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`
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 transcation 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](https://klaytn.gitbook.io/docs/klaytn/design/transactions#sendertxhash).
### 4.3 Check `feepayer_server.js`
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
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.
