What to Expect in the Udaan System Design Interview

Expect a machine coding round first, and a design discussion after it. Udaan's technical loop is unusual among Indian product companies, because candidates report no separate algorithm round for experienced hires. The reported design questions include Twitter with trending topics, and interviewers also ask you to defend design decisions from your own past work, while database questions appear as well, including relational schema design with optimization.

The Machine Coding Round

Prepare for this round first, because candidates report it as the main filter. A machine coding round is a timed session where you build a small working program and are graded on the code that runs. It differs from a system design round, which is graded on the discussion, and the difference is explained in system design versus low level design.

Candidates report about 90 minutes to build and about 30 minutes to explain the result, although longer formats are reported too, with about 30 minutes for requirements before the build starts. Reported problems include a news application built on a public news API, a deck of cards game, a parking lot system, a carpooling service, and a fitness booking service with two APIs. Candidates report sending the code and a README file as a zip archive.

Reviewers check three things: the program must compile and run, every stated feature must work end to end, and the classes must be separated well enough that a new requirement needs new code, not rewritten code. The general format is described in what a machine coding round looks like.

The Design Round: Reported Questions

The reported design questions are standard interview systems rather than Udaan specific ones. Candidates name Twitter with trending topics, while others report a long discussion of the design decisions in their own past projects. Campus candidates report a case study round instead, often about a ride hailing or food delivery product, with the interviewer adding new requirements as the discussion continues.

Udaan's Own Design Problems

Study the company's product, because the follow up questions come from it. Udaan runs a catalog across categories with different rules, including fresh produce, pharma, staples and electronics. It handles bulk orders from business buyers, splits them across sellers, and delivers through its own supply chain, and it also extends working capital to retailers through udaanCapital.

These give the natural topics: catalog and inventory per seller, order splitting, delivery routing, returns, and credit limits.

A Walkthrough: A B2B Order and Fulfillment Flow

Candidates do not report this exact question, but it is built from Udaan's own product, which makes it the most useful practice question available.

1. Requirements, about five minutes. Buyers are shops rather than consumers, sellers list stock per region, and one order can be large and can contain items from several sellers.

2. Catalog and inventory. Give each product one identity, then attach a price and a stock count per seller and per region. Browse and search read from a copy optimized for reads, because catalog reads far outnumber writes.

3. Order placement. Split one buyer cart into one order per seller, and check payment or credit before confirming. Make the place order call idempotent, which means that a repeated call with the same request identifier creates no second order.

4. Fulfillment. Model pick, pack, dispatch and delivery as status events on a queue, so each consumer reads them at its own speed. Handle rejection at delivery, which is common in bulk trade, and return the stock to inventory.

5. Credit. If an order can be funded, the order service must ask a limit service for a decision and record the answer. Never let two concurrent orders each consume the same remaining limit.

6. Scale and failure. Traffic peaks during price drops and festival demand, so cache catalog reads, and degrade browse before checkout. Keep seller tools on separate capacity, so that buyer traffic and seller traffic do not share the same machines.

What the Interviewer Grades

State the requirements before drawing anything, then name each trade-off out loud, and give the reason for the choice. Show the data model, because schema questions are reported in this round, and explain what happens when a part of the system fails.

Connect the design to money: a lost order or a wrong credit decision has a real cost here.

Common Mistakes in This Round

• Treating machine coding as an algorithm round. Optimizing a loop while half the features are missing fails the round.
• Code that does not compile. A partly complete program that runs scores better than a complete program that does not.
• One large class. Extensibility is graded directly, so separate responsibilities from the start.
• Skipping the requirements step. Ask your questions in the first minutes, not after you have built the wrong thing.
• Designing for consumers. A B2B buyer orders in bulk, on credit, on a schedule, and a consumer checkout design misses all three.

How to Prepare

• Drill class design under time pressure. Grokking the Object Oriented Design Interview covers the problems this round reuses.
• Learn the building blocks. Grokking the System Design Interview covers the queues, caches and databases an order flow needs.
• Rehearse the walkthrough. Practice the six steps above out loud in under 40 minutes.
• Know the rest of the loop. The design round is one step of the Udaan interview process, and the motivation question is covered in how to answer why Udaan.
• Plan for the wait after it. How long it takes to hear back gives the reported wait after the machine coding submission.