Why Checks-Effects-Interactions pattern in Blockchain / Solidity? - Purpose & Use Cases

The Big Idea

What if a tiny change in your code order could stop hackers from stealing millions?

The Scenario

Imagine you are writing a smart contract that sends money to users after verifying their balance. You first check the balance, then send the money, and finally update the balance. But what if the sending fails or the user's contract calls back unexpectedly?

The Problem

Doing these steps without order can cause serious problems like reentrancy attacks, where a malicious contract tricks your contract into sending money multiple times. This makes your contract unsafe and can lead to lost funds.

The Solution

The Checks-Effects-Interactions pattern helps by organizing your code: first check all conditions, then update your contract's state, and only after that interact with other contracts. This order prevents attackers from exploiting your contract during external calls.

Before vs After

✗ Before

if(balance >= amount) {
  send(amount);
  balance -= amount;
}

✓ After

if(balance >= amount) {
  balance -= amount;
  send(amount);
}

What It Enables

This pattern makes your smart contracts safer and more reliable by preventing common security bugs related to external calls.

Real Life Example

When building a decentralized bank, using Checks-Effects-Interactions ensures that users can't trick the system into withdrawing more money than they have.

Key Takeaways

Manual ordering of checks and interactions can cause security risks.

Checks-Effects-Interactions enforces a safe order: check, update, then interact.

Following this pattern protects your contract from reentrancy attacks.

Practice

1.

What is the main purpose of the Checks-Effects-Interactions pattern in smart contracts?

easy

A. To reduce the gas cost of transactions

B. To prevent reentrancy attacks by ordering operations safely

C. To increase the speed of contract execution

D. To allow contracts to call external APIs

Solution

1. Step 1: Understand the pattern's purpose

   The Checks-Effects-Interactions pattern is designed to prevent security issues by ordering operations carefully.

2. Step 2: Identify the main security benefit

   By checking conditions first, updating state second, and interacting last, it prevents reentrancy attacks.

3. Final Answer:

   To prevent reentrancy attacks by ordering operations safely -> Option B

4. Quick Check:

   Pattern purpose = Prevent reentrancy [OK]

Hint: Remember: check first, update second, interact last [OK]

Common Mistakes:

• Thinking it speeds up contract execution
• Believing it reduces gas costs
• Assuming it enables external API calls

2.

Which of the following Solidity code snippets correctly follows the Checks-Effects-Interactions pattern?

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  (bool success, ) = msg.sender.call{value: amount}('');
  require(success);
  balances[msg.sender] -= amount;
}

easy

A. Yes, it checks, then interacts, then effects

B. Yes, it updates state before checks

C. No, it interacts before updating state

D. No, it updates state before interaction

Solution

1. Step 1: Analyze the order of operations

   The code first checks the balance, then calls the external address, then updates the balance.

2. Step 2: Compare with pattern order

   The pattern requires checks, then effects (state update), then interactions. Here, interaction happens before effects.

3. Final Answer:

   No, it interacts before updating state -> Option C

4. Quick Check:

   Interaction before effect = Incorrect order [OK]

Hint: State update must come before external calls [OK]

Common Mistakes:

• Ignoring the order of state update and interaction
• Thinking checks alone are enough
• Confusing effects with interactions

3.

What will be the output of the following Solidity function if balances[msg.sender] is 100 and amount is 50?

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  balances[msg.sender] -= amount;
  (bool success, ) = msg.sender.call{value: amount}('');
  require(success);
}

Assume the call succeeds.

medium

A. The balance becomes 50 and 50 ether is sent

B. The balance remains 100 and 50 ether is sent

C. The balance becomes 50 but no ether is sent

D. Transaction reverts due to incorrect order

Solution

1. Step 1: Check balance and update state

   The require passes since 100 >= 50, then balance is reduced to 50.

2. Step 2: Send ether and confirm success

   The call sends 50 ether and returns success, so require(success) passes.

3. Final Answer:

   The balance becomes 50 and 50 ether is sent -> Option A

4. Quick Check:

   Balance updated then ether sent = Correct [OK]

Hint: State updates before sending ether means balance changes [OK]

Common Mistakes:

• Thinking balance stays the same after withdrawal
• Assuming call fails without reason
• Confusing order causing revert

4.

Identify the bug in this Solidity function violating the Checks-Effects-Interactions pattern:

function withdraw(uint amount) public {
  (bool success, ) = msg.sender.call{value: amount}('');
  require(success);
  require(balances[msg.sender] >= amount);
  balances[msg.sender] -= amount;
}

medium

A. It correctly follows the pattern

B. It updates balance before checking conditions

C. It does not check for call success

D. It calls external address before checking balance

Solution

1. Step 1: Review order of operations

   The function calls the external address first, then checks balance, then updates state.

2. Step 2: Compare with pattern rules

   Pattern requires checks first, then effects, then interactions. Here, interaction is first, violating the pattern.

3. Final Answer:

   It calls external address before checking balance -> Option D

4. Quick Check:

   Interaction before checks = Bug [OK]

Hint: Always check before calling external contracts [OK]

Common Mistakes:

• Ignoring the order of require and call
• Assuming call success check is missing
• Thinking balance update order is wrong

5.

You want to write a Solidity function that sends a reward to a user only if they have enough points. Which code snippet correctly applies the Checks-Effects-Interactions pattern?

function rewardUser(address user, uint points) public {
  // Option A
  require(userPoints[user] >= points);
  userPoints[user] -= points;
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);

  // Option B
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);
  require(userPoints[user] >= points);
  userPoints[user] -= points;

  // Option C
  userPoints[user] -= points;
  require(userPoints[user] >= 0);
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);

  // Option D
  require(userPoints[user] >= points);
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);
  userPoints[user] -= points;
}

hard

A. Option A

B. Option B

C. Option C

D. Option D

Solution

1. Step 1: Analyze the order of operations

   The correct snippet first requires userPoints[user] >= points (checks), then subtracts points (effects), then sends ether via call (interactions).

2. Step 2: Identify violations in others

   The second snippet interacts before checks; the third updates state before checks; the fourth interacts before effects.

3. Final Answer:

   Option A -> Option A

4. Quick Check:

   Checks -> Effects -> Interactions = Correct snippet [OK]

Hint: Check first, update second, call last for safety [OK]

Common Mistakes:

• Calling external before checking conditions
• Updating state before checks
• Mixing order of effects and interactions