Fork testing (mainnet fork) in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Fork testing (mainnet fork)

Start Mainnet Fork Setup

↓

Clone Mainnet State

↓

Deploy Forked Chain Locally

↓

Run Tests on Forked Chain

↓

Analyze Test Results

↓

Decide: Fix or Deploy

↓

Fix

This flow shows how a mainnet fork is created locally to test changes safely before deciding to fix issues or deploy.

Execution Sample

Blockchain / Solidity

1. fork = createMainnetFork(rpcUrl)
2. deployContracts(fork)
3. runTests(fork)
4. analyzeResults()
5. decideNextStep()

This code creates a mainnet fork, deploys contracts, runs tests, analyzes results, and decides next steps.

Execution Table

Step	Action	Input/Condition	Result/Output
1	Create mainnet fork	RPC URL to mainnet node	Forked chain state cloned locally
2	Deploy contracts	Forked chain instance	Contracts deployed on fork
3	Run tests	Deployed contracts on fork	Test results collected
4	Analyze results	Test results	Identify pass/fail and issues
5	Decide next step	Analysis outcome	Choose to fix or deploy
6	Exit	Decision made	End of fork testing process

💡 Testing ends after decision to fix issues or deploy changes

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
fork	null	forked chain instance	forked chain instance	forked chain instance	forked chain instance	forked chain instance
contractsDeployed	false	false	true	true	true	true
testResults	null	null	null	results object	results object	results object
decision	null	null	null	null	null	fix or deploy

Key Moments - 3 Insights

Why do we clone the mainnet state instead of testing directly on mainnet?

Cloning mainnet state creates a safe local copy to test without risking real assets or affecting live users, as shown in step 1 of the execution_table.

What happens if tests fail on the forked chain?

If tests fail (step 4), the decision (step 5) is usually to fix issues before deploying, preventing broken code on mainnet.

Is the forked chain connected to the real mainnet after creation?

No, after cloning, the forked chain runs locally and independently, allowing isolated testing as indicated by the 'forked chain instance' variable state.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after step 2?

A Test results collected

B Contracts deployed on fork

C Forked chain state cloned locally

D Identify pass/fail and issues

💡 Hint

Check the 'Result/Output' column for step 2 in the execution_table.

At which step do we analyze the test results?

A Step 3

B Step 5

C Step 4

D Step 1

💡 Hint

Look for the action 'Analyze results' in the execution_table.

If the fork variable was null after step 1, what would that mean?

A Fork creation failed

B Contracts deployed successfully

C Tests passed

D Decision made to deploy

💡 Hint

Refer to variable_tracker for 'fork' variable state after step 1.

Concept Snapshot

Fork testing (mainnet fork):
- Clone mainnet state locally
- Deploy contracts on fork
- Run tests safely
- Analyze results
- Decide to fix or deploy
- Ensures safe testing without risking mainnet

Full Transcript

Fork testing with a mainnet fork means creating a local copy of the mainnet blockchain state. This lets developers test their code safely without affecting the real network. The process starts by cloning the mainnet state using an RPC URL. Then contracts are deployed on this forked chain. Tests run on the fork collect results, which are analyzed to find any issues. Based on the analysis, developers decide whether to fix problems or deploy the code. This approach protects real assets and users by isolating testing from the live mainnet.

Practice

1. What is the main purpose of creating a mainnet fork in blockchain development?

easy

A. To safely test code using real blockchain data without affecting the live network

B. To permanently change the main blockchain rules

C. To mine new blocks faster than the main network

D. To create a new cryptocurrency from scratch

Solution

1. Step 1: Understand what a mainnet fork is

   A mainnet fork is a copy of the live blockchain used for testing.

2. Step 2: Identify the purpose of testing on a fork

   It allows developers to try code changes safely without risking the real network.

3. Final Answer:

   To safely test code using real blockchain data without affecting the live network -> Option A

4. Quick Check:

   Fork testing = safe real-data testing [OK]

Hint: Mainnet fork = safe playground for real blockchain data [OK]

Common Mistakes:

• Thinking a fork changes the live blockchain
• Confusing fork with creating a new coin
• Assuming fork speeds up mining

2. Which of the following commands is commonly used to create a mainnet fork with Hardhat?

easy

A. npx hardhat node --fork https://mainnet.infura.io/v3/YOUR_API_KEY

B. npx hardhat compile --fork mainnet

C. npx hardhat deploy --network mainnet

D. npx hardhat test --fork mainnet

Solution

1. Step 1: Recall Hardhat command for forking

   The command to start a local node forked from mainnet includes node --fork with a mainnet RPC URL.

2. Step 2: Match the correct syntax

   npx hardhat node --fork https://mainnet.infura.io/v3/YOUR_API_KEY uses npx hardhat node --fork with a mainnet URL, which is correct.

3. Final Answer:

   npx hardhat node --fork https://mainnet.infura.io/v3/YOUR_API_KEY -> Option A

4. Quick Check:

   Fork command = hardhat node --fork [OK]

Hint: Fork with 'hardhat node --fork' plus mainnet URL [OK]

Common Mistakes:

• Using compile or deploy commands to fork
• Missing the --fork flag
• Not providing a mainnet RPC URL

3. Given this Hardhat script snippet for mainnet fork testing:

const { ethers } = require("hardhat");

async function main() {
  const balance = await ethers.provider.getBalance("0x742d35Cc6634C0532925a3b844Bc454e4438f44e");
  console.log(ethers.utils.formatEther(balance));
}

main();

What will this script output when run on a mainnet fork?

medium

A. The balance of the address on a testnet like Ropsten

B. The Ether balance of the address on the live mainnet

C. An error because the address is invalid

D. Zero, because the fork has no data

Solution

1. Step 1: Understand mainnet fork data availability

   A mainnet fork copies the live blockchain state, including balances.

2. Step 2: Analyze the script's balance query

   The script queries the balance of a known mainnet address, so it returns the real Ether amount.

3. Final Answer:

   The Ether balance of the address on the live mainnet -> Option B

4. Quick Check:

   Fork has live data = real balance [OK]

Hint: Fork shows real mainnet balances, not zero or errors [OK]

Common Mistakes:

• Assuming fork has empty state
• Confusing mainnet fork with testnet
• Thinking address format is invalid

4. You tried to fork mainnet using Hardhat but got the error: Invalid JSON RPC response. What is the most likely cause?

medium

A. You ran the fork command without internet connection

B. Your Hardhat version does not support forking

C. The address you queried does not exist

D. You forgot to provide a valid mainnet RPC URL with an API key

Solution

1. Step 1: Understand the error context

   "Invalid JSON RPC response" usually means the node cannot connect to the RPC endpoint.

2. Step 2: Identify common causes

   Not providing a valid mainnet RPC URL or missing API key causes this error.

3. Final Answer:

   You forgot to provide a valid mainnet RPC URL with an API key -> Option D

4. Quick Check:

   Missing RPC URL/API key = JSON RPC error [OK]

Hint: Check RPC URL and API key first for JSON RPC errors [OK]

Common Mistakes:

• Blaming Hardhat version without checking URL
• Assuming address errors cause RPC failure
• Ignoring internet connectivity issues

5. You want to test a smart contract upgrade on a mainnet fork. Which sequence correctly describes the steps to do this safely?

hard

A. Fork mainnet, deploy new contract only, ignore old contract state

B. Deploy new contract on mainnet directly, then fork mainnet to test

C. Fork mainnet locally, deploy old contract, run upgrade scripts, test new contract behavior

D. Test upgrade only on testnet, no need to fork mainnet

Solution

1. Step 1: Fork mainnet and deploy old contract

   Start by creating a mainnet fork and deploying the current contract version to replicate live state.

2. Step 2: Run upgrade scripts and test new behavior

   Apply upgrade scripts on the fork and test the new contract to ensure correctness without risk.

3. Final Answer:

   Fork mainnet locally, deploy old contract, run upgrade scripts, test new contract behavior -> Option C

4. Quick Check:

   Fork + old contract + upgrade + test = safe upgrade [OK]

Hint: Fork, deploy old, upgrade, then test for safe contract upgrades [OK]

Common Mistakes:

• Deploying new contract directly on mainnet
• Ignoring old contract state in tests
• Skipping mainnet fork and testing only on testnet