What to Expect in the Allegro System Design Interview

Expect about one hour of conceptual design inside Allegro's technical talks stage. Allegro calls this block the algorithm and architecture conversation. Its careers page says you design a distributed system for a business requirement, then discuss the solutions you chose. The qualities it names include logical thinking, collecting requirements, and running systems in production.

Allegro is the largest online marketplace in Poland, so the problems come from marketplace work: offers, search, orders, payments, and parcel delivery. Candidates report a question that asks you to design chat on an e-commerce page.

You will not get an abstract puzzle in this round. You will get a Polish marketplace problem, and your judgment is graded as closely as your diagram.

The Question Types

Offers and search: design the path from a seller publishing an offer to a buyer finding it. Allegro reports more than 30 million searches a day. The difficulty comes from indexing delay, from ranking, and from keeping the offer service and the search index in agreement.

Buyer and seller messaging: design a conversation between a buyer and a seller, attached to one offer or one order. The difficulty comes from message ordering, from delivery when one side is offline, and from abuse by fraudulent accounts.

Orders and payments: Allegro Pay provides payment and lending, so design an order flow where a payment is charged once, even when the same request is retried. That property is called idempotency, which means that repeating one request produces the same single effect.

Delivery and logistics: Allegro One runs parcel lockers and courier pickup across Poland. Allegro reports about 12,000 delivery predictions per second in this area. Design the service that promises a delivery date to a buyer and then keeps that promise accurate.

Event driven communication: Allegro built and open sourced Hermes, a message bus that it reports at more than 300,000 requests per second. A message bus carries events between services, so a sender never waits for a receiver. Expect questions about queues, retries, and events that arrive twice.

What the Interviewer Grades

Allegro names five qualities in its own description of this round. They are logical thinking, the ability to collect system requirements, and knowledge of the technologies you propose. The other two are knowledge of monitoring, launching, scaling, and maintaining applications in production, and a product approach. Allegro explains that last one as understanding for whom and why you solve the problem.

• The requirements, before any drawing: Allegro grades your ability to collect requirements, so agree on the users, the traffic, and the failure cases first.
• Production work, stated without being asked: explain how you would monitor, launch, and scale the system.
• Trade-offs with reasons: give the reason for every choice, and name its cost in latency, money, or complexity.
• The product view: say who uses the system and why the problem matters to them.
• Depth under follow-up questions: candidate reports describe interviewers who care more about your reasoning than about your knowledge of one specific tool.

A Walkthrough: Chat Between Buyers and Sellers

1. Requirements, about five minutes: buyers message sellers about an offer or an order, from both the web and the mobile apps. Messages must arrive in order, an accepted message must survive a restart, and traffic rises sharply on sale days.
2. The data model: one conversation belongs to one buyer, one seller, and one offer, and every message carries a sequence number inside that conversation. Partition the message store by conversation, so one busy seller cannot slow every other conversation.
3. Delivery while online: clients hold a long lived connection to a gateway service, and the gateway subscribes to the conversations the user has open. Write each message to storage first and publish it second, so a crash cannot drop a message you already accepted.
4. Delivery while offline: publish one event per message to the bus, then let separate consumers handle push notifications, email, and the unread counters. Separate consumers also stop a slow email provider from delaying the chat itself.
5. Abuse and safety: fraudulent accounts use marketplace chat, so apply rate limits per account and filter links and contact details according to policy. Keep an audit record, because a dispute between a buyer and a seller needs evidence afterward.
6. Scale and monitoring: shard the gateway by conversation, and cache the recent messages of the conversations that are open. Report delivery latency, dropped connections, and queue backlog, and alert on each one.

Common Mistakes in This Round

• Drawing before agreeing on the requirement. Allegro grades your ability to collect requirements, so agree on the requirement before you draw.
• Leaving out monitoring. Allegro names monitoring and maintaining applications in production, so a design with no way to observe it is incomplete.
• Ignoring security. Marketplace systems handle money and personal data, so mention authorization and audit records before the interviewer asks for them.
• Naming tools instead of giving reasons. The word Kafka is not an answer, so say what a message bus gives you and what it costs you.
• Forgetting the peak days. Polish e-commerce traffic rises sharply around sales, so explain how your system degrades gradually instead of failing all at once.

How to Prepare

• Learn the building blocks before the patterns. Grokking the System Design Interview covers queues, caches, sharding, and replication.
• Study the harder failure cases next. Advanced System Design Interview, Volume II helps with consistency, isolation, and failure handling between services.
• Rehearse the chat walkthrough out loud. Practice the six steps above in under 40 minutes, without notes in front of you.
• See where this round belongs in the process. The design talk is one part of the Allegro interview process, and the stage after it includes the motivation question.
• Know what the waiting looks like afterward. How long it takes to hear back after an Allegro interview covers the reported waits between stages.