Storage vs memory usage in Blockchain / Solidity - Hands-On Comparison

Storage vs Memory Usage in a Simple Blockchain

📖 Scenario: You are building a simple blockchain to understand how data is stored permanently (storage) versus temporarily (memory) during transactions.

🎯 Goal: Create a small blockchain where blocks are stored permanently in a list (storage), and transactions are held temporarily in a memory list before being added to a block.

📋 What You'll Learn

Create a list called blockchain to store blocks permanently.

Create a list called memory_pool to hold transactions temporarily.

Add transactions to memory_pool.

When memory_pool reaches 2 transactions, create a block and add it to blockchain.

Print the blockchain to show stored blocks.

💡 Why This Matters

🌍 Real World

Blockchains store data permanently in blocks (storage), but transactions first live temporarily in memory pools before being added to blocks.

💼 Career

Understanding storage vs memory in blockchain helps in designing efficient decentralized applications and smart contracts.

1

Create the blockchain storage

Create an empty list called blockchain to store blocks permanently.

Blockchain / Solidity

# Create an empty list called blockchain
# Your code here

Hint

Use blockchain = [] to create an empty list.

2

Create the memory pool for transactions

Create an empty list called memory_pool to hold transactions temporarily.

Blockchain / Solidity

blockchain = []
# Create an empty list called memory_pool
# Your code here

Hint

Use memory_pool = [] to create an empty list.

3

Add transactions and create blocks

Add these two transactions 'tx1' and 'tx2' to memory_pool. Then, when memory_pool has 2 transactions, create a block dictionary with key 'transactions' and value memory_pool. Append this block to blockchain and clear memory_pool.

Blockchain / Solidity

blockchain = []
memory_pool = []
# Add 'tx1' and 'tx2' to memory_pool
# When memory_pool has 2 transactions, create a block and add it to blockchain
# Clear memory_pool after adding the block
# Your code here

Hint

Use append() to add transactions. Check length with len(). Use copy() to avoid reference issues.

4

Print the blockchain storage

Print the blockchain list to show the stored blocks.

Blockchain / Solidity

blockchain = []
memory_pool = []
memory_pool.append('tx1')
memory_pool.append('tx2')
if len(memory_pool) == 2:
    block = {'transactions': memory_pool.copy()}
    blockchain.append(block)
    memory_pool.clear()
# Print the blockchain list
# Your code here

Hint

Use print(blockchain) to display the stored blocks.

Practice

1. Which of the following best describes storage in blockchain smart contracts?

easy

A. Permanent data saved on the blockchain that persists between function calls.

B. Temporary data used only during a function execution and discarded afterward.

C. Data stored off-chain for faster access.

D. Encrypted data that cannot be accessed by the contract.

Solution

1. Step 1: Understand storage purpose

   Storage holds data permanently on the blockchain, so it remains after function execution.

2. Step 2: Compare with memory

   Memory is temporary and only lasts during function execution, unlike storage.

3. Final Answer:

   Permanent data saved on the blockchain that persists between function calls. -> Option A

4. Quick Check:

   Storage = permanent data [OK]

Hint: Storage keeps data after functions finish, memory does not [OK]

Common Mistakes:

• Confusing memory with storage permanence
• Thinking storage is temporary
• Assuming storage is off-chain
• Believing storage is encrypted by default

2. Which of the following is the correct way to declare a variable in Solidity that uses memory?

easy

A. uint memory[] numbers;

B. uint[] storage numbers;

C. memory uint[] numbers;

D. uint[] memory numbers;

Solution

1. Step 1: Recall Solidity syntax for memory arrays

   In Solidity, the correct syntax is type[] memory variableName; for memory arrays.

2. Step 2: Check each option

   uint[] memory numbers; matches the correct syntax: uint[] memory numbers;. Others have wrong order or keywords.

3. Final Answer:

   uint[] memory numbers; -> Option D

4. Quick Check:

   Memory arrays use 'type[] memory name' syntax [OK]

Hint: Memory keyword comes after type and brackets in Solidity [OK]

Common Mistakes:

• Placing 'memory' before type
• Using 'storage' instead of 'memory'
• Incorrect array syntax
• Missing brackets for array type

3. What will be the output of the following Solidity function?

pragma solidity ^0.8.0;
contract Test {
    uint[] storageArray;
    function addAndReturn() public returns (uint) {
        uint[] memory tempArray = new uint[](1);
        tempArray[0] = 5;
        storageArray.push(10);
        return tempArray[0];
    }
}

medium

A. 0

B. 10

C. 5

D. Compilation error

Solution

1. Step 1: Analyze tempArray usage

   The function creates a memory array with one element set to 5, then returns that element.

2. Step 2: Understand storageArray effect

   storageArray.push(10) adds 10 to storage but does not affect the returned value.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Return value from memory array = 5 [OK]

Hint: Return value from memory array, storage change doesn't affect output [OK]

Common Mistakes:

• Confusing storage push with return value
• Expecting storageArray value as output
• Thinking memory array persists after function
• Assuming compilation error due to storage usage

4. Identify the error in this Solidity function that tries to copy a storage array to memory:

pragma solidity ^0.8.0;
contract Example {
    uint[] storageArray;
    function copyArray() public view returns (uint[] memory) {
        uint[] memory tempArray = storageArray;
        return tempArray;
    }
}

medium

A. Cannot assign storage array directly to memory array.

B. Function must be non-view to modify storage.

C. Missing visibility specifier for storageArray.

D. Memory arrays cannot be returned from functions.

Solution

1. Step 1: Understand storage to memory assignment

   In Solidity, you cannot directly assign a storage array to a memory array variable.

2. Step 2: Identify correct copying method

   You must copy elements manually or use a loop to transfer data from storage to memory.

3. Final Answer:

   Cannot assign storage array directly to memory array. -> Option A

4. Quick Check:

   Direct storage to memory assignment is invalid [OK]

Hint: Storage arrays need manual copying to memory arrays [OK]

Common Mistakes:

• Trying direct assignment from storage to memory
• Ignoring need for loops to copy arrays
• Assuming view functions can modify storage
• Believing memory arrays can't be returned

5. You want to write a Solidity function that temporarily modifies an array for calculations without changing the stored data. Which is the best approach?

hard

A. Declare the array as memory in the contract state.

B. Copy the storage array to a memory array, modify the memory array, then discard it.

C. Use a storage pointer to the array and modify it temporarily.

D. Modify the storage array directly and revert changes after calculations.

Solution

1. Step 1: Understand temporary modification needs

   To avoid changing stored data, use a memory copy for temporary changes.

2. Step 2: Evaluate options

   Copy the storage array to a memory array, modify the memory array, then discard it. copies storage to memory, modifies memory, and discards changes, preserving storage.

3. Step 3: Reject incorrect options

   A declares memory in state (invalid syntax); B risks permanent changes if revert fails; C modifies storage directly.

4. Final Answer:

   Copy the storage array to a memory array, modify the memory array, then discard it. -> Option B

5. Quick Check:

   Use memory copy for temporary changes [OK]

Hint: Use memory copy to avoid changing stored data permanently [OK]

Common Mistakes:

• Modifying storage directly for temporary needs
• Trying to declare storage variables as memory in contract
• Assuming revert undoes all changes safely
• Using storage pointers for temporary changes