Let's build your first decentralized App on the MultiversX Blockchain
This guide has been made available in video format as well:
The Ping-Pong app is a very simple decentralized application that will allow the user to deposit a specific number of tokens (default is 1 xEGLD) to a smart contract address and to lock them for a specific amount of time (default is 10 minutes). After this time interval passes, the user can claim back the same amount of tokens.
Sending funds to the contract is called
Claiming the same amount back is called
- the user can only
pong(so no multiple pings).
- only the set amount can be
ping-ed, no more, no less.
Maybe you noticed the default amount for a deposit is 1 xEGLD and not 1 EGLD which is MultiversX official token, this is because, for testing purposes we use MultiversX Devnet, which is a testing environment identical to the Mainnet, the official MultiversX Blockchain. Here, the currency is xEGLD, it's just a test token, it's worth nothing.
MultiversX dApp architecture
Application Layer (The Frontend)
For the web app we'll have two pages:
- The Sign in page - here we can authenticate with ledger, web wallet or with Maiar Wallet
- The Dashboard page - here we can either ping or pong, if we already deposited, then we will see a countdown timer until the time interval clears out.
Blockchain Layer (The Backend)
We will create a smart contract that can handle the deposit (
ping), claim (
pong) and status actions (
Let's say that, for now, this smart contract plays the role of an API in a dApp. Also this is where our business logic resides.
The MultiversX devnet is a public test network maintained by our community where any developer can test their smart contracts and dApps in a real world environment.
Set up the environment
Let's set up the evironment for getting your first dapp up and running.
First let's create a new folder for our project, I'll name it
mkdir -p ping-pong/wallet
In the end, we'll have three subfolders: wallet, contract and dapp. For convenience, we'll save our owner's wallet keystore in the wallet folder.
In our example we'll use Ubuntu 20.04. MacOs works as well (skip to the last paragraph in this section). We'll need to install
libncurses5. For the frontend application, we'll use an app template based on React JS, so we will need
First we make sure the operating system prerequisites are installed:
sudo apt-get update
sudo apt install libncurses5 build-essential python3-pip nodejs npm python3.8-venv
We'll also need
mxpy, the MultiversX command line tool, which is helpful for signing transactions, deploying smart contracts, managing wallets, accounts and validators. We'll use it to deploy our smart contract.
mxpy can be installed using the MultiversX documentation page https://docs.multiversx.com/sdk-and-tools/sdk-py/installing-mxpy
We'll download the
mxpy installer and we run it
wget -O mxpy-up.py https://raw.githubusercontent.com/multiversx/mx-sdk-py-cli/main/mxpy-up.py
Restart the user session to activate
In order to install
mxpy on MacOs, you need to make sure you have installed
python 3.8 and
pip on your system.
Then, install the latest
pip3 install --user --upgrade --no-cache-dir mxpy
If you encounter any error relating to
pynacl package, make sure you have
brew install libsodium
Create an owner wallet
We now have all the prerequisites installed, let's create an owner's wallet PEM file.
The smart contract can only be deployed on the blockchain by an owner, so we will create an owner wallet here. The owner can also update the contract, later on, if needed.
Let's head over to the MultiversX wallet, click "Create new wallet", write down the security phrase (24 words) that can help us retrieve the wallet, and the password for the JSON keystore (that we will save in the
~/ping-pong/wallet folder). We should be able to see our new MultiversX wallet owner address which is, in this case, erd1.......
We can also generate a private key PEM file, like this we won't need to enter our wallet password each time we want to confirm a transaction.
mxpy --verbose wallet derive ./wallet-owner.pem --mnemonic
We will enter our 24 secret words when prompted and a new PEM file will be created. This command requires that you enter all 24 words each separated by a space.
In order to initiate transactions on the blockchain, we need some funds, every transaction costs a very small fee, on the blockchain this is called gas. On the devnet wallet we have a faucet that allows you to get free test funds for our applications. We can request 10 xEGLD every 24 hours, so let's request 10 xEGLD now. We now check if the transaction was successful, and yes, we see that we now have 10 xEGLD in our devnet wallet.
The Blockchain Layer - The Smart Contract
Our owner wallet is completely set now, we can move on to our backend, the blockchain layer.
Clone the template
Clone the Ping-Pong Sample Smart Contract
Let's start with our smart contract. We'll first clone the sample contract repository from here https://github.com/multiversx/mx-ping-pong-sc
git clone https://github.com/multiversx/mx-ping-pong-sc contract
Build the Smart Contract
We now have the source code for the smart contract, but we need to compile it into a binary that the MultiversX Virtual Machine can run. The VM can run Web Assembly code, so we need to compile our Rust source code into Web Assembly (WASM).
We now have the source code for the smart contract, but we need to compile it into a binary that the MultiversX (previously Elrond) Virtual Machine can run. The VM can run Web Assembly code, so we need to compile our Rust source code into Web Assembly (WASM).
Run the following command in order to build the rust smart contract into a wasm file.
mxpy contract build
On the last line of the output we'll have:
INFO:projects.core:WASM file generated: output/ping-pong.wasm
After running this command line, we see that a wasm file was generated. This file contains the runtime code for our smart contract.
Customize and Deploy
Deploy the smart contract on MultiversX Devnet Next step is to deploy the contract to the blockchain.
Customize the variables
Optionally we can customize the default params in
"arguments": [1000000000000000000, 600],
Make sure your owner wallet PEM file is in the right folder, the smart contract is build and let's get to the deployment. For now let's continue with the default values. We will run:
mxpy contract deploy
We'll take a look at the log output. We have 2 elements that need our attention: the contract address and the transaction hash. Let's check them in the Devnet Explorer.
Devnet Explorer will be your best friend in developing dApps on the MultiversX Blockchain, as you'll first deploy and test your dApps on Devnet.
INFO:accounts:Account.sync_nonce() INFO:accounts:Account.sync_nonce() done: 32 INFO:cli.contracts:Contract address: erd1qqqqqqqqqqqqqpgq0hmfvuygs34cgqsvgg6fpq9c5mffh4y04cysagr6cn INFO:utils:View this contract address in the MultiversX Devnet Explorer: https://devnet-explorer.multiversx.com/accounts/erd1qqqqqqqqqqqqqpgq0hmfvuygs34cgqsvgg6fpq9c5mffh4y04cysagr6cn INFO:transactions:Transaction.send: nonce=32 INFO:transactions:Hash: ee84f3e833d439e159c9619fd76e26d2afcdad62c197d87e4940072f18558153 INFO:utils:View this transaction in the MultiversX Devnet Explorer: https://devnet-explorer.multiversx.com/transactions/ee84f3e833d439e159c9619fd76e26d2afcdad62c197d87e4940072f18558153
The smart contract is now deployed on the blockchain. We can interact with it using blockchain transactions in order to invoke smart contract functions
The smart contract source code resides in
There are two main functions:
pong, these are invoked using blockchain transactions.
We also have two other functions defined in the smart contract:
did_user_ping, these view functions are invoked using MultiversX API (https://devnet-api.multiversx.com/vm-values/query).
The Application Layer - The Web App
All right, let's move on to the application layer.
Clone the Sample App
We will clone a very simple dApp template that implements the calls to our newly deployed smart contract.
git clone https://github.com/multiversx/mx-template-dapp dapp
Configure the app
Customize the Smart Contract Address
We'll take a look on the first instruction:
export const contractAddress = "erd1qqqqqqqqqqqqq...";
and we'll change it to our contract address that was shown after mxpy contract deploy:
export const contractAddress =
Save and close
config.tsx and we're ready for the first build.
Build the dApp
We'll first install the dependencies:
and then we'll start a development server to test our new dApp
npm run start
Run it on local machine (or host it on your server) If you start the development server on the local machine, then open http://localhost:3000 in your browser. If you start it on your own server, then you can access http://ip:3000. The built version only contains static files, so any hosting provider will do.
After you start the development server, when you see the Sign in screen, this means the application is up and running.
Test Your Application
We will sign in with a test wallet. You can reuse the same owner's wallet if you want to, or create a new one, following the same steps you followed when creating the owner's wallet.
After signing in, we'll see the dashboard where we can see the Ping button.
Click the Ping button and you'll be redirected to the authentication page on the web wallet, maiar wallet or your authentication device.
A new transaction will be created and you'll be asked to confirm it. This transaction transfers balance from your wallet to the smart contract address. Those funds will be locked for the specified period of time. Pay attention to the data field, where we call the smart contract function
After you confirm the transaction, a success message will appear and the funds are locked.
Wait the time interval You can see the amount of time you'll have to wait until you can pong.
After the time interval has passed, you can claim the funds by clicking the Pong button.
Another blockchain transaction will wait to be processed, this time the amount will be zero, as we only have to invoke the
pong function (specified in the data field).
The transaction will trigger a success message and the funds will be returned to the wallet.
Where to go next?
The purpose of this guide is to provide a starting point for you to discover the MultiversX technology capabilities and devkit. Keep reading the next docs to dive in deeper. We welcome your questions and inquiries on Stack Overflow: https://stackoverflow.com/questions/tagged/elrond.
Break down this guide and learn more about how to extend the smart contract, the wallet and the MultiversX tools. https://docs.multiversx.com