Blockchain Fundamentals Interview Questions for Blockchain
25 Blockchain Fundamentals questions with answers for blockchain roles — 8 easy, 13 medium, 4 hard.
About Blockchain Fundamentals
Understand what a blockchain is, how blocks link through hashes, and how public, private and consortium networks differ. This topic covers nodes, the mempool, forks, finality, UTXO versus account models and enterprise frameworks like Hyperledger Fabric, plus how to judge whether a business problem really needs a blockchain at all.
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A blockchain is a distributed, append-only ledger where records are grouped into blocks and each block stores the hash of the previous block, forming a chain. Many independent nodes keep a copy and agree on new blocks through a consensus protocol, so no single party controls the data. Changing an old block would change its hash and break every later link, which makes tampering easy to detect. Bitcoin and Ethereum are the best known public examples.
02
How is a blockchain different from a traditional database?
A traditional database is controlled by one administrator who can create, update and delete rows, and trust comes from that owner. A blockchain is shared by many parties, is append-only, and trust comes from cryptography and consensus. The trade-off is performance: a database handles thousands of writes per second with low latency, while a public chain is slower and costlier. Use a blockchain only when several parties who do not fully trust each other must share one record.
03
What does a block contain and what is inside a block header?
A block has a header and a body. The body holds the list of transactions. The header holds metadata: the previous block hash, a timestamp, the Merkle root (or state and transaction roots in Ethereum) summarising the transactions, and consensus fields such as the difficulty target and nonce in Bitcoin. Because the header commits to all transactions through the root hash, nodes can verify a block's integrity by hashing just the header.
04
Explain public, private and consortium blockchains.
A public blockchain such as Bitcoin or Ethereum lets anyone read, transact and run a validator. A private blockchain is run by one organisation that controls who joins, useful for internal audit trails. A consortium (permissioned) blockchain is governed by a group of organisations, for example several banks sharing trade-finance data on Hyperledger Fabric. Public chains give maximum decentralisation; permissioned chains give privacy, known identities and higher throughput.
Immutability comes from hash linking plus consensus. Each block includes the previous block's hash, so editing a past transaction changes that block's hash and every block after it. An attacker would have to redo the work or stake for all those blocks faster than the honest network, which is economically impractical on a large chain. Strictly it is tamper-evident and practically immutable rather than impossible to change; a hard fork by social agreement can still rewrite history.
06
You open a transaction on a block explorer. Which fields do you check and what do they mean?
Check the status (success or failed), the block number and confirmations, from and to addresses, and the value transferred. For contract calls, check the decoded input data and emitted event logs, which show token transfers. Check the gas limit, gas used and effective gas price, which together give the fee paid, plus the nonce. If the status failed, the revert reason tells you why, while fees are still charged for gas used.
07
How would you set up a local blockchain for development and testing?
For EVM work, run a local node with Anvil from Foundry or the Hardhat network. Each gives about ten pre-funded test accounts, instant mining and console logging. You can fork mainnet at a given block to test against real contracts and balances without spending money. For integration tests, deploy to a public testnet such as Sepolia, using faucet ETH. Keep deploy scripts and RPC URLs in config so the same code targets local, testnet and mainnet.
Use a specific, sincere answer. For example: I first got interested when I built a small token contract in college and saw that code could hold and move value without a middleman. Since then I have completed Solidity projects, learnt Foundry testing and read audit reports. I like that the field mixes cryptography, distributed systems and finance. I want to grow as a smart contract developer and help build secure, real-world applications such as payments, tokenisation or supply chain.
09
What is the difference between a full node, a light node and an archive node?
A full node downloads and validates every block and keeps the current state, so it can verify everything independently. A light node stores only block headers and asks full nodes for Merkle proofs of the data it needs, which suits mobile wallets. An archive node keeps all historical states, so you can query a balance at any past block; indexers and block explorers need it, but it requires many terabytes of storage.
10
What is double spending and how does a blockchain prevent it?
Double spending is trying to spend the same digital money twice, for example sending the same coins to two merchants. A blockchain prevents it by keeping one shared ordering of transactions agreed through consensus. Once a transaction is included in a block, any conflicting transaction that spends the same funds is rejected as invalid. Merchants wait for several confirmations so that the chance of the block being replaced in a reorg becomes negligible.
In the UTXO model used by Bitcoin, coins exist as unspent transaction outputs; a transaction consumes some outputs and creates new ones, which makes parallel validation and privacy easier. In the account model used by Ethereum, the state stores a balance and nonce per account, which is simpler for smart contracts and wallets. The trade-off: UTXO avoids replay issues and scales verification well, while accounts make complex stateful logic far easier to write.
12
What is the difference between a soft fork and a hard fork?
A soft fork tightens the rules so that new blocks are still valid for old nodes; it is backward compatible, like Bitcoin's SegWit upgrade. A hard fork changes the rules so that old nodes reject new blocks, so every node must upgrade or the chain splits, as with Ethereum and Ethereum Classic in 2016. Planned network upgrades on Ethereum, such as Dencun and Pectra, are coordinated hard forks that all clients adopt together.
The trilemma says a blockchain finds it hard to maximise decentralisation, security and scalability at the same time on a single layer. Raising block size or using few validators increases throughput but reduces decentralisation. Ethereum's answer is a rollup-centric roadmap: keep the base layer decentralised and secure, and move execution to Layer 2 rollups that post data back to Layer 1. Many high-throughput chains instead accept fewer, more powerful validators.
The mempool is each node's waiting area for valid transactions that have been broadcast but not yet included in a block. Block producers usually pick transactions paying the highest fees or priority tips. Every node has its own mempool, so contents can differ slightly. Because pending transactions are public, bots can watch the mempool and front-run trades, which is why private transaction relays and order-flow auctions are used for sensitive transactions.
15
What are the main components of Hyperledger Fabric?
Hyperledger Fabric is a permissioned enterprise framework. Peers hold the ledger and run chaincode (smart contracts written in Go, Java or JavaScript). Orderers, usually using Raft, sequence transactions into blocks. A Membership Service Provider issues identities through certificate authorities. Channels give separate private ledgers to subsets of members, and private data collections hide data inside a channel. Fabric uses an execute-order-validate flow with endorsement policies instead of mining.
16
An HR team wants to store employee records on a blockchain. Would you recommend it?
Usually no. Employee records are owned by one organisation, change often and contain personal data that must be correctable and deletable under the DPDP Act 2023. A normal database with role-based access and audit logs is cheaper, faster and compliant. I would ask who else needs to trust the data. If several employers or background-check agencies must verify certificates, I would store only hashes of credentials on a permissioned chain and keep the actual data off-chain.
17
Design a blockchain solution for farm-to-store supply chain traceability. What goes on-chain?
I would use a permissioned or low-cost chain where farmers, transporters, warehouses and retailers are known members. On-chain I would record batch IDs, ownership handovers, timestamps, signer identity and hashes of certificates or sensor data. Large documents, images and IoT streams stay off-chain in object storage or IPFS, referenced by hash. QR codes on packs link consumers to the trail. The main risk is bad input data, so I would add signed device readings and audits.
18
A user's transaction has been pending for an hour. How do you troubleshoot it?
First check it on a block explorer: if it is not found, it was never broadcast. If pending, the fee is probably below current market rates, or an earlier transaction with a lower nonce is stuck and blocking it. On Ethereum you fix it by sending a replacement with the same nonce and a higher max fee and priority fee, or cancel by sending zero ETH to yourself with that nonce. Then make the wallet estimate fees dynamically.
19
Write a minimal Python example showing how blocks are linked by hashes.
Each block stores the previous hash, so changing any earlier data breaks the chain.
import hashlib, json
def block_hash(block):
data = json.dumps(block, sort_keys=True).encode()
return hashlib.sha256(data).hexdigest()
chain = [{'index': 0, 'data': 'genesis', 'prev': '0'}]
for i, tx in enumerate(['A pays B 5', 'B pays C 2'], 1):
chain.append({'index': i, 'data': tx, 'prev': block_hash(chain[-1])})
valid = all(chain[i]['prev'] == block_hash(chain[i-1]) for i in range(1, len(chain)))
print(valid)
20
How do you get the latest block number from an Ethereum node using JSON-RPC?
Send a JSON-RPC POST request to the node or provider URL. The result is a hex string that you convert to decimal.
curl -s -X POST https://rpc.example.org \
-H 'Content-Type: application/json' \
-d '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":1}'
# Response: {"jsonrpc":"2.0","id":1,"result":"0x15d3a1c"}
# In Python: int('0x15d3a1c', 16)
Other common methods are eth_getBalance, eth_call and eth_getTransactionReceipt.
21
Tell me about a time you explained blockchain to a non-technical stakeholder.
Situation: a business head wanted 'blockchain' in a vendor portal because competitors mentioned it. Task: help him decide. Action: I compared it to a shared online spreadsheet that nobody can secretly edit, then showed cost, speed and privacy trade-offs against a normal database with audit logs. Result: we agreed to keep the portal on a database and anchor monthly hashes of invoices on a chain for proof, saving months of work while meeting the trust goal.
22
What is finality, and how does probabilistic finality differ from deterministic finality?
Finality is the guarantee that a confirmed transaction will not be reversed. In proof of work, finality is probabilistic: each new block on top makes reversal exponentially less likely, so exchanges wait for several confirmations. In BFT-style and Ethereum proof of stake, finality is economic and deterministic: once two-thirds of stake attests to a checkpoint, reverting it would require burning at least one-third of all staked ETH. Ethereum finalises in about two epochs, roughly 13 minutes.
23
A product team wants to store user documents and personal details directly on a public chain. What do you advise?
Do not put personal data on a public chain. Data there is permanent, public and replicated globally, which conflicts with the right to erasure under India's DPDP Act and GDPR, and storage is extremely expensive. Store documents encrypted in off-chain storage, keep only a salted hash or commitment on-chain for integrity, and control access through the application. If the data must be deleted, delete the off-chain copy and the encryption key; the hash alone reveals nothing.
24
A group of banks wants a shared KYC ledger. Would you choose a public chain, a Layer 2 or Hyperledger Fabric?
For shared KYC among regulated banks I would lean to a permissioned framework such as Hyperledger Fabric or a private EVM network. Members are known, data is confidential, and regulators expect access control and data residency in India. Fabric channels and private data collections restrict who sees what, and Raft ordering gives fast finality without gas fees. A public chain or L2 fits only if we need open verification, and even then only hashes and attestations would go on it.
25
Your exchange credited a deposit, but the block was later dropped in a reorg and the funds vanished. How do you prevent this?
Credit deposits only after enough confirmations for the chain's risk profile, and on Ethereum prefer waiting for finalized or at least safe block tags rather than the latest block. Make the deposit indexer reorg-aware: track block hashes, detect when a parent hash changes, roll back credits from orphaned blocks and reprocess. Scale required confirmations with deposit size, and alert on unusually deep reorgs on smaller chains, which can signal a 51 percent attack.
Frequently asked questions
How important is Blockchain Fundamentals in Blockchain interviews?
Understand what a blockchain is, how blocks link through hashes, and how public, private and consortium networks differ. This topic covers nodes, the mempool, forks, finality, UTXO versus account models and enterprise frameworks like Hyperledger Fabric, plus how to judge whether a business problem really needs a blockchain at all.
How should I use the Blockchain Fundamentals sheet?
Go top to bottom — easy to hard. Answer before opening each answer, tick what you solved and save misses to Last-Minute Revision.