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How to create an upgradeable smart contract in Celo

· 15 min read
Viral Sangani

What are upgradeable smart contracts?

Usually, when we deploy a smart contract, it’s impossible to update or change the code since it’s on-chain, and that’s how it should be. This increases the safety and trust of users who interact with that contact.

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But there might be cases where you want to upgrade your smart contract, like fixing severe bugs and adding some critical features for the users. Traditionally doing this is not possible. The best one can deploy is a new smart contract with bug fixes with all the information migrated. This does not end here; one must update the references where the old contract address was used and inform existing users to use the new contract.

This can be overwhelming, but there is a way to handle these cases by using Upgradeable contracts. An upgradeable smart contract can be updated and edited after the deployment. This can be achieved by a plugin/tool created by OpenZeppelin.

In a nutshell

The plugin is used to deploy the contracts on Hardhat or truffle. Whenever in the future you wish to upgrade the smart contract, use the same address that you used to deploy the first contract via the plugin, and the plugin will handle the transferring of any state and data from the old contract while keeping the same contract address to interact with.

Basic Upgradeable example

OpenZeppelin uses a proxy pattern where they deploy three smart contracts to manage the storage layer and implement smart contracts. Whenever a contract call is invoked, the user indirectly calls the proxy contract, and the proxy contract passes the parameters to the implemented smart contract before sending the output back to the user. Now since we have a proxy contract as a middle man, imagine how you want to change something in your implemented contract. All you have to do is deploy the new contract and tell your proxy contract to refer to the latest smart contract, and voila. All users are using the updated contract.

How do upgradeable contracts work?

When we use OpenZeppelin’s upgradeable plugin to deploy the contract, three contracts are deployed —

  • Implemented Contract — The contract the developers create which contains all the logic and functionalities.
  • Proxy Contract — The contract that the end-user interacts with. All the data and state of the contract are stored in the context of the Proxy contract. This Proxy Contract is an implementation of the EIP1967 standard.
  • ProxyAdmin Contract — This contract links the Proxy and Implementation contract.

What is ProxyAdmin? (According to OpenZeppelin docs) A ProxyAdmin is a contract that acts as the owner of all your proxies. Only one per network gets deployed. When you start your project, the ProxyAdmin is owned by the deployer address, but you can transfer ownership of it by calling transferOwnership.

When a user calls the proxy contract, the call is delegated to the implementation contract. Now to upgrade the contract, what we have to do is:

  1. Deploy the updated Implementation Contract.
  2. Updating the ProxyAdmin so that all the calls are redirected to the newly implemented contract.

OpenZeppelin has created a plugin for Hardhat and Truffle to handle these jobs for us. Let’s see step by step how to create and test the upgradeable contracts.

Writing upgradeable smart contracts

We will be using Hardhat’s localhost to test the contract locally and Celo Alfajores test network.

Initialize project and install dependencies

mkdir upgradeable-contract && cd upgradeable-contract
yarn init -y
yarn add hardhat
yarn hardhat // choose typescript

Add plugin and update hardhat.config.ts

yarn add @openzeppelin/hardhat-upgrades

// hardhat.config.ts
import '@openzeppelin/hardhat-upgrades';

To understand the flow, we will be using Greeter.sol a contract. We will create and deploy several versions of this contract.

  • Greeter.sol
  • GreeterV2.sol
  • GreeterV3.sol

NOTE — The big difference between a normal contract and an upgradeable smart contract is upgradeable smart contracts do not have a constructor.

//contracts/Greeter.sol
//SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.0;
contract Greeter {
string public greeting;
// Emitted when the stored value changes
event ValueChanged(string newValue);
function greet() public view returns (string memory) {
return greeting;
}
function setGreeting(string memory _greeting) public {
greeting = _greeting;
emit ValueChanged(_greeting);
}
}

This is a very simple Greeter contract that returns the value of greeting whenever we call greet() method.

Unit testing for Greeter.sol

Create a file called 1.Greeter.test.ts and add the following content.

// test/1.Greeter.test.ts
import { expect } from "chai";
import { Contract } from "ethers";
import { ethers } from "hardhat";
describe("Greeter", function() {
let greeter: Contract;
beforeEach(async function() {
const Greeter = await ethers.getContractFactory("Greeter");
greeter = await Greeter.deploy();
await greeter.deployed();
});
it("should greet correctly before and after changing value", async function() {
await greeter.setGreeting("Celo to the Moon");
expect(await greeter.greet()).to.equal("Celo to the Moon");
});
});

Run the test:

yarn hardhat test test/1.Greeter.test.ts

Results:

Greeter
✔ should greet correctly before and after changing value
1 passing (334ms)
✨ Done in 1.73s.

Let’s write a deploy script and deploy Greeter.sol it to Hardhat’s local node. Create a file named Greeter.deploy.ts in scripts directory and paste the following code.

// scripts/Greeter.deploy.ts
import { ethers, upgrades } from "hardhat";
async function main() {
const Greeter = await ethers.getContractFactory("Greeter");
console.log("Deploying Greeter...");
const greeter = await upgrades.deployProxy(Greeter);
console.log(greeter.address, " greeter(proxy) address");
console.log(
await upgrades.erc1967.getImplementationAddress(greeter.address),
" getImplementationAddress"
);
console.log(
await upgrades.erc1967.getAdminAddress(greeter.address),
" getAdminAddress"
);
}
main().catch((error) => {
console.error(error);
process.exitCode = 1;
});

Run the local node and deploy the Greeter.sol contract:

yarn hardhat node
yarn hardhat run scripts/Greeter.deploy.ts --network localhost

Results:

Deploying Greeter...
0x9A676e781A523b5d0C0e43731313A708CB607508 greeter(proxy) address
0xA51c1fc2f0D1a1b8494Ed1FE312d7C3a78Ed91C0 getImplementationAddress
0x0DCd1Bf9A1b36cE34237eEaFef220932846BCD82 getAdminAddress
✨ Done in 2.74s.

Note: If you run the deployment command several times you can notice that adminAddress does not change.

Now we need to implement the increment feature in the existing contract. Instead of replacing the current contract we will create a new contract and change the proxy to refer to the new contract.

Creating GreeterV2.sol

// contracts/GreeterV2.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "./Greeter.sol";
contract GreeterV2 is Greeter {
uint256 public counter;
// Increments the counter value by 1
function increment() public {
counter++;
}
}

Writing unit tests for GreeterV2 before deploying.

import { expect } from "chai";
import { BigNumber, Contract } from "ethers";
import { ethers } from "hardhat";
describe("Greeter V2", function() {
let greeterV2: Contract;
beforeEach(async function() {
const GreeterV2 = await ethers.getContractFactory("GreeterV2");
greeterV2 = await GreeterV2.deploy();
await greeterV2.deployed();
});
it("should retrieve value previously stored", async function() {
await greeterV2.setGreeting("Celo to the Moon");
expect(await greeterV2.greet()).to.equal("Celo to the Moon");
});
it("should increment value correctly", async function() {
expect(await greeterV2.counter()).to.equal(BigNumber.from("0"));
await greeterV2.increment();
expect(await greeterV2.counter()).to.equal(BigNumber.from("1"));
});
});

Run the test —

yarn hardhat test test/2.GreeterV2.test.ts

Results —

Greeter V2
✔ should retrieve value previously stored
✔ should increment value correctly
2 passing (442ms)
✨ Done in 6.43s.

The above test was designed to only test GreeterV2.sol not the upgraded version of Greeter.sol. Let’s write and deploy the GreeterV2 in proxy patter and test if GreeterV2 works correctly.

// test/3.GreeterV2Proxy.test.ts
import { expect } from "chai";
import { Contract } from "ethers";
import { ethers, upgrades } from "hardhat";
describe("Greeter (proxy) V2", function() {
let greeter: Contract;
let greeterV2: Contract;
beforeEach(async function() {
const Greeter = await ethers.getContractFactory("Greeter");
const GreeterV2 = await ethers.getContractFactory("GreeterV2");
greeter = await upgrades.deployProxy(Greeter);
// setting the greet value so that it can be checked after upgrade
await greeter.setGreeting("WAGMI");
greeterV2 = await upgrades.upgradeProxy(greeter.address, GreeterV2);
});
it("should retrieve value previously stored correctly", async function() {
expect(await greeterV2.greet()).to.equal("WAGMI");
await greeter.setGreeting("Celo to the Moon");
expect(await greeterV2.greet()).to.equal("Celo to the Moon");
});
});

Here we are setting the greeting value to WAGMI in Greeter (V1) and after upgrading, checking the greeting value in GreeterV2.

Run the test —

yarn hardhat test test/3.GreeterV2Proxy.test.ts

Results —

Greeter (proxy) V2
✔ should retrieve value previously stored correctly
1 passing (521ms)
✨ Done in 6.45s.

Writing a script to update Greeter to GreeterV2

Note that the greeter proxy address is 0x9A676e781A523b5d0C0e43731313A708CB607508 that we got when we deployed Greeter.sol. We need a proxy address to deploy GreeterV2.sol

Create a file called GreeterV2.deploy.ts and add the following code.

// scripts/2.upgradeV2.ts
import { ethers, upgrades } from "hardhat";
const proxyAddress = "0x9A676e781A523b5d0C0e43731313A708CB607508";
async function main() {
console.log(proxyAddress, " original Greeter(proxy) address");
const GreeterV2 = await ethers.getContractFactory("GreeterV2");
console.log("upgrade to GreeterV2...");
const greeterV2 = await upgrades.upgradeProxy(proxyAddress, GreeterV2);
console.log(greeterV2.address, " GreeterV2 address(should be the same)");
console.log(
await upgrades.erc1967.getImplementationAddress(greeterV2.address),
" getImplementationAddress"
);
console.log(
await upgrades.erc1967.getAdminAddress(greeterV2.address),
" getAdminAddress"
);
}
main().catch((error) => {
console.error(error);
process.exitCode = 1;
});

Running deployment script for GreeterV2.sol

We need to start from the beginning, i.e., deploy Greeter.sol first, get the proxy address, and use that to deploy GreeterV2.sol

yarn hardhat node
yarn hardhat run scripts/Greeter.deploy.ts --network localhost

Here we got 0x9A676e781A523b5d0C0e43731313A708CB607508 as a proxy address. (Update the proxy address in scripts/GreeterV2.deploy.ts ).

yarn hardhat run scripts/GreeterV2.deploy.ts --network localhost

Results —

0x9A676e781A523b5d0C0e43731313A708CB607508  original Greeter(proxy) address
upgrade to GreeterV2...
0x9A676e781A523b5d0C0e43731313A708CB607508 GreeterV2 address(should be the same)
0x0B306BF915C4d645ff596e518fAf3F9669b97016 getImplementationAddress
0x0DCd1Bf9A1b36cE34237eEaFef220932846BCD82 getAdminAddress
✨ Done in 2.67s.

Overriding the existing methods

Now let’s say you want to add a custom name field in with the greeting such that when greet() is called, the name is added to the returned string.

Create a new file called GreeterV3.sol in contracts directory and add the following code —

// contracts/GreeterV3.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "./GreeterV2.sol";
contract GreeterV3 is GreeterV2 {
string public name;
function setName(string memory _name) public {
name = _name;
}
function greet() public view override returns (string memory) {
return string(abi.encodePacked(greeting, " ", name));
}
}

Let’s write test cases to deploy and test GreeterV3 . Create a file called 4.GreeterV3Proxy.test.ts and add the following test cases.

// test/.GreeterV3Proxy.test.ts
import { expect } from "chai";
import { BigNumber, Contract } from "ethers";
import { ethers, upgrades } from "hardhat";
describe("Greeter (proxy) V3 with name", function() {
let greeter: Contract;
let greeterV2: Contract;
let greeterV3: Contract;
beforeEach(async function() {
const Greeter = await ethers.getContractFactory("Greeter");
const GreeterV2 = await ethers.getContractFactory("GreeterV2");
const GreeterV3 = await ethers.getContractFactory("GreeterV3");
greeter = await upgrades.deployProxy(Greeter);
// setting the greet value so that it can be checked after upgrade
await greeter.setGreeting("WAGMI");
greeterV2 = await upgrades.upgradeProxy(greeter.address, GreeterV2);
greeterV3 = await upgrades.upgradeProxy(greeter.address, GreeterV3);
});
it("should retrieve value previously stored and increment correctly", async function() {
expect(await greeterV2.greet()).to.equal("WAGMI ");
expect(await greeterV3.counter()).to.equal(BigNumber.from("0"));
await greeterV2.increment();
expect(await greeterV3.counter()).to.equal(BigNumber.from("1"));
});
it("should set name correctly in V3", async function() {
expect(await greeterV3.name()).to.equal("");
const name = "Viral";
await greeterV3.setName(name);
expect(await greeterV3.name()).to.equal(name);
expect(await greeterV3.greet()).to.equal(`WAGMI ${name}`);
});
});

Note 1 — Since all the data and state is stored in Proxy contract you can see in the test cases that we are calling increment() method in GreeterV2 and checking if the value is incremented or not in GreeterV3 . Note 2 — Since in our GreeterV3.sol the contract, we have added space and name to the greeting If the name is null, then the greet the call will return the greeting with an extra space appended to it.

Run the tests —

yarn hardhat test test/4.GreeterV3Proxy.test.ts

Results —

Greeter (proxy) V3 with name
✔ should retrieve value previously stored and increment correctly
✔ should set name correctly in V3
2 passing (675ms)
✨ Done in 2.38s.

Deployment script for GreeterV3.sol

Create a file called GreeterV3.deploy.ts in scripts and paste the following code —

// scripts/3.upgradeV3.ts
import { ethers, upgrades } from "hardhat";
const proxyAddress = "0x4ed7c70F96B99c776995fB64377f0d4aB3B0e1C1";
async function main() {
console.log(proxyAddress, " original Greeter(proxy) address");
const GreeterV3 = await ethers.getContractFactory("GreeterV3");
console.log("upgrade to GreeterV3...");
const greeterV3 = await upgrades.upgradeProxy(proxyAddress, GreeterV3);
console.log(greeterV3.address, " GreeterV3 address(should be the same)");
console.log(
await upgrades.erc1967.getImplementationAddress(greeterV3.address),
" getImplementationAddress"
);
console.log(
await upgrades.erc1967.getAdminAddress(greeterV3.address),
" getAdminAddress"
);
}
main().catch((error) => {
console.error(error);
process.exitCode = 1;
});

Deploy the GreeterV3

yarn hardhat run scripts/GreeterV3.deploy.ts --network localhost

Note — If you get “Error: Proxy admin is not the one registered in the network manifest” when you try to deploy GreeterV3 , you need to run Greeter.deploy.ts and GreeterV2.deploy.ts again and copy the new proxy address to be used to deploy the upgraded contracts.

Results -

0x4ed7c70F96B99c776995fB64377f0d4aB3B0e1C1  original Greeter(proxy) address
upgrade to GreeterV3...
0x4ed7c70F96B99c776995fB64377f0d4aB3B0e1C1 GreeterV3 address(should be the same)
0x3Aa5ebB10DC797CAC828524e59A333d0A371443c getImplementationAddress
0x59b670e9fA9D0A427751Af201D676719a970857b getAdminAddress
✨ Done in 2.63s.

Deploying the upgraded contract manually

Let’s write a new contract — GreeterV4 where we need to —

  • Change the state variable name from public to private.
  • Add getName method to fetch name value.

Create a file called GreeterV4.sol in contracts directory and add the following code —

// contracts/GreeterV4.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "./GreeterV2.sol";
contract GreeterV4 is GreeterV2 {
string private name;
event NameChanged(string name);
function setName(string memory _name) public {
name = _name;
}
function getName() public view returns (string memory) {
return name;
}
}

Here we need to inherit from GreeterV2 instead of GreeterV3 because GreeterV3 already has a state variable called name and we cannot change the visibility but what we can do is inherit from GreeterV3 which does not have name variable and set the visibility as we want.

Let’s write test cases for GreeterV4.sol

// test/5.GreeterV4Proxy.test.ts
/* eslint-disable no-unused-vars */
import { expect } from "chai";
import { Contract } from "ethers";
import { ethers, upgrades } from "hardhat";
describe("Greeter (proxy) V4 with getName", function() {
let greeter: Contract;
let greeterV2: Contract;
let greeterV3: Contract;
let greeterV4: Contract;
beforeEach(async function() {
const Greeter = await ethers.getContractFactory("Greeter");
const GreeterV2 = await ethers.getContractFactory("GreeterV2");
const GreeterV3 = await ethers.getContractFactory("GreeterV3");
const GreeterV4 = await ethers.getContractFactory("GreeterV4");
greeter = await upgrades.deployProxy(Greeter);
greeterV2 = await upgrades.upgradeProxy(greeter.address, GreeterV2);
greeterV3 = await upgrades.upgradeProxy(greeter.address, GreeterV3);
greeterV4 = await upgrades.upgradeProxy(greeter.address, GreeterV4);
});
it("should setName and getName correctly in V4", async function() {
expect(await greeterV4.getName()).to.equal("");
const greetername = "Celo";
await greeterV4.setName(greetername);
expect(await greeterV4.getName()).to.equal(greetername);
});
});

Run the test —

yarn hardhat test test/5.GreeterV4Proxy.test.ts

Results —

Greeter (proxy) V4 with getName
✔ should setName and getName correctly in V4
1 passing (608ms)
✨ Done in 6.18s.

Preparing for the upgrade, but not really upgrading

When we call upgrades.upgradeProxy() there are a couple of things happening —

  • Your contract is deployed first,
  • ProxyAdmin called the upgrade() method and link the proxy to the newly implemented contract’s address.

To do these steps manually, we can call upgrades.prepareUpgrade() which only deploys your contract but does not link it with the proxy YET. This is left for the developers to link manually. This is useful when you want to test in production before wanting all the users to use the new contract.

Create a file called GreeterV4Prepare.deploy.ts in scripts directory and add the following code —

import { ethers, upgrades } from "hardhat";
const proxyAddress = "0xc5a5C42992dECbae36851359345FE25997F5C42d";
async function main() {
console.log(proxyAddress, " original Greeter(proxy) address");
const GreeterV4 = await ethers.getContractFactory("GreeterV4");
console.log("Preparing upgrade to GreeterV4...");
const greeterV4Address = await upgrades.prepareUpgrade(
proxyAddress,
GreeterV4
);
console.log(greeterV4Address, " GreeterV4 implementation contract address");
}
main().catch((error) => {
console.error(error);
process.exitCode = 1;
});

Note — You might want to run all the deployment scripts again if you face any issues while running this script.

Run the script —

yarn hardhat run scripts/GreeterV4Prepare.deploy.ts --network localhost

Results —

0xc5a5C42992dECbae36851359345FE25997F5C42d  original Greeter(proxy) address
Preparing upgrade to GreeterV4...
0x9E545E3C0baAB3E08CdfD552C960A1050f373042 GreeterV4 implementation contract address
✨ Done in 2.56s.

Deploying all the contracts to Celo’s Alfajores Network

First, we need to add Alfajores RPC details in hardhat.config.ts. Refer to this site for the latest RPC details — /blog/tutorials/3-simple-steps-to-connect-your-metamask-wallet-to-celo

alfajores: {
url: "https://alfajores-forno.celo-testnet.org",
accounts:
process.env.PRIVATE_KEY !== undefined ? [process.env.PRIVATE_KEY] : [],
},

Run the script and deploy Greeter.sol

Three contracts are deployed

Implementation Contract

Admin Contract

Proxy Contract

For your reference, here are the three deployed contracts —

0xeE5Dc1234Cdb585F54cc51ec08d67f6f43d0183B — Implementation Contract 0xc2E6c44e9eA569C6D1EfF7Ce0229Fb86a4A1d2d1 — ProxyAdmin Contract 0xd15100A570158b7EEbE196405D8a456d56807F2d — Proxy Contract

Upgrade Greeter to GreeterV2

Run the following command to deploy the GreeterV2. Make sure before running update the proxyAddress in GreeterV2.deploy.ts with your deployed proxy contract address.

yarn hardhat run scripts/GreeterV2.deploy.ts --network alfajores

Since we called upgrades.upgradeProxy in the deploy script, it did two things, deployed the new implementation contact and called proxyAdmin.upgrade() the method to link proxy and new implementation contract.

Greeter > GreeterV2

Upgrading GreeterV2 to GreeterV3

Run the following command —

yarn hardhat run scripts/GreeterV3.deploy.ts --network alfajores

GreeterV2 > GreeterV3

Upgrading GreeterV3 to GreeterV4

In GreeterV4Prepare.deploy.ts we had only called prepareUpgrade which only deploys the implementation contract. We will use a hardhat console to manually call update() the method to link the proxy with the implementation contract. Run the following command to deploy the implementation contract.

yarn hardhat run scripts/GreeterV4Prepare.deploy.ts --network alfajores

Results —

0xd15100A570158b7EEbE196405D8a456d56807F2d  original Greeter(proxy) address
Preparing upgrade to GreeterV4...
0x8843F73D7c761D29649d4AC15Ee9501de12981c3 GreeterV4 implementation contract address
✨ Done in 5.81s.

We will use a hardhat console to link the proxy with the implementation contract. Run the following command to start the console.

yarn hardhat console --network alfajores

For linking, we need two things —

  • ProxyAdmin address
  • GreeterV4 contract factory instance

Run the following commands in the console —

> const GreeterV4 = await ethers.getContractFactory("GreeterV4");
> await upgrades.upgradeProxy("0xd15100A570158b7EEbE196405D8a456d56807F2d", GreeterV4);

This will point the proxy contract to the latest GreeterV4 .

Conclusion

= Congratulations 💪 on making it to the end. I know this article is a bit long, but now you know how upgradeable smart contracts work under the hood. You have built and deployed proxy contracts in the local testnet and Alfajores testnet. If you have doubts or just fancy saying Hi 👋🏻, you can reach out to me on Twitter or Discord[0xViral (Celo)#6692].

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