PhonePe DevOps System Design & Architecture interview questions

The Indian e-commerce interview question, and autoscaling is the wrong first answer. A 10x spike arriving in seconds outruns any scaling loop, so the design has to absorb rather than scale.

Every candidate says replication and failover. The scoring answer names the replication mode, states the resulting data loss window, and explains how the cluster avoids two primaries.

The Google-style NALSD round in miniature. The boxes are easy; what is being scored is whether the numbers fall out of Little's law before you draw them.

Cron is the most-deployed scheduler on earth and the worst-behaved at scale. The scored content is everything crontab cannot do: exactly-once firing, thundering herds, and what happens when the scheduler itself dies.

CAP is not a menu of two letters per system. The scored answer knows what the theorem claims (narrowly), what most engineers misquote, and what a split-brain costs when both halves accept writes.

Timeouts guarantee uncertainty, not failure: the first request may have succeeded after the client gave up. The idempotency key pattern is the industry answer; the scored details are storage, expiry and concurrency.

The vocabulary question hiding real depth: which primitive replaces ORDER BY, why sharding by player is wrong, and what a 45,000 updates-per-second storm actually costs.

Sharding buys write headroom and bills you in joins, foreign keys, and every future migration. The key choice is a query-pattern bet; the resharding choreography separates operators from diagram artists.

Size stored sessions from expiry and creation rates, then decide how quickly logout must take effect and what failover may forget.

The difficult decisions are preserving offline intent and enforcing rules that span devices. Interviewers want conflict classes named and handled deliberately, not a CRDT framework proposed on instinct.

Listing the match job is table stakes. The scored material is the breaks taxonomy, the suspense account, and why every match must be replayable from immutable raw files.

The classic napkin-math question. The number in the prompt is the question, and candidates who draw the architecture before doing the arithmetic design something the physics forbids.

The design round where the honest answer is mostly about data. Stateless traffic across regions is straightforward; the database is where active-active is either solved or pretended.

The migration question, and the trap is answering with a target architecture. What is being scored is sequencing: how you get there without a rewrite and without a release freeze.

The default answer is 'horizontal' because interviews reward it. The correct answer names the constraint that makes distribution expensive and knows when one big machine is the engineering choice.

Everyone names token bucket. The scored part is the rest: drift between nodes, the 429 contract with clients, and whether your quota service should fail open or closed.

The catalog is the easy half. The scored half is stampede dynamics: one hot key expiring can multiply origin load by a thousand in a second, and there are three distinct fixes.