Upgrade strategies in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a Proxy Upgrade Pattern

Consider a simple proxy contract pattern where the proxy delegates calls to an implementation contract. What will be the output of the following Solidity code snippet when calling getVersion() after upgrading the implementation?

Blockchain / Solidity

contract ImplementationV1 {
    function getVersion() public pure returns (string memory) {
        return "V1";
    }
}

contract ImplementationV2 {
    function getVersion() public pure returns (string memory) {
        return "V2";
    }
}

contract Proxy {
    address public implementation;

    constructor(address _impl) {
        implementation = _impl;
    }

    function upgrade(address _newImpl) public {
        implementation = _newImpl;
    }

    fallback() external payable {
        (bool success, bytes memory data) = implementation.delegatecall(msg.data);
        require(success);
        assembly {
            return(add(data, 0x20), mload(data))
        }
    }
}

A "V1"

B Compilation error due to delegatecall

C "V2"

D Runtime revert due to fallback

💡 Hint

Think about what happens when the proxy's implementation address is changed and a function is called.

The proxy delegates the call to the current implementation address. After upgrading to ImplementationV2, calling getVersion() returns "V2".

🧠 Conceptual

intermediate

Choosing Upgrade Strategy for Immutable Contracts

Which upgrade strategy is best suited for smart contracts that cannot be changed after deployment but require new features?

A Use a proxy contract with delegatecall to the implementation contract

B Use self-destruct to remove the old contract and redeploy

C Modify the contract bytecode directly on the blockchain

D Deploy a new contract and use a registry to point users to the latest version

💡 Hint

Immutable contracts cannot be changed, so think about how to redirect users.

Since contracts are immutable, deploying a new contract and updating a registry or pointer is a common upgrade strategy.

🔧 Debug

advanced

Identify the Bug in Upgradeable Contract Storage

Given the following Solidity contracts, what is the cause of the unexpected behavior where the stored value does not persist after upgrading?

Blockchain / Solidity

contract StorageV1 {
    uint256 public value;

    function setValue(uint256 _value) public {
        value = _value;
    }
}

contract StorageV2 {
    uint256 public newValue;
    uint256 public value;

    function setValue(uint256 _value) public {
        value = _value;
    }

    function setNewValue(uint256 _newValue) public {
        newValue = _newValue;
    }
}

A Storage layout mismatch between V1 and V2 causes data corruption

B Delegatecall is not used in the proxy contract

C The upgrade function does not update the implementation address

D The fallback function is missing in the proxy contract

💡 Hint

Think about how storage variables are ordered and how that affects upgradeable contracts.

Adding new variables in the middle or changing order breaks storage layout, causing data corruption after upgrade.

📝 Syntax

advanced

Syntax Error in Upgradeable Contract Code

Which option contains the correct syntax for a Solidity function that upgrades the implementation address in a proxy contract?

A function upgrade(address newImpl) public { implementation = newImpl; }

B function upgrade(address newImpl) public { implementation = newImpl }

C function upgrade(address newImpl) public { implementation == newImpl; }

D function upgrade(address newImpl) public { implementation := newImpl; }

💡 Hint

Remember Solidity statements end with semicolons and assignment uses a single equals sign.

Option A uses correct assignment syntax with a semicolon. Option A misses semicolon, C uses comparison operator, D uses invalid operator.

🚀 Application

expert

Calculate Number of Upgrade Steps in a Multi-Stage Upgrade

A blockchain project uses a multi-stage upgrade strategy where each upgrade deploys a new implementation contract and updates the proxy. If the project starts at version 1 and plans to reach version 5, how many upgrade transactions are needed to reach version 5 from version 1?

A 5

B 4

C 3

D 1

💡 Hint

Count how many times the implementation must be changed to go from version 1 to 5.

To move from version 1 to 5, you must upgrade 4 times: 1->2, 2->3, 3->4, 4->5.

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