Design PayPal
PremiumProduct architectureHard
- Double entry ledger
- Idempotency keys
- Authorize and capture
Quick answer
This product architecture interview question asks you to design PayPal. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Design PayPal, a digital wallet where people send money to each other, pay online merchants with a checkout button and withdraw their balance to a bank. The hard parts are moving money exactly once under retries and failures, keeping a ledger that always balances, and handling bank transfers that can fail days later.
What must the system do?
- Send money: A user sends an amount to another user's email or phone, paid from balance first and then a linked card; the recipient sees it at once.
- Merchant checkout: The merchant creates an order, the buyer approves it on a hosted page, and the merchant captures up to the approved amount within 29 days.
How well must it do it?
- Exactly-once money movement: A retried request never moves money twice, and a failure never leaves money half-moved.
- Balanced ledger: Every transaction's ledger entries sum to zero, and every balance can be rebuilt from its entries.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Double entry ledger
- Idempotency keys
- Authorize and capture
- Payment state machine
- Reconciliation
- Hot accounts
How you’ll practise
- Requirements
- Core entities
- API design
- Client and server flow
- Scale and edge cases
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any product architecture design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a product architecture question. You design PayPal and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Hard. It suits engineers preparing for senior interviews, where you are expected to drive the discussion and go deep on trade-offs.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team