Proxy pattern (upgradeable contracts) in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Proxy pattern in upgradeable contracts?

The Proxy pattern is a design that separates contract logic from data storage. It uses a proxy contract to forward calls to a logic contract, allowing the logic to be upgraded without changing the contract address.

beginner

Why do upgradeable contracts use a proxy?

Because blockchain contracts are immutable, a proxy allows changing the logic by pointing to a new implementation contract, while keeping the same address and stored data.

beginner

What is the role of the implementation contract in the Proxy pattern?

The implementation contract contains the actual business logic and functions. The proxy delegates calls to it while storing the data itself.

intermediate

How does the proxy forward calls to the implementation contract?

The proxy uses a special low-level function called 'delegatecall' to execute the implementation's code in the proxy's context, preserving storage and address.

intermediate

What is a common risk when using the Proxy pattern in upgradeable contracts?

Incorrect storage layout between proxy and implementation can cause data corruption. Also, malicious upgrades can change logic unexpectedly.

What does the proxy contract store in upgradeable contracts?

A Only the contract data

B Neither logic nor data

C Both logic and data

D Only the contract logic

The proxy contract stores the data (state variables), while the logic is in the implementation contract.

Which function is used by the proxy to execute the implementation's code?

A transfer

B send

C call

D delegatecall

The proxy uses 'delegatecall' to run the implementation's code in its own context.

Why is the Proxy pattern important for upgradeable contracts?

A It prevents any changes to the contract

B It allows changing the contract address

C It allows changing the contract logic without changing the address

D It stores all contract data off-chain

The Proxy pattern lets you upgrade logic while keeping the same contract address and data.

What can happen if storage layouts differ between proxy and implementation?

A Data corruption or unexpected behavior

B The contract will run faster

C The proxy will ignore the implementation

D The implementation will be deleted

Different storage layouts can cause data to be read or written incorrectly, leading to errors.

Which of these is NOT a component of the Proxy pattern?

A Proxy contract

B Storage contract

C Implementation contract

D Delegatecall mechanism

There is no separate 'Storage contract'; storage is held in the proxy contract.

Explain how the Proxy pattern enables contract upgrades on blockchain.

Think about separating data and logic and how calls are forwarded.

Describe the risks involved in using the Proxy pattern for upgradeable contracts.

Consider what can go wrong if data or logic changes unexpectedly.

Practice

1.

What is the main purpose of using the Proxy pattern in smart contracts?

easy

A. To upgrade contract logic without changing the contract address

B. To reduce gas fees by optimizing code

C. To create multiple copies of the same contract

D. To prevent any changes to the contract after deployment

Solution

1. Step 1: Understand the Proxy pattern role

   The Proxy pattern allows a contract to forward calls to another contract, enabling upgrades.

2. Step 2: Identify the main benefit

   This forwarding lets you change the logic contract without changing the proxy's address.

3. Final Answer:

   To upgrade contract logic without changing the contract address -> Option A

4. Quick Check:

   Proxy pattern = Upgrade logic without address change [OK]

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

Common Mistakes:

• Thinking proxy reduces gas fees
• Believing proxy creates contract copies
• Assuming proxy prevents all changes

2.

Which Solidity keyword is used inside a proxy contract to forward calls to the implementation contract?

easy

A. delegatecall

B. call

C. transfer

D. send

Solution

1. Step 1: Recall Solidity call types

   Solidity has several low-level calls: call, delegatecall, send, transfer.

2. Step 2: Identify forwarding call for proxy

   Proxy contracts use delegatecall to run implementation code in proxy's context.

3. Final Answer:

   delegatecall -> Option A

4. Quick Check:

   Proxy forwarding uses delegatecall [OK]

Hint: Proxy uses delegatecall to keep storage context [OK]

Common Mistakes:

• Confusing call with delegatecall
• Using transfer or send which are for Ether
• Not knowing delegatecall preserves storage

3.

Consider this simplified proxy contract snippet in Solidity:

contract Proxy {
    address implementation;
    
    fallback() external payable {
        (bool success, ) = implementation.delegatecall(msg.data);
        require(success);
    }
}

What happens if implementation address is zero?

medium

A. The contract will self-destruct

B. The call will succeed but do nothing

C. The fallback function will be ignored

D. The call will fail and revert the transaction

Solution

1. Step 1: Understand delegatecall to zero address

   Calling delegatecall on address zero means no code to execute.

2. Step 2: Effect of delegatecall failure

   delegatecall returns false on failure; require(success) then reverts transaction.

3. Final Answer:

   The call will fail and revert the transaction -> Option D

4. Quick Check:

   delegatecall to zero address = revert [OK]

Hint: delegatecall to zero address always fails [OK]

Common Mistakes:

• Assuming call succeeds silently
• Thinking fallback is skipped
• Believing contract self-destructs

4.

Identify the bug in this proxy upgrade function:

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

What is the main issue?

medium

A. Implementation address is not validated

B. Missing event emission after upgrade

C. No access control, anyone can upgrade implementation

D. Function should be external, not public

Solution

1. Step 1: Check function access control

   The function is public, so anyone can call it and change implementation.

2. Step 2: Understand security risk

   Without restricting access, attackers can hijack the contract logic.

3. Final Answer:

   No access control, anyone can upgrade implementation -> Option C

4. Quick Check:

   Upgrade function needs access control [OK]

Hint: Always restrict upgrade function access [OK]

Common Mistakes:

• Ignoring access control importance
• Focusing only on event emission
• Thinking public vs external affects security

5.

You want to upgrade a proxy contract to a new implementation that adds a new state variable. What must you ensure to avoid breaking storage layout?

hard

A. Rearrange all variables in the new implementation for optimization

B. Add new variables only at the end of existing storage variables

C. Remove unused variables from the old implementation

D. Change variable types to reduce storage size

Solution

1. Step 1: Understand storage layout importance

   Proxy pattern requires storage layout consistency between implementations.

2. Step 2: Correct way to add variables

   New variables must be appended to avoid overwriting existing storage slots.

3. Final Answer:

   Add new variables only at the end of existing storage variables -> Option B

4. Quick Check:

   Storage layout consistency = append variables [OK]

Hint: Append new variables to preserve storage layout [OK]

Common Mistakes:

• Rearranging variables breaks storage
• Removing old variables causes data loss
• Changing types shifts storage slots