Curated interview practice questions with answers. These are practice scenarios, not a verified list of past interview questions.
About Deloitte hiring for Blockchain roles
Deloitte is one of the Big Four professional services networks, offering audit, tax, consulting, risk advisory and financial advisory services. Its Indian entities, including Deloitte USI, support global clients with technology consulting, analytics, cyber and testing work. It hires in India for analyst and consultant roles in data analytics, business intelligence, QA, ERP and risk advisory.
For Blockchain roles, this Deloitte practice set focuses most on Ethereum and the EVM (6), DeFi, Tokens and NFTs (6), Solidity Programming (5), Blockchain Fundamentals (4). It has 5 easy, 22 medium, 3 hard questions, so you can start easy and work up to the harder ones.
Company type
Consulting
Headquarters
London, United Kingdom
Main India offices
Hyderabad, Bengaluru, Mumbai, Gurugram, Pune
Deloitte Blockchain interview process
Online assessment (aptitude + technical)
Group discussion or case discussion
Technical interview
Partner or HR round
Typical pattern based on public candidate experiences. Rounds vary by role, team and year.
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01
Your dApp's users are paying high gas fees on Ethereum mainnet. How would you choose a Layer 2?
I would compare candidate rollups on security (proof system live, stage of decentralisation, upgrade keys), EVM compatibility so our Solidity code deploys unchanged, ecosystem fit (wallets, oracles, liquidity, bridges our users already use), fees and throughput, and withdrawal experience. I would deploy to testnets first, measure real costs and latency, and check that our dependencies such as Chainlink feeds exist there. Then migrate gradually, keeping L1 contracts for high-value settlement if needed.
Gas measures the computational work an operation needs; for example a simple ETH transfer uses 21,000 gas, and writing a new storage slot costs about 20,000. The gas limit is the maximum gas the sender allows a transaction to consume. The gas price, per unit, is what the sender pays. The fee equals gas used times the effective gas price. If execution exceeds the limit, it reverts but the used gas is still charged.
03
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.
04
The business wants to change contract logic after launch. How do you plan for it?
Decide early whether upgradeability is needed, because it adds trust and storage risks. Options are a proxy (UUPS or Transparent) controlled by a multisig with a timelock, a parameter-driven design where only settings like fees change through governed setters, or a migration plan to a new immutable version. Whatever you choose, document it for users, protect upgrade rights, keep storage layout append-only, and test each upgrade on a fork before executing.
05
How did you handle a time you received critical code review feedback?
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.
06
You need to send ETH to hundreds of winners. Should you push payments or let them pull?
Prefer pull payments. If a contract pushes ETH in a loop, one recipient that reverts, perhaps a contract without receive, can block everyone, and the loop may exceed the gas limit. With pull payments, the contract records each winner's amount in a mapping and winners call withdraw, which zeroes their balance before sending. This isolates failures and limits reentrancy risk. Push only for small fixed lists using call with a checked result.
07
How do you connect a browser wallet to a dApp using ethers.js?
Browser wallets inject an EIP-1193 provider at window.ethereum. Wrap it with BrowserProvider and request accounts.
import { BrowserProvider } from 'ethers';
async function connect() {
if (!window.ethereum) throw new Error('Install a wallet');
const provider = new BrowserProvider(window.ethereum);
const signer = await provider.getSigner(); // prompts the user
const address = await signer.getAddress();
const { chainId } = await provider.getNetwork();
return { signer, address, chainId };
}
In production, use EIP-6963 discovery or a connector library so users with several wallets can choose.
The ABI (Application Binary Interface) is the JSON description of a contract's functions, events and errors, and it defines how arguments are encoded. A function selector is the first four bytes of the Keccak-256 hash of the function signature, for example transfer(address,uint256) gives 0xa9059cbb. Calldata is the selector followed by ABI-encoded arguments in 32-byte words. Wallets, ethers.js and explorers use the ABI to encode calls and decode results and events.
09
You need the same contract at the same address on five chains. How do you do it?
Use CREATE2 through a deterministic deployment factory that already exists at the same address on all those chains. Keep the init code identical: same compiler version, optimiser settings, constructor arguments and metadata. Pick a salt, compute the expected address off-chain, then deploy on each chain. Foundry scripts support this by passing a salt. Avoid constructor arguments that differ per chain, or move chain-specific settings into an initialiser restricted to the deployer.
10
Write a simple escrow contract where a buyer deposits and releases payment to a seller.
The UI is probably reading cached results or a lagging RPC node. After tx.wait() resolves, explicitly refetch or invalidate the relevant queries, for example with wagmi or TanStack Query invalidation. Show optimistic updates while pending. If you use an indexer, it may lag by a few blocks, so poll until it reaches the receipt's block number. Use the same RPC endpoint for writes and immediate reads, and avoid long caching for balances.
12
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.
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.
Fiat-backed stablecoins hold cash and short-term government bonds off-chain, one dollar per token, and depend on the issuer's audits and redemptions. Crypto-collateralised stablecoins are minted against overcollateralised crypto in smart contracts, such as vaults requiring 150 percent collateral. Algorithmic stablecoins try to hold the peg with supply rules and incentives; several have collapsed, the most famous in 2022. Choose based on trust model, liquidity, transparency and regulatory status.
15
A sharp market fall causes cascading liquidations in your lending protocol and bad debt. How do you design against it?
Set conservative loan-to-value ratios and liquidation thresholds per asset based on volatility and on-chain liquidity. Use supply and borrow caps for illiquid tokens, isolation mode for risky assets, and a liquidation bonus that attracts liquidators without over-penalising users. Allow partial liquidations so positions shrink gradually. Keep reliable oracles, an insurance or reserve fund to absorb bad debt, and run stress tests simulating 30 to 50 percent price drops.
16
A user is angry that withdrawing from an optimistic rollup to Ethereum takes seven days. How do you explain and solve it?
Explain that the seven-day window lets anyone challenge a fraudulent batch; it protects their funds. For faster exits, offer third-party fast bridges or intent-based bridges where a liquidity provider pays them on L1 within minutes for a small fee and later claims the slow withdrawal. Also show the expected timeline clearly in the UI and send a notification when funds are claimable, because most anger comes from uncertainty, not the delay itself.
17
What are value types and reference types in Solidity, and what are data locations?
Value types such as uint, int, bool, address, bytes32 and enums are copied when assigned or passed. Reference types such as arrays, structs, strings and mappings point to data, so you must state a data location: storage for persistent contract state, memory for temporary data inside a function, or calldata for read-only external inputs. Assigning storage to a local storage pointer modifies state, while copying to memory creates an independent copy.
18
Explain Byzantine Fault Tolerance and the 3f+1 rule.
Byzantine Fault Tolerance is the ability of a system to reach agreement even when some nodes lie or act arbitrarily. Classical BFT protocols such as PBFT, Tendermint and QBFT need at least 3f+1 nodes to tolerate f Byzantine nodes, because a quorum of 2f+1 votes guarantees that any two quorums overlap in at least one honest node. They give instant deterministic finality but involve heavy message passing, so they work best with tens to low hundreds of validators.
19
A transaction keeps failing with an out of gas error. How do you fix it?
First see whether the gas limit was too low or the function is genuinely unbounded. Run eth_estimateGas or simulate with cast run or Tenderly to see actual gas used and where it fails. If estimation is fine, raise the limit with a buffer of 10 to 20 percent. If the function loops over a growing array, redesign it with pagination or pull-based patterns, because the block gas limit will eventually make it impossible to call.
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.
21
Why do you want to work in DeFi, given its risks and volatility?
I am drawn to DeFi because it turns financial products into open, auditable code, and that combination of finance and engineering motivates me. I understand the risks: hacks, market crashes and regulatory change. That is why I have focused on security, testing and risk thinking, not just writing contracts. I see volatility as a reason to build carefully. I want to work on a team that values audits, monitoring and user protection.
22
What are storage collision and uninitialised implementation risks in proxies?
With delegatecall, the implementation's code writes to the proxy's storage. If the proxy stores its admin or implementation address in a normal slot, implementation variables can overwrite it, so EIP-1967 uses pseudo-random slots. New implementation versions must only append variables, or use namespaced storage (ERC-7201). Implementations must also disable initialisers in their constructor; otherwise an attacker can initialise the implementation directly, take ownership and, in UUPS, potentially destroy or hijack it.
Events are declared in Solidity and emitted with emit; the EVM writes them as logs in the transaction receipt. Each log has the contract address, up to three indexed topics plus the event signature hash, and non-indexed data. Logs are much cheaper than storage but contracts cannot read them. Front-ends and indexers subscribe to events to update the UI, track token transfers and build history, so emitting clear events for every state change is good practice.
24
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.
25
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.
26
Calculate the output of a swap in a constant product pool with a 0.3 percent fee.
Formula: amountOut = (amountIn x 997 x reserveOut) / (reserveIn x 1000 + amountIn x 997).
Example: reserves 1,000 ETH and 3,000,000 USDC, swap 10 ETH.
amountInWithFee = 10 x 997 = 9,970
numerator = 9,970 x 3,000,000 = 29,910,000,000
denominator = 1,000 x 1000 + 9,970 = 1,009,970
amountOut = 29,615 USDC (approx)
The spot price is 3,000, but the trader gets about 2,961.5 per ETH because of fee and price impact.
27
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.
28
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.
29
How do you calculate a borrower's health factor in a lending protocol?
Health factor = (sum of collateral value x liquidation threshold) / total debt value. Below 1, the position can be liquidated.
Example: collateral 10 ETH at 3,000 dollars = 30,000 dollars, liquidation threshold 80 percent, debt 18,000 dollars of stablecoins.
HF = (30,000 x 0.80) / 18,000 = 1.33.
If ETH falls to 2,250, collateral is 22,500, so HF = 18,000 / 18,000 = 1.0 and liquidation begins. Users raise HF by repaying debt or adding collateral.
30
How do you handle unclear or changing requirements from a client?
Situation: a client asked for a marketplace dApp but kept changing royalty and fee rules. Task: deliver without endless rework. Action: I wrote a short spec with examples and edge cases, got sign-off, and made fees configurable parameters with limits instead of hard-coded logic. Changes went through a simple change log with impact estimates. Result: we delivered in eight weeks, and later fee changes needed only a governance transaction, not a redeployment.
Tips to crack the Deloitte Blockchain interview
Practise guesstimates and simple business cases, since consulting interviews often test structured thinking alongside technical skills.
Be ready to explain dashboards or analyses you built in Excel, SQL, Power BI or Tableau and the business decisions they supported.
Read about Deloitte's service lines such as Risk Advisory and Consulting so you can say which practice suits you.
Keep a one-minute story ready for every project on your CV: problem, your part, tools, result.
Practise the hard questions on this page aloud; Deloitte interviewers usually ask follow-ups on your first answer.
Frequently asked questions
How many rounds are there in the Deloitte Blockchain interview?
Candidates usually go through 4 stages: Online assessment (aptitude + technical); Group discussion or case discussion; Technical interview; Partner or HR round. The exact rounds change with the role, team, location and your experience, so confirm the process with the recruiter.
Which topics should I prepare for the Deloitte Blockchain interview?
This practice set leans on Ethereum and the EVM (6), DeFi, Tokens and NFTs (6), Solidity Programming (5), Blockchain Fundamentals (4). Revise those first, then practise explaining one or two of your own projects end to end, because most Deloitte interviewers ask you to walk through real work.
Are these the exact questions asked at Deloitte?
No. These are curated practice questions that match the skills and difficulty usually tested in consulting interviews like Deloitte's. They are not a leaked or verified list of past questions.
Where does Deloitte hire Blockchain professionals in India?
Deloitte has large teams in Hyderabad, Bengaluru, Mumbai, Gurugram, Pune. Openings and locations change often, so check the Deloitte careers page and filter by city and experience.