Upgrade strategies in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a new version number for a smart contract upgrade.

Blockchain / Solidity

uint256 public constant VERSION = [1];

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Av1

B"1"

Cversion1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a number for the version.

Using an undefined variable name.

The version number should be a numeric constant, so 1 is correct.

2fill in blank

medium

Complete the code to emit an event when the contract is upgraded.

Blockchain / Solidity

event Upgraded(address indexed [1]);

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Asender

Bowner

Cupgrader

Dcaller

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic names like owner which may not reflect the upgrader.

Using sender which is less descriptive here.

The event should log the address of the upgrader, so upgrader is the best choice.

3fill in blank

hard

Fix the error in the upgrade function to correctly call the new implementation.

Blockchain / Solidity

function upgradeTo(address newImplementation) external [1] {
    require(newImplementation != address(0), "Invalid address");
    _implementation = newImplementation;
    emit Upgraded(msg.sender);
}

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Apublic

BonlyOwner

Cexternal onlyOwner

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the function public without access control.

Using private which prevents external calls.

The upgrade function should be restricted to the contract owner, so the onlyOwner modifier is needed.

4fill in blank

hard

Fill both blanks to create a mapping that stores upgrade timestamps and a function to check if upgrade is allowed.

Blockchain / Solidity

mapping(address => uint256) public [1];

function canUpgrade(address upgrader) public view returns (bool) {
    return block.timestamp >= [2][upgrader] + 1 days;
}

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AlastUpgradeTime

BupgradeTimes

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the mapping and in the function.

Choosing unclear or inconsistent variable names.

The mapping and the function should use the same name lastUpgradeTime to track timestamps.

5fill in blank

hard

Fill all three blanks to implement a function that upgrades the contract only if the new version is higher and updates the version number.

Blockchain / Solidity

function upgradeToVersion(uint256 newVersion, address newImplementation) external [1] {
    require(newVersion > [2], "Version must be higher");
    upgradeTo(newImplementation);
    [3] = newVersion;
}

Drag options to blanks, or click blank then click option'

AonlyOwner

BVERSION

CcurrentVersion

D_version

Attempts:

3 left

💡 Hint

Common Mistakes

Not restricting the function to the owner.

Comparing with the wrong version variable.

Not updating the version after upgrade.

The function should be restricted to the owner (onlyOwner), compare with currentVersion, and update _version.

Practice

1. Which of the following is a common upgrade strategy in blockchain development?

easy

A. Changing the blockchain consensus algorithm without notifying nodes

B. Using proxy contracts to allow logic changes without changing the contract address

C. Deleting old blocks to save space

D. Ignoring backward compatibility during upgrades

Solution

1. Step 1: Understand upgrade strategies

   Common upgrade strategies include proxy contracts, hard forks, and soft forks.

2. Step 2: Identify the correct method

   Proxy contracts allow changing logic while keeping the same address, enabling safe upgrades.

3. Final Answer:

   Using proxy contracts to allow logic changes without changing the contract address -> Option B

4. Quick Check:

   Proxy contracts = safe upgrade method [OK]

Hint: Proxy contracts keep address same for upgrades [OK]

Common Mistakes:

• Confusing hard forks with proxy contracts
• Thinking deleting blocks is an upgrade
• Ignoring backward compatibility

2. Which syntax correctly declares a proxy contract upgrade function in Solidity?

easy

A. function upgradeTo(address newImplementation) external onlyOwner {}

B. upgradeTo(address newImplementation) public {}

C. function upgrade(address newImplementation) private {}

D. function upgradeTo() external {}

Solution

1. Step 1: Check function declaration syntax

   In Solidity, functions must start with 'function' keyword and specify visibility.

2. Step 2: Match upgrade function signature

   The upgrade function usually takes an address and is external with access control like 'onlyOwner'.

3. Final Answer:

   function upgradeTo(address newImplementation) external onlyOwner {} -> Option A

4. Quick Check:

   Correct Solidity function syntax = function upgradeTo(address newImplementation) external onlyOwner {} [OK]

Hint: Solidity functions need 'function' and visibility keywords [OK]

Common Mistakes:

• Omitting 'function' keyword
• Using wrong visibility like private for upgrade
• Missing function parameters

3. Given this Solidity proxy upgrade snippet, what will be the output of implementation() after calling upgradeTo(newAddress)?

contract Proxy {
  address private _implementation;
  function implementation() public view returns (address) {
    return _implementation;
  }
  function upgradeTo(address newImplementation) public {
    _implementation = newImplementation;
  }
}

medium

A. Compilation error due to missing visibility

B. Always zero address (0x0)

C. The address of the Proxy contract itself

D. The address stored in _implementation after upgradeTo is called

Solution

1. Step 1: Understand state variable update

   The function upgradeTo sets _implementation to newImplementation address.

2. Step 2: Check implementation() return value

   implementation() returns the current _implementation address, which changes after upgradeTo call.

3. Final Answer:

   The address stored in _implementation after upgradeTo is called -> Option D

4. Quick Check:

   State variable updated = returned address [OK]

Hint: State variable returns updated address after upgrade [OK]

Common Mistakes:

• Assuming implementation() returns Proxy address
• Thinking _implementation stays zero
• Confusing visibility keywords

4. Identify the bug in this upgrade function and how to fix it:

function upgradeTo(address newImplementation) public {
  _implementation = newImplementation;
}

medium

A. Incorrect parameter type; should be uint256 instead of address

B. Function should be private to prevent external calls

C. Missing access control; add 'onlyOwner' modifier to restrict upgrades

D. No bug; function is correct as is

Solution

1. Step 1: Analyze function security

   The function allows anyone to call upgradeTo and change implementation, which is unsafe.

2. Step 2: Add access control

   Adding 'onlyOwner' modifier restricts upgrades to contract owner, preventing unauthorized changes.

3. Final Answer:

   Missing access control; add 'onlyOwner' modifier to restrict upgrades -> Option C

4. Quick Check:

   Access control needed for upgrade functions [OK]

Hint: Always restrict upgrade functions with access control [OK]

Common Mistakes:

• Ignoring security risks of public upgrade functions
• Changing parameter type incorrectly
• Making function private disables upgrades

5. You want to upgrade a deployed smart contract without changing its address or losing stored data. Which upgrade strategy should you use and why?

hard

A. Use a proxy contract pattern to separate logic and data storage

B. Perform a hard fork to replace the entire blockchain state

C. Deploy a new contract and ask users to switch manually

D. Delete the old contract and deploy a new one at the same address

Solution

1. Step 1: Understand upgrade goals

   We want to keep the same contract address and preserve stored data during upgrade.

2. Step 2: Evaluate upgrade strategies

   Proxy contracts separate logic and data, allowing logic upgrades without changing address or data loss.

3. Step 3: Compare other options

   Hard forks replace blockchain state, deploying new contracts requires user action, deleting contracts is impossible on blockchain.

4. Final Answer:

   Use a proxy contract pattern to separate logic and data storage -> Option A

5. Quick Check:

   Proxy pattern = upgrade without address or data loss [OK]

Hint: Proxy pattern upgrades logic, keeps data and address [OK]

Common Mistakes:

• Thinking hard forks preserve contract address
• Assuming users will always switch to new contract
• Trying to delete deployed contracts