Scalability and Reliability Interview Questions

Questions 201–353 of 353

201. How would you handle a server crash during a chunked file upload?

     Asked once · last asked Sep 8 · Scenario · Senior

202. How would you scale an API that jumps from 500 to 5,000 RPS?

     Asked once · last asked Sep 8 · Design · Senior

203. How would you make sure the generated short code is unique across multiple callers or services?

     Asked once · last asked Sep 8 · Design · Senior

204. How do you generate unique short URLs and keep the service scalable and highly available?

     Asked once · last asked Sep 8 · Design · Senior

205. How do you handle millions of requests per second, especially on the create and get paths?

     Asked once · last asked Sep 8 · Design · Senior

206. As a service grows and receives more traffic, what steps would you take to keep it scalable?

     Asked once · last asked Sep 9 · Design · Mid-level

207. How would you deploy an application behind a load balancer without downtime?

     Asked once · last asked Sep 10 · Design · Senior

208. How would you optimize a large-list API if 1,000 users accessed it at the same time?

     Asked once · last asked Sep 10 · Design · Senior

209. Besides rate limiting, how would you manage high demand for a large-list API while still allowing users to query it?

     Asked once · last asked Sep 10 · Design · Senior

210. How do you guarantee message ordering and idempotent deduplication across retries when publishing from the outbox?

     Asked once · last asked Sep 11 · Design · Senior

211. When testing a nondeterministic system, what oracle would you use to verify correctness, including no double-spend and exactly-once effects under varied event ordering and retries?

     Asked once · last asked Sep 11 · Scenario · Senior

212. How would you precisely specify the correctness requirements for the outbox processor and test requirements around ordering, delivery guarantees, deduplication keys, and retry backoff?

     Asked once · last asked Sep 16 · Scenario · Senior

213. How would you detect and recover from outbox failure modes, like DB commit succeeding but broker publish failing, or broker publish succeeding but DB update failing, without duplicates or reordering?

     Asked once · last asked Sep 16 · Scenario · Senior

214. How should the system handle an alert that is not delivered?

     Asked once · last asked Sep 17 · Scenario · Mid-level

215. How would you handle a 10x traffic increase in a full-stack application?

     Asked once · last asked Sep 17 · Scenario · Senior

216. How would you prevent data loss and duplicate processing when delivering messages through Azure Service Bus?

     Asked once · last asked Sep 18 · Scenario · Senior

217. How would you handle interruptions and connection loss in a real-time voice chat to maintain a good user experience?

     Asked once · last asked Sep 18 · Scenario · Senior

218. How would you reduce the impact of blocking external HTTP calls without a full architectural redesign?

     Asked once · last asked Sep 18 · Scenario · Senior

219. Why can endlessly increasing retries during failures of external services make the problem worse?

     Asked once · last asked Sep 18 · Concept · Mid-level

220. How would you prepare a single-instance ASP.NET Core API for horizontal scaling in the cloud?

     Asked once · last asked Sep 18 · Design · Senior

221. How would you handle duplicate payment webhooks or messages to prevent crediting tokens twice?

     Asked once · last asked Sep 18 · Scenario · Senior

222. Where should duplicate detection happen when processing payment webhooks?

     Asked once · last asked Sep 18 · Design · Senior

223. How would you design a Salesforce integration that needs to handle a large and unpredictable spike in API traffic?

     Asked once · last asked Sep 18 · Design · Senior

224. If an external API call succeeds but the subsequent Salesforce database update fails, how would you detect and recover from this inconsistent state?

     Asked once · last asked Sep 18 · Scenario · Senior

225. When a system is stable, how do you choose its next scaling goal: higher throughput, lower latency, or another objective?

     Asked once · last asked Sep 21 · Scenario · Senior

226. If your application had 10 million users and received 10,000 requests per second, how would you handle the load?

     Asked once · last asked Sep 21 · Design · Senior

227. How would you trace an end-to-end request through a microservices system, from the user's action through service calls and data reads and writes?

     Asked once · last asked Sep 22 · Design · Mid-level

228. How should you choose a TTL for an idempotency key stored in Redis?

     Asked once · last asked Sep 22 · Scenario · Senior

229. How would you adjust retry behavior when retries are increasing request latency?

     Asked once · last asked Sep 22 · Scenario · Mid-level

230. What should happen if a consumer crashes while processing a message?

     Asked once · last asked Sep 23 · Scenario · Senior

231. How would you keep a containerized web application scalable and optimize its performance?

     Asked once · last asked Sep 23 · Scenario · Senior

232. How would you scale a production application under heavier loads and ensure high availability?

     Asked once · last asked Sep 23 · Design · Senior

233. What techniques do you use to optimize the performance of a large-scale application?

     Asked once · last asked Sep 23 · Design · Senior

234. How would you build an early warning system that detects service problems before users report them, and which services would you use?

     Asked once · last asked Sep 24 · Design · Senior

235. How do you safely retry failed external API calls?

     Asked once · last asked Sep 25 · Scenario · Senior

236. How would you safely retry a transaction-creation request after a timeout?

     Asked once · last asked Sep 25 · Scenario · Senior

237. How would you handle transient errors when creating a transaction?

     Asked once · last asked Sep 25 · Scenario · Senior

238. How would you handle a transaction-creation request if the remote system times out?

     Asked once · last asked Sep 25 · Scenario · Senior

239. How would you prevent duplicate transactions and recover safely after a timeout?

     Asked once · last asked Sep 25 · Scenario · Senior

240. What would you do if an external API does not support an idempotency key and a request times out?

     Asked once · last asked Sep 25 · Scenario · Senior

241. How can a diagnostic system automatically identify issues from telemetry, and what is the minimum input it needs?

     Asked once · last asked Sep 25 · Design · Senior

242. How would you handle skewed issue volumes and alert floods in an automated diagnosis system?

     Asked once · last asked Sep 25 · Scenario · Senior

243. How would you deduplicate recurring issues reported across many robots and prevent duplicate tickets?

     Asked once · last asked Sep 25 · Design · Senior

244. If you were testing a webhook endpoint that sometimes returns 200 but silently drops events, what tests and logs would you require to prove that end-to-end processing happened exactly once?

     Asked once · last asked Sep 28 · Scenario · Senior

245. How would you implement idempotency and retry handling in an integration?

     Asked once · last asked Sep 29 · Scenario · Senior

246. When creating multiple events, how can you prevent sending duplicate notification emails to the organizer?

     Asked once · last asked Sep 29 · Scenario · Mid-level

247. Why would you choose HAProxy instead of native AWS load balancers?

     Asked once · last asked Sep 29 · Concept · Senior

     Other ways it was asked

     • “Why do you use HAProxy instead of native AWS load balancers?”

248. How would you handle a survey submission being sent twice because a student has a weak signal?

     Asked once · last asked Sep 30 · Scenario · Mid-level

249. How would you preserve a student's survey progress if they disconnect and then reconnect?

     Asked once · last asked Sep 30 · Scenario · Mid-level

250. How would you handle two identical requests arriving simultaneously before either request has finished?

     Asked once · last asked Sep 30 · Scenario · Senior

251. How would you store and use a unique key to prevent duplicate payment requests across tabs and devices?

     Asked once · last asked Oct 1 · Design · Senior

252. If some leads receive a broadcast message twice, how would you debug the issue and find its root cause?

     Asked once · last asked Oct 1 · Scenario · Senior

253. An export of 1,000,000 leads crashes halfway through. How would you diagnose the failure and make the export recoverable?

     Asked once · last asked Oct 1 · Scenario · Senior

254. How would you achieve the required performance in a real-time system?

     Asked once · last asked Oct 2 · Scenario · Senior

255. How would you design a background job, such as an inventory update or settlement job, to remain idempotent if it retries before the database update and completion status are recorded?

     Asked once · last asked Oct 5 · Design · Senior

256. How can you prevent or mitigate cascading failures in a distributed system?

     Asked once · last asked Oct 6 · Scenario · Senior

257. How would you scale a web application to approximately one million users using access tokens, refresh tokens, Redis, local storage, and horizontal scaling?

     Asked once · last asked Oct 7 · Design · Mid-level

258. How would you handle a slow or failed API request in a frontend application?

     Asked once · last asked Oct 8 · Scenario · Mid-level

259. How would you handle a complex project with multiple interacting components and high concurrency?

     Asked once · last asked Oct 8 · Behavioral · Senior

     Other ways it was asked

     • “How have you handled complex projects with multiple interacting components and high concurrency?”

260. How would you handle a microservice that depends on another service that is down?

     Asked once · last asked Oct 9 · Scenario · Senior

261. What should happen when a customer places an order while a downstream microservice is down?

     Asked once · last asked Oct 9 · Scenario · Senior

262. What should you consider when horizontally scaling a message consumer that processes messages, uses a database, and calls an external API?

     Asked once · last asked Oct 9 · Scenario · Senior

263. What would be your step-by-step diagnostic first, and then the remedy plan across the database, application, frontend, and network layers to resolve a bottleneck?

     Asked once · last asked Oct 10 · Scenario · Senior

264. If a webhook provider can deliver events out of order, how would you design your processing so the final state is still correct and you don't drop legitimate updates?

     Asked once · last asked Oct 13 · Design · Senior

265. How do you load-balance traffic across multiple production front-end instances?

     Asked once · last asked Oct 13 · Design · Mid-level

266. How do you design a system for high availability?

     Asked once · last asked Oct 13 · Design · Mid-level

267. How would you optimize the app to handle 1,000,000 users on the front end without server-side pagination?

     Asked once · last asked Oct 14 · Design · Senior

268. How do you ensure an application remains available after deployment?

     Asked once · last asked Oct 14 · Design · Mid-level

269. How would you prevent a transaction from being processed twice when a client retries a request?

     Asked once · last asked Oct 14 · Scenario · Senior

270. What could happen to a system if its fraud-detection service slows from 200 milliseconds to 10 seconds, and how would you prevent the system from degrading?

     Asked once · last asked Oct 14 · Scenario · Senior

271. How would you prevent system degradation if an external provider's response time increased from 200 milliseconds to 10 or 15 seconds?

     Asked once · last asked Oct 14 · Scenario · Senior

272. When an external service becomes slow, how can that cause a user-facing error, and what might the user see?

     Asked once · last asked Oct 14 · Scenario · Mid-level

273. What HTTP status code or response information does a client receive when a load balancer terminates a request due to a timeout?

     Asked once · last asked Oct 14 · Concept · Mid-level

274. An external provider sends a transaction for 50,000 pesos, but a system error causes the same transaction to arrive again. How would you prevent the customer from receiving 100,000 pesos?

     Asked once · last asked Oct 15 · Scenario · Senior

275. When an external provider sends the same transaction more than once, how would you prevent a customer from being credited twice?

     Asked once · last asked Oct 15 · Scenario · Senior

276. How would you handle a late-arriving event in a payment-processing workflow?

     Asked once · last asked Oct 15 · Scenario · Senior

277. If a workload's CPU remains high after a release because it genuinely needs more capacity, how would you address it?

     Asked once · last asked Oct 15 · Scenario · Mid-level

278. What are the usual issues during auto-scaling or sudden traffic, how do they manifest, and how do you determine whether the cause is the application or the scaling infrastructure?

     Asked once · last asked Oct 15 · Scenario · Senior

279. A banking core sends a payment order via a REST POST request, but the request times out without a response. Can the POST be retried, and what determines whether doing so is safe?

     Asked once · last asked Oct 15 · Scenario · Senior

280. A message instructs a banking core to credit 10,000, but the server crashes after updating the account and before acknowledging the message. The message is redelivered after restart. How should the application handle it?

     Asked once · last asked Oct 15 · Scenario · Senior

281. An MQ consumer receives transfer events in this order: authorized, settled, then in progress. What should the final state be, and how would you handle the late event?

     Asked once · last asked Oct 15 · Scenario · Senior

282. How would you implement a retry mechanism with exponential backoff for a message processing system?

     Asked once · last asked Oct 15 · Scenario · Senior

283. What technical challenges can arise in an integration, such as idempotency, retries, consistency, or timeouts, and how would you design for them?

     Asked once · last asked Oct 17 · Scenario · Senior

284. How would you implement idempotency across steps in an integration? What key would you use, where would you store it, and how would you handle concurrent duplicate requests?

     Asked once · last asked Oct 17 · Design · Senior

285. How would you systematically generate gateway scenarios involving timeouts, partial failures, and retries? Would you use a model, state machine, or property-based approach, and what properties would you enforce?

     Asked once · last asked Oct 17 · Scenario · Senior

286. How would you redesign session management when sessions are stored in memory and servers are restarted during updates?

     Asked once · last asked Oct 19 · Design · Senior

287. How would you store network logs for a backend system?

     Asked once · last asked Oct 19 · Scenario · Mid-level

288. How would you approach load balancing and event-bus design for this application?

     Asked once · last asked Oct 19 · Design · Senior

289. An external service sends the same payment notification twice. How would you prevent the payment from being recorded or applied twice, even if both notifications arrive simultaneously?

     Asked once · last asked Oct 20 · Scenario · Senior

290. A customer makes a 200-sol payment toward a 500-sol debt, and the payment gateway sends two notifications for the same payment simultaneously. How would you prevent duplicate recording and ensure the debt is updated correctly, even if an error occurs?

     Asked once · last asked Oct 20 · Scenario · Senior

291. What performance strategies would you use to reduce latency when building integrations?

     Asked once · last asked Oct 20 · Scenario · Senior

     Other ways it was asked

     • “What performance strategies have you applied when building integrations to reduce latency?”

292. How would you investigate and resolve a difficult technical issue in a full-stack multiplayer app?

     Asked once · last asked Oct 20 · Behavioral · Mid-level

     Other ways it was asked

     • “What was the hardest technical issue you encountered while building a full-stack multiplayer app, and how did you resolve it?”

293. How would you measure the impact of separating client responsibilities, server-side coordination, and persistence on reliability or latency?

     Asked once · last asked Oct 20 · Design · Mid-level

     Other ways it was asked

     • “What impact did separating client responsibilities, server-side coordination, and persistence have on reliability or latency?”

294. If a web app suddenly receives 10 times more traffic, what are the first two or three architectural changes you would consider to keep it reliable?

     Asked once · last asked Oct 20 · Design · Mid-level

295. How do you document retryability, idempotency, and backoff for errors and retries so engineers do not accidentally double-charge?

     Asked once · last asked Oct 20 · Scenario · Mid-level

296. How do you decide whether an API operation should be idempotent, and what are two concrete techniques you would use to enforce idempotency end to end?

     Asked once · last asked Oct 20 · Design · Mid-level

297. How would you prevent a payment from being recorded twice if a network error occurs or the system sends the amount twice?

     Asked once · last asked Oct 21 · Scenario · Mid-level

298. Explain how you make an API operation idempotent. Give me a real business example where this matters.

     Asked once · last asked Oct 21 · Concept · Mid-level

299. Can you describe a system you worked on that had significant scale, reliability, or performance requirements? How did you ensure you met those requirements?

     Asked once · last asked Oct 21 · Behavioral · Senior

300. How should worker instances scale as a queue grows?

     Asked once · last asked Oct 22 · Design · Mid-level

301. How would you support load balancing within a Spring Boot application before using server-side or cloud-side load balancing?

     Asked once · last asked Oct 22 · Scenario · Senior

302. How would you handle a critical reliability issue, such as webhook retries, idempotency, out-of-order events, or duplicate payments, from detection through resolution?

     Asked once · last asked Oct 22 · Behavioral · Senior

     Other ways it was asked

     • “How did you handle a critical reliability issue, such as webhook retries, idempotency, out-of-order events, or duplicate payments, from detection through resolution?”

303. If a payment transitions quickly from pending to failed to succeeded, how would you preserve ordering and correctness so users don't see stale status from out-of-order events?

     Asked once · last asked Oct 22 · Scenario · Senior

304. If the notification queue is down for thirty minutes, what should your system do to avoid impacting the payment API and how would it recover afterward?

     Asked once · last asked Oct 22 · Scenario · Senior

305. If leadership says you need to cut payments-to-notification delay from minutes to under ten seconds end to end, what exact plan would you drive around metrics, bottlenecks, isolation, changes, and rollout validation?

     Asked once · last asked Oct 22 · Scenario · Senior

306. If a notification provider's rate limits and occasional 30–60 second spikes are the main bottlenecks, what changes would you make to reduce payments-to-notification delay to under ten seconds?

     Asked once · last asked Oct 22 · Scenario · Senior

307. You have a Java service handling millions of requests a day. How would you design it for high availability?

     Asked once · last asked Oct 23 · Design · Senior

308. How would you deploy a high-availability Java service across instances and regions, and what would happen to in-flight requests if an instance failed?

     Asked once · last asked Oct 23 · Design · Senior

309. How would you design a disaster recovery architecture for an application?

     Asked once · last asked Oct 23 · Design · Senior

310. How would you implement an event-driven failover policy?

     Asked once · last asked Oct 23 · Scenario · Senior

311. If you ran a web crawler on a massive website, what would likely break first?

     Asked once · last asked Oct 23 · Design · Senior

312. Why might you choose a distributed queue and a distributed data store for a production web crawler?

     Asked once · last asked Oct 23 · Concept · Senior

313. How would you handle an unreliable legacy mainframe so users still get a proper experience when it is down?

     Asked once · last asked Oct 23 · Scenario · Senior

314. How can a CDN help an ecommerce platform handle high traffic, and how does it work?

     Asked once · last asked Oct 23 · Concept · Mid-level

315. How would you handle payment retries and delayed webhook confirmations in a purchase flow?

     Asked once · last asked Oct 23 · Scenario · Mid-level

316. When designing idempotency for a payment creation API, what key would you choose, where would you store it, and how would you handle requests with the same key concurrently?

     Asked once · last asked Oct 24 · Design · Senior

317. How would you make the event handler idempotent across database writes and external API calls so reprocessing after a crash doesn't duplicate side effects?

     Asked once · last asked Oct 24 · Scenario · Senior

318. How do you handle errors or exceptions in a back-office system?

     Asked once · last asked Jun 19 · Scenario · Mid-level

319. How would you scale a system to handle 2.5 million requests?

     Asked once · last asked Aug 22 · Design · Senior

320. How many servers can be added when scaling horizontally?

     Asked once · last asked Feb 22 · Concept · Junior

321. What can you do to increase memory when scaling vertically?

     Asked once · last asked Feb 22 · Concept · Junior

322. Why might an organization deploy redundant network equipment but only one Riverbed appliance at each site?

     Asked once · last asked Feb 24 · Scenario · Mid-level

323. How should a system respond when one of its components crashes?

     Asked once · last asked Mar 16 · Scenario · Senior

324. How should a payment-card system respond if its card service crashes?

     Asked once · last asked Mar 16 · Scenario · Senior

325. What do you mean by scalability in IoT?

     Asked once · last asked Mar 24 · Concept · Junior

326. How would you use load balancing to handle concurrent user account creation requests?

     Asked once · last asked Apr 9 · Scenario · Mid-level

327. If load balancing were unavailable, how would you use multithreading in an application to handle concurrent users?

     Asked once · last asked Apr 9 · Scenario · Mid-level

328. How would you account for lower merchant usage at midnight when designing an image management system?

     Asked once · last asked Apr 13 · Scenario · Senior

329. What are timeouts, and how should an application handle them?

     Asked once · last asked Apr 13 · Concept · Mid-level

330. What does availability mean for an object storage service, and why is it relevant?

     Asked once · last asked Apr 22 · Concept · Mid-level

331. What response-time target would you set for queries in a cloud application?

     Asked once · last asked Apr 23 · Scenario · Mid-level

332. What does the closed state represent in a circuit breaker?

     Asked once · last asked May 4 · Concept · Junior

333. What is the purpose of the half-open state in a circuit breaker?

     Asked once · last asked May 4 · Concept · Junior

334. Which web server would you choose for a high-load project?

     Asked once · last asked May 12 · Design · Senior

335. How would you design a workflow to resume processing after a manual intervention?

     Asked once · last asked May 21 · Scenario · Senior

336. Can an architecture serving customer profile lookups from data in S3, Glue, and Athena meet a sub-10-millisecond p99 response-time target?

     Asked once · last asked Jun 19 · Design · Senior

337. When scaling an application to 100,000 users, what networking measures would you consider?

     Asked once · last asked Jul 16 · Scenario · Mid-level

338. What role does a load balancer play between a data pipeline and third-party platforms?

     Asked once · last asked Jul 20 · Concept · Mid-level

339. How would you configure retry logic in an application?

     Asked once · last asked Jul 28 · Concept · Mid-level

340. When scaling an application horizontally, what component is needed to distribute traffic across instances?

     Asked once · last asked Aug 5 · Concept · Junior

341. What is the use of a fault-tolerant agent in load balancing?

     Asked once · last asked Dec 4 · Concept · Mid-level

342. What is a fault-tolerant agent?

     Asked once · last asked Dec 4 · Concept · Junior

343. How do you make sure what you do is production ready and highly available?

     Asked once · last asked Aug 20 · Scenario · Senior

344. What is a server cluster?

     Asked once · last asked Aug 21 · Concept · Junior

345. How would you handle a failed processing job?

     Asked once · last asked Sep 3 · Scenario · Mid-level

346. What is vertical scaling?

     Asked once · last asked Sep 4 · Concept · Junior

347. Which failure mode in a database-outbox architecture concerns you most, and why?

     Asked once · last asked Sep 16 · Scenario · Senior

348. How would you handle heavy application load in a cloud architecture?

     Asked once · last asked Sep 17 · Scenario · Senior

349. How would you handle retrying an operation?

     Asked once · last asked Oct 12 · Scenario · Mid-level

350. What does high system availability mean?

     Asked once · last asked Oct 13 · Concept · Junior

351. How would you configure load balancing for a production application?

     Asked once · last asked Oct 13 · Design · Mid-level

352. What is auto scaling?

     Asked once · last asked Jan 24 · Concept · Junior

353. How would you make the event handler idempotent?

     Asked once · last asked Oct 24 · Scenario · Mid-level

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Every question here was asked by a real interviewer in a real job interview that a candidate ran MeetAssist in. After the call, the interviewer's questions are pulled from the transcript. Practice sessions, tests, exams and anything that was not a job interview are dropped.

Questions are rewritten so they make sense on their own: names of people, employers and clients are removed, and speech-to-text mistakes are fixed. Small talk, logistics and questions about one candidate's own résumé are left out. Rephrasings of the same question are merged, and the count is the number of different interviews it came up in.

The interviews come from candidates who use MeetAssist, so they lean towards software, data and AI roles: the shares describe these interviews, not every System Design interview. Answers to the most asked questions are written and checked by an editor; a "common mistake" is shown only when reviews of real answers in these interviews show it, with the count.

942 interviews, 495 candidates, Oct 2025 to Oct 2026. Updated every month with new interviews.