A well-structured set of interview questions for a Solana development Program position.

Below, I’ve provided detailed answers to all 50 interview questions for a Solana Smart Contract Developer position. I’ve explain technical concepts clearly and thoroughly, while addressing each question as if I am preparing you for an interview. Let’s dive in!

Section 1: Technical Questions (Solana & Anchor Framework)

1. Smart Contract Fundamentals

1. Explain how Solana’s Proof-of-History (PoH) consensus works.

Solana uses Proof-of-History (PoH) to make its blockchain faster and more efficient. PoH is not a full consensus mechanism like Proof-of-Work (PoW) or Proof-of-Stake (PoS); it’s a way to timestamp transactions so validators (computers that confirm transactions) can process them quickly. Here’s how it works:

• A special function called a Verifiable Delay Function (VDF) runs on a leader node (the computer in charge at that moment).
• It creates a sequence of hashes, where each hash depends on the one before it—like a chain.
• This chain acts like a clock: it proves that time has passed and events happened in a specific order.
• Validators use this “history” to agree on the order of transactions without needing to talk to each other a lot.

So, PoH speeds things up by letting Solana process thousands of transactions per second because the order is already clear.

2. How does Solana differ from Ethereum in terms of transaction execution and state management?

Solana and Ethereum are both blockchains, but they work differently:

• Transaction Execution:
  • Ethereum processes transactions one by one (sequentially). This can make it slow when lots of people use it.
  • Solana uses parallel execution, meaning it can process many transactions at the same time. It does this by splitting work across multiple threads using a system called Sealevel.
• State Management:
  • Ethereum stores all its data (like account balances) in one big “state.” Every time a transaction happens, this state updates, and every node must keep a copy of it. This can get slow and expensive.
  • Solana splits its state into accounts. Each account holds specific data (like a user’s balance), and only the accounts involved in a transaction get updated. This makes Solana faster and cheaper because not everything needs to change at once.

In short, Solana is built for speed and scale, while Ethereum is slower but more battle-tested.

3. What is Parallel Transaction Execution in Solana, and how does it impact performance?

Parallel Transaction Execution means Solana can process many transactions at the same time instead of one after another. This happens because of Sealevel, Solana’s runtime:

• Transactions that don’t affect the same accounts (like two people sending SOL to different friends) can run together.
• Solana uses a trick called account locking: it checks which accounts a transaction touches and only locks those, leaving others free for other transactions.

Impact on Performance:

• It makes Solana super fast—up to 65,000 transactions per second (TPS) in theory.
• It reduces waiting time, so apps like games or trading platforms feel snappy.
• But if lots of transactions hit the same account (like a popular trading pool), it can still slow down.

4. What are Program Derived Addresses (PDAs), and how do they differ from normal accounts?

A Program Derived Address (PDA) is a special type of account in Solana that belongs to a smart contract (called a program) instead of a person.

• How it’s made: A PDA is created using seeds (like a name or ID) and a program ID. A mathematical formula combines these to generate the address.
• Key Feature: PDAs don’t have private keys. Only the program that owns them can control them by signing with a bump seed (more on that later).
• Use: They’re great for things like user profiles or vaults that a program needs to manage.

Difference from Normal Accounts:

• Normal accounts (like your wallet) have a private key and public key pair. You control them with your key.
• PDAs are controlled by the program, not a person, and can’t be created manually—they’re “derived” from code.

2. Solana Smart Contract Development (Anchor & Rust)

5. How do you initialize a new Anchor project?

Anchor is a framework that makes Solana development easier. Here’s how to start a new project:

1. Install Tools:
   • Make sure you have Rust (a programming language), Solana CLI, and Node.js installed.
   • Install Anchor with: cargo install –git https://github.com/coral-xyz/anchor anchor-cli.
2. Create Project:
   • Run anchor init my_project_name in your terminal.
   • This creates a folder with files like Cargo.toml (for Rust), Anchor.toml (Anchor settings), and a basic program in src/lib.rs.
3. Set Up:
   • Open the folder in a code editor (like VSCode).
   • Run anchor build to compile your program.
   • Run anchor test to set up a local Solana network and test it.

Now you’re ready to write smart contracts!

6. Explain how account serialization works in Anchor and why it’s needed.

Serialization is turning data (like a struct in Rust) into a format that Solana can store or read.

• How it works in Anchor:
  • You define a struct with #[account] in Rust, like a User with fields (e.g., balance, owner).
  • Anchor uses a library called Borsh to convert this struct into a string of bytes (e.g., 32 bytes for Pubkey + 8 bytes for u64).
  • These bytes get saved in a Solana account.
• Why it’s needed:
  • Solana only stores raw bytes, not fancy objects like Rust structs.
  • Serialization lets your program save and load data properly.
  • It also ensures the data stays consistent when you read it back (deserialization).

Without this, your program couldn’t talk to Solana’s storage system.

7. Given the following struct, what is the exact space required for the account?

rust

#[account]
pub struct User {
    pub owner: Pubkey,  // 32 bytes
    pub balance: u64,   // 8 bytes
    pub is_active: bool // 1 byte
}

To calculate the space:

• Pubkey: 32 bytes (a public key is always 32 bytes in Solana).
• u64: 8 bytes (an unsigned 64-bit integer).
• bool: 1 byte (true/false takes 1 byte).

Total = 32 + 8 + 1 = 41 bytes.

But wait! Solana adds an extra 8 bytes at the start of every account for a discriminator (a unique ID Anchor uses to identify account types). So:

Final Answer: 41 + 8 = 49 bytes.

8. How do you derive a PDA using seeds and bump in an Anchor program?

Here’s how to create a PDA in Anchor:

• Seeds: These are inputs like a string (e.g., “user”) or a public key that make the PDA unique.
• Bump: A number that ensures the PDA is off the elliptic curve (a math thing Solana uses for keys).

Example Code:

rust

let (pda, bump) = Pubkey::find_program_address(
    &[b"user", ctx.accounts.user.key().as_ref()], // Seeds
    &ctx.program_id                               // Program ID
);

• find_program_address tries different bumps (255 down to 0) until it finds a valid PDA.
• You use this PDA as an account in your program, like a storage spot.

In Anchor: You can mark an account as a PDA with #[account(mut, seeds = [b”user”], bump)].

9. What is the bump seed, and why is it important in Solana development?

The bump seed is a number (usually 255 to 0) added to seeds when creating a PDA.

• Why it’s needed:
  • Solana uses a math curve (Ed25519) for valid keys. Most random addresses don’t fit this curve.
  • The bump adjusts the address until it’s “off the curve,” making it a valid PDA without a private key.
• Importance:
  • It lets programs control PDAs securely.
  • It ensures every PDA is unique but reproducible with the same seeds and bump.

Without the bump, you couldn’t create usable PDAs.

10. Write an Anchor instruction to transfer SOL from one account to another securely.

Here’s an example:

rust

use anchor_lang::prelude::*;

#[program]
pub mod transfer_sol {
    use super::*;

    pub fn transfer(ctx: Context<Transfer>, amount: u64) -> Result<()> {
        // Check if sender has enough SOL
        let sender_balance = ctx.accounts.sender.lamports();
        require!(sender_balance >= amount, ErrorCode::InsufficientFunds);

        // Transfer SOL
        **ctx.accounts.sender.lamports.borrow_mut() -= amount;
        **ctx.accounts.recipient.lamports.borrow_mut() += amount;

        Ok(())
    }
}

#[derive(Accounts)]
pub struct Transfer<'info> {
    #[account(mut)]
    pub sender: Signer<'info>,        // Sender must sign the transaction
    #[account(mut)]
    pub recipient: AccountInfo<'info>, // Recipient account
    pub system_program: Program<'info, System>, // Needed for SOL transfers
}

#[error_code]
pub enum ErrorCode {
    #[msg("Not enough funds to transfer")]
    InsufficientFunds,
}

• How it works:
  • The sender signs the transaction (proving they agree).
  • We check if they have enough SOL (lamports are SOL’s smallest unit).
  • We subtract from sender and add to recipient using direct lamport changes.
  • It uses the System Program (built into Solana) to handle SOL.
• Security: The Signer check ensures only the sender can start this.

3. Security & Best Practices

11. How would you prevent reentrancy attacks in a Solana smart contract?

A reentrancy attack is when a program calls another program, and that second program calls back into the first one before it’s done, messing things up (common in Ethereum).

• In Solana: Reentrancy is less of a problem because:
  • Transactions are atomic (all or nothing), so they finish before anything else happens.
  • Solana doesn’t have recursive calls by default.
• Prevention:
  • Use checks-effects-interactions: Check conditions (e.g., balance), update state (e.g., subtract SOL), then call other programs.
  • Lock accounts with mut only when needed and finish updates first.
  • Avoid external calls (CPI) until all internal state changes are done.

12. Explain the role of CPI (Cross-Program Invocation) in Solana. When should you use it?

CPI is when one Solana program calls another program.

• Role:
  • It lets programs work together. For example, your program might call the Token Program to move tokens.
  • It’s like a phone call between smart contracts.
• When to use it:
  • To use Solana’s built-in programs (e.g., System Program for SOL, Token Program for SPL tokens).
  • To interact with other deployed programs (e.g., a staking program calling a reward program).
  • When you need modularity—splitting big logic into smaller programs.

Example: Transferring tokens with invoke() to call the Token Program.

13. How does Solana handle rent exemption, and what happens when an account runs out of funds?

Solana charges rent to store data in accounts (like paying rent for storage space).

• Rent Exemption:
  • If an account has enough SOL to cover 2 years of rent, it’s “rent-exempt” and doesn’t pay ongoing fees.
  • The amount depends on data size (e.g., ~0.00089 SOL for a 1 KB account).
• What happens if funds run out:
  • If an account’s balance drops below the rent-exempt amount and can’t pay rent, Solana deletes it.
  • The data is gone, and the account is closed unless someone adds more SOL.

Tip: Always fund accounts with enough SOL when creating them.

14. Describe how to implement access control in a Solana smart contract.

Access control means deciding who can do what in your program.

• How to do it:
  • Use Signer to ensure only the account owner can call an instruction (e.g., #[account(signer)]).
  • Check public keys: require!(ctx.accounts.authority.key() == expected_key, ErrorCode::Unauthorized);.
  • Use a PDA as an “admin” account and only let it call certain functions.
  • Add a field like is_admin: bool in an account and check it.

Example: Only let an admin update settings:

rust

if !ctx.accounts.admin.is_admin {
    return Err(ErrorCode::NotAdmin.into());
}

15. What are the best practices to optimize Compute Budget in Solana programs?

Solana limits each transaction to 1.4 million Compute Units (CU). Here’s how to use less:

• Minimize Loops: Loops eat CU fast—avoid them or keep them small.
• Avoid Heavy Math: Things like encryption or big calculations use lots of CU.
• Use PDAs Efficiently: Deriving PDAs takes CU, so reuse them instead of recreating.
• Set Compute Budget: Add ComputeBudgetInstruction::SetComputeUnitLimit(500_000) to cap usage.
• Profile: Use solana logs to see where CU goes and fix heavy spots.

This keeps your program fast and cheap.

4. Solana Program Debugging & Testing

16. How do you debug a failed transaction in Solana?

When a transaction fails:

1. Check Logs: Run solana logs to see the program’s output. Look for error codes or messages.
2. Transaction ID: Use the transaction hash (from your wallet or CLI) in solana transaction <hash> to get details.
3. Explorer: Paste the hash into Solana Explorer (explorer.solana.com) for a breakdown.
4. Local Testing: Replay the transaction on a local validator with anchor test to see what’s wrong.
5. Add Prints: Add msg!(“Checkpoint 1”); in your code to trace where it fails.

17. Explain how to write unit tests for Anchor programs using Mocha & Chai.

Anchor uses JavaScript with Mocha and Chai for testing. Here’s how:

1. Set Up: After anchor init, tests go in tests/my_program.js.
2. Example Test:

javascript

const anchor = require("@coral-xyz/anchor");
const { expect } = require("chai");

describe("my_program", () => {
  const provider = anchor.AnchorProvider.env();
  anchor.setProvider(provider);
  const program = anchor.workspace.MyProgram;

  it("Transfers SOL", async () => {
    const recipient = anchor.web3.Keypair.generate();
    await program.rpc.transfer(new anchor.BN(1000000), {
      accounts: {
        sender: provider.wallet.publicKey,
        recipient: recipient.publicKey,
        systemProgram: anchor.web3.SystemProgram.programId,
      },
    });
    const balance = await provider.connection.getBalance(recipient.publicKey);
    expect(balance).to.equal(1000000);
  });
});

3. Run: Use anchor test to execute it on a local Solana network.

• Mocha runs the tests; Chai checks if results match expectations.

18. What tools can be used to analyze Solana smart contract performance?

• Solana CLI: solana logs shows CU usage and errors.
• Solana Explorer: Check transaction details and timing.
• Anchor Test: Profiles local runs for speed and CU.
• Rust Profilers: Tools like cargo flamegraph measure Rust code performance.
• Solana Playground: A web IDE to simulate and analyze programs.

19. How do you deploy a Solana smart contract to Devnet?

1. Build: Run anchor build to compile your program.
2. Set Network: Use solana config set –url devnet to switch to Devnet.
3. Deploy: Run anchor deploy—it uploads your program and gives a Program ID.
4. Fund: Ensure your wallet has Devnet SOL (solana airdrop 2).
5. Verify: Check the Program ID on Devnet Explorer.

20. How would you handle error propagation in an Anchor program?

In Anchor, errors are handled with Result<()> and custom error codes:

• Define Errors:

rust

#[error_code]
pub enum ErrorCode {
    #[msg("Not enough funds")]
    InsufficientFunds,
}

• Return Errors:

rust

if balance < amount {
    return Err(ErrorCode::InsufficientFunds.into());
}

• Propagate: Use ? to pass errors up: let result = some_function()?;.
• Client Side: The JS client sees the error message from logs (e.g., “Not enough funds”).

This keeps errors clear and manageable.

Section 2: Real-World Problem-Solving Questions

5. DeFi & NFT Use Cases

21. How would you design a staking smart contract where users earn rewards based on lock-up periods?

• Structure:
  • A StakeAccount PDA per user with fields: amount, start_time, lock_period, owner.
  • A RewardPool account holding reward tokens.
• Logic:
  • Stake: User sends SOL/tokens to the PDA, sets start_time and lock_period (e.g., 30 days).
  • Reward Calculation: (amount * lock_period * reward_rate) / time_unit. Longer lock = more rewards.
  • Unstake: After lock_period, transfer staked amount + rewards back.
• Security: Check current_time >= start_time + lock_period before unstaking.

22. What steps are needed to build an NFT minting smart contract on Solana?

1. Set Up Token: Use the SPL Token Program to create a token with supply = 1 (unique NFT).
2. Metadata: Use the Metaplex Token Metadata Program to add name, image URI, etc.
3. Mint Logic:
   • User pays SOL to a minting PDA.
   • Program mints the NFT to their wallet with CPI to Token Program.
4. Security: Limit minting to authorized users or a max supply.

23. How would you prevent double-spending in a token transfer function?

• Check Balance: Before transfer, ensure sender has enough tokens.
• Atomicity: Solana transactions are atomic, so the transfer either fully happens or fails.
• State Update: Subtract tokens from sender and add to recipient in one step.
• Replay Protection: Solana’s transaction signatures are unique, preventing reuse.

24. Design a DEX (Decentralized Exchange) contract for token swaps. What key components would you include?

• Components:
  • Liquidity Pool: A PDA holding token pairs (e.g., SOL/USDC).
  • Pricing: Use a formula like x * y = k (constant product) to set swap rates.
  • Swap Instruction: User sends Token A, gets Token B based on pool ratio.
  • Fees: Take a small % (e.g., 0.3%) to reward liquidity providers.
  • Liquidity Tracking: Accounts for providers to claim rewards.
• Security: Check pool balances and prevent flash loan attacks.

25. How would you implement a dynamic royalty system for NFT creators on Solana?

• Structure:
  • Add a royalty_percentage field (e.g., 5%) in NFT metadata via Metaplex.
  • Create a RoyaltyAccount PDA to track creators.
• Logic:
  • On each sale, calculate royalty = sale_price * royalty_percentage.
  • Use CPI to transfer royalty SOL/tokens to the creator’s wallet.
  • Allow creators to update royalty % (with access control).

6. Performance & Scalability

26. How would you handle thousands of concurrent transactions in a Solana program?

• Use parallel execution: Ensure transactions touch different accounts so they run simultaneously.
• Batch Instructions: Combine multiple actions into one transaction to save fees.
• Optimize CU: Keep logic lean to fit within 1.4M CU per transaction.
• Sharding: Split data across multiple PDAs (e.g., per user or region).

27. What strategies can you use to minimize Solana transaction fees?

• Reduce CU: Simplify logic (fewer loops, less math).
• Batch: Put multiple instructions in one transaction.
• State Compression: Store less data on-chain (see Q28).
• Priority Fees: Avoid overpaying unless needed for speed.

28. What are state compression techniques, and how do they help reduce costs?

State Compression: Storing data off-chain or in a smaller format.

• How:
  • Use Merkle Trees to store data (e.g., NFT metadata) off-chain, only keeping a root hash on-chain.
  • Programs verify data with proofs instead of storing everything.
• Benefits:
  • Cuts rent costs (less data = less SOL needed).
  • Speeds up transactions (less to process).

29. How would you optimize a Solana smart contract to run efficiently on mobile devices?

• Light Logic: Keep program instructions simple (low CU).
• Client-Side: Move heavy work (e.g., UI math) to the mobile app, not the contract.
• Batch Calls: Reduce transaction frequency with multi-instruction transactions.
• Compression: Use state compression for smaller data.

30. Explain how priority fees work in Solana and how they affect transactions.

• Priority Fees: Extra SOL you pay to make your transaction process faster.
• How: Add ComputeBudgetInstruction::SetComputeUnitPrice(micro_lamports) to your transaction.
• Effect: Validators prioritize high-fee transactions, reducing wait time during network congestion.

Section 3: Behavioral & Situational Questions

7. Problem-Solving & Debugging

31. Describe a time when you encountered a critical bug in a smart contract. How did you fix it?

Example: I once had a staking contract where users got double rewards due to a loop error.

• Found It: Used solana logs to see reward calculations repeating.
• Fixed It: Added a check to ensure rewards were paid only once per period.
• Tested: Ran unit tests to confirm it worked.

32. If your contract is experiencing high compute unit consumption, how would you optimize it?

• Profile: Check logs for CU usage per instruction.
• Simplify: Remove loops or heavy math.
• Split: Break big instructions into smaller ones.
• Set Limit: Cap CU with SetComputeUnitLimit.

33. How would you respond to an exploit found in a live Solana smart contract you deployed?

• Pause: Add an emergency pause function to stop the contract.
• Analyze: Use logs and Explorer to find the issue.
• Fix: Deploy a patched version with a new Program ID.
• Migrate: Move funds/data to the new contract.

34. What steps would you take if a Solana transaction keeps failing due to an unknown reason?

• Check logs with solana logs.
• Verify account balances and signatures.
• Test locally with anchor test.
• Look for CU overflow or invalid accounts.

35. Your smart contract needs to support millions of users. What design choices would you make?

• Use PDAs per user for scalable storage.
• Enable parallel execution with unique accounts.
• Compress state to cut costs.
• Batch transactions for efficiency.

8. Teamwork & Collaboration

36. How do you handle code reviews in a blockchain development team?

• Submit: Share clean, commented code.
• Review: Check others’ code for bugs, security, and efficiency.
• Discuss: Talk through feedback respectfully.
• Improve: Fix issues before merging.

37. Have you ever contributed to open-source Solana projects? What was your contribution?

Example: I added a staking tutorial to Anchor’s docs. It helped new devs understand PDAs and rewards.

38. How do you explain Solana smart contracts to someone with no blockchain background?

I’d say: “Imagine a smart contract as a vending machine. You put in money (SOL), pick an option (like staking), and it automatically gives you something (rewards) based on rules written in code. Solana makes this fast and cheap.”

39. How do you stay updated with Solana ecosystem changes and upgrades?

• Follow Solana’s blog and Twitter.
• Join Discord/Reddit communities.
• Read GitHub updates (e.g., Anchor repo).
• Test new features on Devnet.

40. How would you handle a disagreement with another developer on smart contract architecture?

• Listen: Understand their view.
• Explain: Share my reasoning with examples.
• Test: Prototype both ideas and compare.
• Compromise: Pick the best for security and performance.

Section 4: Advanced & Expert-Level Questions

9. Advanced Solana Concepts

41. What are Atomic Transactions in Solana, and how can they be used?

• Atomic Transactions: Everything in a transaction succeeds or fails together—no partial changes.
• Use: Combine actions (e.g., swap tokens then stake) so if one fails, none happen.

42. Explain how to build a multi-signature wallet on Solana.

• Structure: A PDA with a list of owner keys and a threshold (e.g., 2/3 must sign).
• Logic:
  • Users propose a transaction.
  • Collect signatures via instructions.
  • Execute when threshold is met.

43. What is Zero-Knowledge Proof (ZKP), and how could it be implemented in Solana smart contracts?

• ZKP: Proving something is true without revealing details (e.g., “I’m over 18” without showing my age).
• In Solana: Use a program with a ZKP library (like zk-SNARKs) to verify off-chain proofs on-chain.

44. How would you design a cross-chain bridge to transfer assets between Solana and Ethereum?

• Components:
  • Lock assets on Ethereum via a contract.
  • Mint wrapped assets on Solana via a program.
  • Use oracles (e.g., Chainlink) to verify cross-chain events.
  • Reverse process to burn and unlock.

45. Explain Triton VM and its significance in the Solana ecosystem.

• Triton VM: A virtual machine for running zero-knowledge proofs efficiently.
• Significance: Could enable private transactions or proofs on Solana, boosting privacy.

10. Emerging Trends & Innovations

46. What is Solana’s State Compression, and how does it benefit NFT projects?

• State Compression: Storing data (like NFT metadata) in Merkle Trees off-chain, with roots on-chain.
• Benefits: Cuts storage costs (e.g., millions of NFTs for pennies) and speeds up minting.

47. How would you implement a privacy-preserving DeFi protocol on Solana?

• Use ZKPs to hide amounts or identities.
• Store sensitive data off-chain, verify with proofs.
• Encrypt transactions with a privacy layer.

48. What is the role of MEV (Maximal Extractable Value) in Solana?

• MEV: Profit validators make by reordering transactions (e.g., arbitrage).
• In Solana: Less common due to high speed, but still happens in DeFi pools.

49. What are Geyser Plugins, and how can they enhance Solana data streaming?

• Geyser Plugins: Tools to stream blockchain data (e.g., transactions) to external systems.
• Enhance: Real-time analytics, faster indexing for apps.

50. How do you see the future of Solana’s scalability and adoption in Web3?

• Scalability: State compression and parallel execution will keep it fast and cheap.
• Adoption: More DeFi, NFTs, and mobile apps will grow it, but competition (e.g., Ethereum L2s) is a challenge.