Session 3.3 - Usage Patterns

Learning Objectives

Apply common smart contract design patterns
Understand when and how to use different patterns
Analyze pattern trade-offs and best practices
Implement secure and efficient contract designs

Security Patterns

Access Control Pattern

Restrict function access to authorized users only

contract AccessControl {
    address public owner;
    mapping(address => bool) public admins;
    
    modifier onlyOwner() {
        require(msg.sender == owner, "Not owner");
        _;
    }
    
    modifier onlyAdmin() {
        require(admins[msg.sender], "Not admin");
        _;
    }
    
    function addAdmin(address _admin) public onlyOwner {
        admins[_admin] = true;
    }
}

Circuit Breaker Pattern

Emergency stop mechanism for critical situations

bool public stopped = false;

modifier stopInEmergency() {
    require(!stopped, "Contract is stopped");
    _;
}

function emergencyStop() public onlyOwner {
    stopped = true;
}

Financial Patterns

Withdrawal Pattern

mapping(address => uint) pendingWithdrawals;

function withdraw() public {
    uint amount = pendingWithdrawals[msg.sender];
    pendingWithdrawals[msg.sender] = 0;
    payable(msg.sender).transfer(amount);
}

Rate Limiting

mapping(address => uint) lastWithdrawal;
uint constant COOLDOWN = 1 days;

modifier rateLimited() {
    require(block.timestamp >= 
        lastWithdrawal[msg.sender] + COOLDOWN);
    lastWithdrawal[msg.sender] = block.timestamp;
    _;
}

State Management Patterns

State Machine Pattern

enum State { Created, Locked, Released, Inactive }
State public state;

modifier inState(State _state) {
    require(state == _state, "Invalid state");
    _;
}

modifier transitionTo(State _state) {
    _;
    state = _state;
}

function lock() public inState(State.Created) transitionTo(State.Locked) {
    // Lock logic
}

Pattern Animation Lab

Visualize how patterns change behavior. Choose a pattern and press Play to see how messages, state and funds move across components.

Security Financial State

Select Pattern

Speed

A

B

Tx

Summary

Key Takeaways

Design patterns solve common smart contract problems
Security patterns prevent vulnerabilities and attacks
Financial patterns handle money transfers safely
State patterns manage contract lifecycle effectively
Proper pattern usage improves code quality and security

What's Next?

Next, we'll explore DLT-based Smart Contracts across different platforms.

Next: DLT-based Smart Contracts Previous: Life Cycle Back to Course Home