System Design Interview: Design Amazon Kindle Payments

Karan Pratap Singh (Senior Software Engineer, Curebase) talks about how to design a system for the Amazon Kindle shop to purchase books.

Chapters -
00:00 - Introduction
00:48 - Question
01:14 - Requirements
10:16 - Design
18:06 - API
26:30 - Follow-up questions
32:22 - Interview Analysis
32:47 - Tips

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[TheKshitijagrawal]

This is probably not a great example. It starts with listing consistency in functional requirements, then does not highlight the basic requirements properly (eg what all payment channels), not asking about retention period for payment data .. .Similarly in non-functional security, durability, observability etc could have been added. Specially security is key NFR for payments. Estimate calculations are mostly wrong. Not covering any payment failure scenarios and areas like refunds, reconciliation of payment etc. There is no proper reason given for DB sharding but to me sharding is more about understanding query patterns. For example if sharding is on payment id hash then query to list out all payments by given user will lead to query across shards.

[Rhy-vddg]

The thing these interviews always get wrong is starting with functional requirements. Start with identifying your user first, and base the functional requirements around the user’s use cases

[ravipradeep]

This is purely for a SDE1/SDE2 at FAANG, i have been working as a tech lead at microsoft and taken multiple interviews.
There are significant gaps which shows immaturity .
Starting from structure to the technical details .

[grovervishal]

10 million active users. and all of them are buying 2 books every day. I would say either that is a mistake or his readers just read books all the time !!

[ayushigupta]

This is great, thanks for explaining it so well in such an easy manner!

[hegdevishwa]

if the requirement was there are billions of records, for the database can we use something like MongoDB that provides consistency and portion tolerance?
I think even the availability isn't as bad as RDBMS where if the primary node fails it brings up the replica set. would that be a fair assumption? MongoDB4 also supports transactions.

[piyushjaiswal]

Is it necessary to go into ISO details about currency for someone who never worked on payment system but get this question in the interview?

[codingmonkey]

I think there are several critical component is missing in this design:
1. Given we are design payment system for Kindle, a book merchant website, then we probably should have the abstraction of the orders. So that when there is payment failure, user could retry explicitly later;
2. If the payment service fail to make the payment with PSP, then there probably should have some retry mechanism; or even we don't provide auto retry, then we still need to prevent several concurrent request to PSP happens; which implies that some mechanism to ensure the idempotent property needs to be provided
3. Actually I'm not sure how distributed transaction going to help in the current design even we try to use eventual consistent database such as Cassandra.

[piyushjaiswal]

'What we will be storing in DB?' I don't know what's there with 500KB. They just skipped and went forward with it.

[antonignatenko]

Consistency refers to non-functional requirements, as it's about how the system acts rather than what it does
and also no use cases identified by the speaker.
Who he designs this system for and what they will do, so that the system, is prepared to handle it.

[sdasarathan]

Does consistency goes in Funtional Requirement ?

[emanik]

Which software did you use for whiteboard?

[nlke]

500 bytes per tansaction * 20 million Transactions. Comes out to 10GB a day I think or am I missing something? I don't understand where 2e8 came from either.

[mick]

may I know what's the drawing app you're using ? very handy

[chaowang]

I think before you dive into what type of database is prefered, you should probably demonstrate what kind of tables, columns you need.

[arunraj]

The estimation is wrong => 20M/56400 =231 not 600

[sdasarathan]

Since this is a payment sytem, is nt it good to use RDBMS?

[grovervishal]

I think the API section should have the first API where you check with the Bank the User is legitimate and the card he is using is not hotlisted and has credit available to proceed with the txn. Only then you can go and actually do the txn

[deepeshnitk]

adding consistency under functional requirements. Is that right ?

[valdecigs]

This guy demonstrates limited experience, although they have studied some topics. He would be suitable for a Junior or Mid-level Engineer role, but not for a Senior position based on the experience shown. It's surprising and quite telling that he became a Senior Software Engineer after only two years of experience.

He is currently a Cloud Engineer at Apple, but they wouldn't qualify for a Senior Software Development Engineer role at Amazon, for example.