Rollups (Optimistic vs ZK) in Blockchain / Solidity - Hands-On Comparison

Understanding Rollups: Optimistic vs ZK

📖 Scenario: You are learning about blockchain scaling solutions called rollups. Rollups help process many transactions off the main blockchain to make it faster and cheaper. There are two main types: Optimistic Rollups and Zero-Knowledge (ZK) Rollups.In this project, you will create a simple program to compare how many transactions each rollup type can handle and decide which one is better for a given situation.

🎯 Goal: Build a Python program that stores transaction data for Optimistic and ZK rollups, sets a minimum transaction threshold, filters rollups that meet this threshold, and prints the filtered results.

📋 What You'll Learn

Create a dictionary with exact rollup names and their transaction speeds

Add a threshold variable to filter rollups

Use a dictionary comprehension to select rollups meeting the threshold

Print the filtered rollups clearly

💡 Why This Matters

🌍 Real World

Rollups are important in blockchain to speed up transactions and reduce costs. Understanding how to compare their speeds helps developers choose the right solution.

💼 Career

Blockchain developers and engineers often analyze rollup performance to optimize decentralized applications and improve user experience.

1

Create the rollup transaction speeds dictionary

Create a dictionary called rollup_speeds with these exact entries: 'Optimistic': 200, 'ZK': 1500, 'Other': 100

Blockchain / Solidity

# Create the rollup_speeds dictionary with given values
# Your code here

Hint

Use curly braces {} to create a dictionary with keys and values separated by colons.

2

Set the minimum transaction speed threshold

Create a variable called min_speed and set it to 300 to filter rollups with speeds at least this value

Blockchain / Solidity

rollup_speeds = {'Optimistic': 200, 'ZK': 1500, 'Other': 100}
# Create min_speed variable with value 300
# Your code here

Hint

Just assign the number 300 to the variable min_speed.

3

Filter rollups using dictionary comprehension

Use a dictionary comprehension to create a new dictionary called fast_rollups that includes only rollups from rollup_speeds with speeds greater than or equal to min_speed. Use for name, speed in rollup_speeds.items() in your comprehension.

Blockchain / Solidity

rollup_speeds = {'Optimistic': 200, 'ZK': 1500, 'Other': 100}
min_speed = 300
# Create fast_rollups dictionary with rollups having speed >= min_speed
# Your code here

Hint

Use dictionary comprehension syntax: {key: value for key, value in dict.items() if condition}

4

Print the filtered rollups

Write a print statement to display the fast_rollups dictionary

Blockchain / Solidity

rollup_speeds = {'Optimistic': 200, 'ZK': 1500, 'Other': 100}
min_speed = 300
fast_rollups = {name: speed for name, speed in rollup_speeds.items() if speed >= min_speed}
# Print the fast_rollups dictionary
# Your code here

Hint

Use print(fast_rollups) to show the filtered dictionary.

Practice

1.

What is the main difference between Optimistic Rollups and ZK Rollups?

easy

A. Optimistic Rollups assume transactions are valid until challenged; ZK Rollups use proofs to verify immediately.

B. Optimistic Rollups use zero-knowledge proofs; ZK Rollups wait for challenges.

C. Both rollups require a waiting period before finalizing transactions.

D. ZK Rollups do not move any work off the main blockchain.

Solution

1. Step 1: Understand Optimistic Rollups behavior

   Optimistic Rollups trust transactions are valid initially and allow a challenge period to dispute invalid ones.

2. Step 2: Understand ZK Rollups behavior

   ZK Rollups generate cryptographic proofs that transactions are valid immediately, so no waiting period is needed.

3. Final Answer:

   Optimistic Rollups assume transactions are valid until challenged; ZK Rollups use proofs to verify immediately. -> Option A

4. Quick Check:

   Trust first vs proof first [OK]

Hint: Optimistic trusts first, ZK proves first instantly [OK]

Common Mistakes:

• Confusing which rollup uses proofs immediately
• Thinking both rollups have the same waiting period
• Assuming ZK Rollups do not move work off-chain

2.

Which of the following is the correct syntax to describe a ZK Rollup in a blockchain smart contract comment?

// This rollup uses ______ to verify transactions instantly

easy

A. manual challenges

B. a waiting period

C. optimistic assumptions

D. zero-knowledge proofs

Solution

1. Step 1: Identify ZK Rollup verification method

   ZK Rollups use zero-knowledge proofs to verify transactions instantly.

2. Step 2: Match the correct phrase in the comment

   The comment should mention "zero-knowledge proofs" to describe ZK Rollups.

3. Final Answer:

   zero-knowledge proofs -> Option D

4. Quick Check:

   ZK Rollups = zero-knowledge proofs [OK]

Hint: ZK means zero-knowledge proofs, not waiting [OK]

Common Mistakes:

• Choosing 'waiting period' which applies to Optimistic Rollups
• Confusing optimistic assumptions with ZK proofs
• Selecting manual challenges which are for Optimistic Rollups

3.

Consider this pseudocode for an Optimistic Rollup transaction verification:

function verifyTransaction(tx) {
  assume tx is valid
  wait 7 days for challenge
  if no challenge then finalize tx
  else revert tx
}

What will happen if a fraudulent transaction is submitted and no one challenges it?

medium

A. The system will generate a zero-knowledge proof to verify it.

B. The transaction will be rejected immediately.

C. The fraudulent transaction will be finalized after 7 days.

D. The transaction will be finalized instantly without waiting.

Solution

1. Step 1: Analyze the verification logic

   The function assumes transactions are valid and waits 7 days for any challenge.

2. Step 2: Consider no challenge scenario

   If no challenge occurs within 7 days, the transaction is finalized regardless of validity.

3. Final Answer:

   The fraudulent transaction will be finalized after 7 days. -> Option C

4. Quick Check:

   Optimistic waits then finalizes if no challenge [OK]

Hint: No challenge means finalize after wait in Optimistic Rollups [OK]

Common Mistakes:

• Thinking fraudulent tx is rejected immediately
• Assuming zero-knowledge proofs are used here
• Believing finalization is instant without waiting

4.

Identify the bug in this ZK Rollup pseudocode snippet:

function verifyZKProof(proof) {
  if (proof.isValid) {
    finalizeTransaction()
  } else {
    wait 7 days for challenge
  }
}

What is the main issue?

medium

A. finalizeTransaction() should be called only after waiting.

B. ZK Rollups should not wait for challenges; proof validity is immediate.

C. The proof.isValid check is missing a negation operator.

D. The function should always wait 7 days regardless of proof validity.

Solution

1. Step 1: Understand ZK Rollup verification

   ZK Rollups use immediate proof verification and do not require waiting periods.

2. Step 2: Analyze the code logic

   The code incorrectly waits 7 days if proof is invalid, which contradicts ZK Rollup design.

3. Final Answer:

   ZK Rollups should not wait for challenges; proof validity is immediate. -> Option B

4. Quick Check:

   ZK Rollups = instant proof, no wait [OK]

Hint: ZK proofs mean no waiting, immediate finalize [OK]

Common Mistakes:

• Thinking waiting is needed for ZK Rollups
• Misreading proof validity condition
• Assuming finalizeTransaction requires delay

5.

You want to design a rollup system that minimizes user waiting time but can handle complex computations off-chain. Which rollup type should you choose and why?

hard

A. ZK Rollup, because it provides immediate proof and faster finality.

B. Optimistic Rollup, because it finalizes instantly without proofs.

C. Optimistic Rollup, because it uses zero-knowledge proofs for speed.

D. ZK Rollup, because it waits for challenges before finalizing.

Solution

1. Step 1: Identify rollup goals

   The goal is to minimize waiting time and handle complex off-chain computations.

2. Step 2: Compare rollup features

   ZK Rollups provide immediate validity proofs, enabling faster finality without waiting periods, suitable for complex computations.

3. Final Answer:

   ZK Rollup, because it provides immediate proof and faster finality. -> Option A

4. Quick Check:

   Minimize wait + complex work = ZK Rollup [OK]

Hint: ZK Rollups = fast finality with proofs, best for complex tasks [OK]

Common Mistakes:

• Choosing Optimistic Rollup for instant finality incorrectly
• Confusing which rollup uses zero-knowledge proofs
• Thinking ZK Rollups wait for challenges