36 practice questions with answers for Blockchain interviews in Delhi.
Blockchain jobs in Delhi
Delhi NCR has one of India's biggest pools of IT, consulting, banking and start-up jobs. Delhi itself hosts many corporate, government-tech and consulting offices, and most candidates also apply across Noida and Gurgaon, so interviews here mix service-company and product-company styles.
Below are 36 questions that match the mix of service, product and domain employers hiring blockchain developers in Delhi.
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01
What is the difference between optimistic rollups and zk-rollups?
Optimistic rollups assume batches are valid and allow a challenge window, typically about seven days, during which anyone can submit a fraud proof; this makes them EVM-compatible and simpler but slows native withdrawals. Zk-rollups submit a validity proof with each batch, so L1 verifies correctness mathematically and withdrawals finalise once the proof is accepted. Zk proving is computationally heavy and EVM equivalence was harder, though zkEVMs have matured a lot by 2026.
Validators deposit 32 ETH (after the Pectra upgrade a validator can hold up to 2048 ETH) to join. Time is divided into 12-second slots and 32-slot epochs. In each slot a randomly chosen validator proposes a block and committees attest to it. The fork-choice rule LMD-GHOST picks the head, and Casper FFG finalises checkpoints when two-thirds of stake agrees. Honest validators earn rewards; offline ones lose small amounts, and provably malicious ones are slashed.
03
What happens, step by step, when you send an Ethereum transaction?
The wallet builds the transaction with nonce, to, value, data, gas limit and fee fields, signs it with the private key, and sends it to a node via eth_sendRawTransaction. The node validates and gossips it to other mempools. A block builder includes it, and the proposer publishes the block. Every node executes it in the EVM, updating state and producing a receipt with status, gas used and logs. After about two epochs the block is finalised.
A multisig wallet is a smart contract, such as a Safe, that needs M of N owner signatures to execute a transaction; rules are visible on-chain and easy to audit, but every action costs extra gas and works per chain. An MPC wallet splits one private key into shares held by different parties who jointly produce a normal signature; it looks like a regular address, works on any chain and allows key rotation, but the security depends on the off-chain implementation.
05
How do you call read and write functions of a contract with ethers v6?
Situation: a senior reviewer rejected my pull request for a staking feature, saying the storage layout would break future upgrades. Task: fix it and learn. Action: I thanked him, asked him to explain the risk, read about EIP-1967 and storage gaps, rewrote the contract with namespaced storage, and added a storage-layout check to CI. Result: the PR merged, and that check later caught a teammate's layout change before an upgrade.
07
A user lost funds on our DeFi app due to their own mistake and is angry. How would you handle it?
Situation: a user sent tokens to the contract address directly instead of using deposit, and they were stuck. Task: support him honestly. Action: I listened, explained calmly what happened without blaming him, checked whether a recovery function existed, and escalated to the team. We had a rescue function for stray tokens behind the multisig, so after verification we returned them. Result: he thanked us publicly, and we added a front-end warning.
08
Users complain they must pay gas to approve tokens before every trade. How can signatures help?
Use off-chain signed approvals. With EIP-2612 permit, the user signs a typed EIP-712 message granting an allowance, and the dApp submits it with the trade in one transaction, so no separate approve call is needed. For tokens without permit, Permit2-style contracts give the same experience after one approval. EIP-712 shows readable fields in the wallet, reducing phishing risk. Always include a nonce, deadline, chain ID and verifying contract to prevent replays.
09
How do you get current fee data and send an EIP-1559 transaction with ethers.js?
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.
11
How can a smart contract suffer denial of service?
Common causes: loops over arrays that grow with users until they exceed the block gas limit; push payments in a loop where one reverting recipient blocks everyone; relying on an external call that an attacker can make fail; and griefing by filling storage cheaply. For example, an auction that refunds the previous bidder will freeze if that bidder is a contract that reverts. Use pull payments, bounded loops, pagination and failure isolation.
Typical bugs include missing onlyOwner or role checks on sensitive functions such as mint, withdraw or setOracle, initialiser functions in upgradeable contracts that anyone can call, misconfigured roles, and a single EOA owning critical powers. Use OpenZeppelin Ownable2Step or AccessControl, give each role the minimum rights, protect admin actions with a multisig and timelock, and write tests confirming that every privileged function reverts for unauthorised callers.
13
How is an Ethereum address derived from a private key?
Start with a random 256-bit private key. Multiply it by the secp256k1 generator point to get a 64-byte uncompressed public key. Take the Keccak-256 hash of that public key and keep the last 20 bytes; that is the address, shown as 0x plus 40 hex characters. EIP-55 adds a mixed-case checksum so typos can be detected. The process is one-way: you cannot get the private key back from the address or public key.
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.
Testnets are public networks that run the same software as mainnet but with valueless ETH, so developers can test deployments and dApps safely. Sepolia is the main testnet for application developers, and Hoodi replaced Holesky for validator and staking testing in 2025. Test ETH comes from faucets, which often require a small mainnet balance or a login to stop abuse. Many Layer 2s run their own Sepolia-based testnets bridged from Ethereum Sepolia.
16
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.
17
Your contract fails to deploy because it exceeds the 24 KB size limit. What do you do?
The EIP-170 limit is 24,576 bytes of runtime code. First enable the optimiser and try a lower runs value or via-IR. Replace revert strings with custom errors, move repeated logic out of modifiers into internal functions, and remove unused code. If still too big, split logic into separate contracts or external libraries, or use a modular pattern like the Diamond (EIP-2535). Splitting also often improves clarity and auditability.
18
How do you compute a function selector and decode calldata with cast?
cast handles ABI work from the command line.
# Function selector
cast sig 'transfer(address,uint256)'
# 0xa9059cbb
# Encode calldata
cast calldata 'transfer(address,uint256)' 0xAbc... 1000000
# Decode calldata from a transaction
cast 4byte-decode 0xa9059cbb000000...
# Decode with a known signature
cast decode-calldata 'transfer(address,uint256)' 0xa9059cbb...
This is useful when checking what a wallet is really asking you to sign.
19
Tell me about a bug you missed and what you learned from it.
Situation: I shipped a rewards contract where a rounding error let users claim slightly more than earned when claiming very often. Task: fix it and prevent repeats. Action: I owned the mistake in the team call, wrote a failing test reproducing it, changed the division order to round down in the protocol's favour, and added fuzz tests on claim frequency. Result: losses were tiny, and I now always fuzz any maths involving division.
20
What is a nonce in blockchain? Explain both common meanings.
In proof of work, the nonce is a number miners change repeatedly in the block header until the header hash falls below the difficulty target. In Ethereum's account model, the account nonce is a counter of transactions sent from an address; each transaction must use the next nonce, which orders transactions and prevents replay. A stuck transaction with a low nonce blocks all later ones from that account until it is mined or replaced.
21
Tell me about a technical decision you made where you had to balance trade-offs.
Situation: our loyalty-points product needed on-chain points but users would not pay mainnet fees. Task: pick a chain. Action: I compared three rollups and a sidechain on fees, security model, wallet support and bridge maturity, built a quick benchmark, and wrote a one-page decision record with risks. Result: we chose a rollup with live fraud proofs, cut per-transaction cost by over 95 percent, and documented a migration path if our requirements changed.
22
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.
23
What is MEV and what is proposer-builder separation?
MEV, maximal extractable value, is profit a block producer or searcher earns by including, excluding or reordering transactions, for example arbitrage, liquidations and sandwich attacks on swaps. On Ethereum, most validators use MEV-Boost, where specialised builders assemble profitable blocks and bid for them through relays, separating block building from proposing. This spreads MEV revenue to validators but concentrates building power. Enshrined PBS and inclusion lists are being researched to reduce censorship risk.
24
What are the basic Foundry commands to create, build, test and work with a project?
curl -L https://foundry.paradigm.xyz | bash && foundryup
forge init my-dapp # new project with src, test, script
forge install OpenZeppelin/openzeppelin-contracts
forge build # compile
forge test -vvv # run tests with traces
forge coverage # coverage report
forge snapshot # gas snapshot
anvil # local node
cast call ADDR 'count()(uint256)' --rpc-url $RPC
Foundry tests are written in Solidity and run fast; Hardhat is the JavaScript or TypeScript alternative with a large plugin ecosystem.
25
Design the custody setup for an exchange's hot and cold wallets.
Keep a small float, around a few percent of assets, in hot wallets for withdrawals, secured by HSMs or MPC with rate limits, address allowlists and automated monitoring. Keep the rest in cold storage: air-gapped or multisig/MPC with keys held by different officers in different locations, plus time delays. Refill hot wallets through a manual, multi-approver process. Add proof of reserves, regular key ceremonies, insurance, and incident runbooks for freezing withdrawals if anomalies appear.
26
Write a basic voting contract using a struct, mapping and event.
Since the Merge, an Ethereum node runs two pieces of software. The execution client, such as Geth, Nethermind, Besu, Erigon or Reth, runs the EVM, holds state and handles transactions. The consensus client, such as Lighthouse, Prysm, Teku, Nimbus or Lodestar, runs Proof of Stake. They talk over the Engine API. Client diversity matters because a bug in a client used by over two-thirds of validators could finalise an invalid chain and cause mass slashing.
28
Show how to sign and verify a message with ethers.js.
With ethers v6, a wallet signs using EIP-191 personal sign, and verifyMessage recovers the address.
import { Wallet, verifyMessage } from 'ethers';
const wallet = Wallet.createRandom();
const message = 'Login nonce: 48213';
const signature = await wallet.signMessage(message);
const recovered = verifyMessage(message, signature);
console.log(recovered === wallet.address); // true
This pattern powers Sign-In with Ethereum; the server issues a fresh nonce each time to stop replay.
29
Your staking validator is missing attestations and losing rewards. How do you investigate?
Check that both execution and consensus clients are synced and peering well, since low peer count or a lagging execution client causes missed duties. Look at system clock sync through NTP, CPU, memory and disk I/O, as slow disks often cause delays. Review client logs and metrics in Grafana, check for pending client upgrades before a fork, and verify network ports are open. Never start a second copy of the same validator key as a fix, because that causes slashing.
30
What is a consensus mechanism and why does a blockchain need one?
A consensus mechanism is the set of rules that lets thousands of independent nodes agree on one order of transactions and one current state, even when some nodes are faulty or malicious. Without it, two nodes could accept conflicting transactions and double spending would be possible. Common mechanisms are Proof of Work, Proof of Stake, Proof of Authority and BFT protocols like PBFT, Tendermint and Raft, each trading off decentralisation, speed and fault tolerance.
31
Your dApp needs a full history of user transactions and fast queries. Reading events from the node is too slow. What do you do?
Build or use an indexer. An indexer listens to contract events from a start block, transforms them and stores them in a database like PostgreSQL, so the front-end runs normal fast queries. Options are a subgraph on The Graph, hosted indexing services, or a custom service with ethers and a queue. It must handle reorgs by tracking block hashes, support backfilling, and expose an API. Design events up front with indexed fields that queries need.
32
How do you read the latest finalized block with ethers.js, and why use it?
Pass the block tag to getBlock. The finalized tag returns the latest block that Proof of Stake has finalised, which cannot be reverted without slashing a third of stake.
import { JsonRpcProvider } from 'ethers';
const provider = new JsonRpcProvider(process.env.RPC_URL);
const latest = await provider.getBlock('latest');
const safe = await provider.getBlock('safe');
const finalized = await provider.getBlock('finalized');
console.log(latest.number - finalized.number); // usually about 64-96
Use finalized for deposits and accounting; use latest for UI updates.
33
What is the difference between call, delegatecall and staticcall?
call runs the target contract's code in the target's own context, with its own storage, and msg.sender becomes the caller contract. delegatecall runs the target's code in the caller's context, using the caller's storage, balance and original msg.sender; this is how proxies and libraries work, and storage layouts must match. staticcall is like call but reverts on any state change, used for view functions. Delegatecall to untrusted code is extremely dangerous.
34
What is Bitcoin halving and difficulty adjustment?
Halving cuts the Bitcoin block reward in half every 210,000 blocks, roughly every four years, which caps total supply at 21 million BTC. After the April 2024 halving the reward is 3.125 BTC per block. Difficulty adjustment happens every 2016 blocks: if blocks came faster than ten minutes on average, difficulty rises, and if slower, it falls. Together they make issuance predictable regardless of how much mining power joins or leaves.
35
Compare interfaces, abstract contracts and libraries.
An interface only declares external functions, events and errors, with no state or implementation; it is used to call other contracts, like IERC20. An abstract contract can have state and implemented functions but leaves at least one unimplemented or is marked abstract, so it cannot be deployed alone. A library holds reusable functions without state; internal library functions are inlined, while external ones are deployed once and called via delegatecall. Libraries can attach to types with using for.
36
How does a constant product automated market maker work, and what is impermanent loss?
A constant product AMM holds two tokens with reserves x and y and keeps x times y equal to k. A trader adds one token and removes the other so that k stays constant after fees, which makes price equal to the reserve ratio and rise with trade size. Liquidity providers earn fees, but when prices move, arbitrage rebalances the pool, leaving them worse off than simply holding; that gap is impermanent loss.
Frequently asked questions
Which companies hire Blockchain professionals in Delhi?
Companies with large offices in Delhi include many IT services and product firms. Use their company pages below to practise employer-specific sets.
Are Blockchain interviews in Delhi different from other cities?
The questions depend more on the company than the city. Delhi's mix of employers decides the style: Delhi itself hosts many corporate, government-tech and consulting offices, and most candidates also apply across Noida and Gurgaon, so interviews here mix service-company and product-company styles
How do I find Blockchain jobs in Delhi?
Shortlist companies with Delhi offices, keep your CV focused on Blockchain projects, apply on company career pages and job portals with the Delhi filter, and practise the questions on this page before each interview.