Right now I'm learning a bit of domain knowledge about how Payment Gateways like Stripe work to not fuckup when I publish System Design Solution here. SO:

Terminology:
Stripe - payment gateway
Merchant - the guy selling goods or services
Customer - the guy buying goods or services from Merchant

Issuing / customer's bank - the bank that issued card to customer, that he pays with
Merchant's bank - the bank where merchant receives money when customer pays for it

Don't worry, by the end of the day the answers to the poll will be revealed :)
But that is interesting how people's opinions diverge

Here the correct answer is Card Network, e.g. Visa or Mastercard, this is what first would be called. Cause Stripe might not have integration with customer's bank + it has only card details, it does not have an bank account. Whenever you charge from card you pretty much always work through the Card Network.

You can imagine Mastercard or Visa as a bridge that transfers money from customer's account (card holder) to Stripe's account

The money is transferred to Stripe's bank account (it has multiple in different regions). If it would directly transfer to merchant - how would it collect fee/interest then?

Daily

Mid, but it is close to hard I would say for building an intuition

https://leetcode.com/problems/minimum-operations-to-convert-all-elements-to-zero/description/?envType=daily-question&envId=2025-11-10

#daily #medium

Gmail, seems to put some sort of "agent" inside. It detected that I mentioned "attached", then detected that nothing is attached and warned me with alert()

I would understand if it was Outlook, but GMAIL, I want to believe they didn't do it like that xD

Daily

Eventually, good old DP problem, love it.

https://leetcode.com/problems/ones-and-zeroes/description/?envType=daily-question&envId=2025-11-11

#daily #medium

Around 5-6 years ago when I started learning "Clean Code", so finished "Code Complete" by Steve McConnell, then "Clean Architecture" by uncle Bob, then bunch of Domain Drive Design books, I was under impression that this is IT, I know how to write code and keep it simple and clean.

But the experience always was the following

Question before sleep for System Design. We can afford to be eventually consistent, but, we cannot afford data loss and want to be highly available.

Answer to the above's will be soon

Daily

Not that bad, but a bit bullshit. It takes a bit long to build an intuition (I will leave hint for that in comments)

https://leetcode.com/problems/minimum-number-of-operations-to-make-all-array-elements-equal-to-1/description/?envType=daily-question&envId=2025-11-12

#daily #medium

So, there is only 1 correct answer for all the cases:

✅ Consensus(raft, paxos), it will guarantee everything.

❌ Async leader follower - might loose data on leader election
❌ Sync leader follower - does not give high availability, as any follower crash will make system unresponsive, and not able to process writes

✅❌ Leader Leader - depends, if operations are not concurrent we will not loose the data and guarantee high availability, once we write to any leader it will be synced. Concurrency might bring data loss only when Last Write Wins conflict resolution.
✅❌ Leaderless - same as leaderfull, if no concurrent operations - we will not have any conflict with w + r > n, but with concurrent operation, again on read repair when there is a conflict we might override some writes, that got accepted by quorum.

Do you remember this epic game? I need to play it NOW!

Pls, give me feedback about such polls. Did you like it, was it not useful at all, was it too hard, ambiguous? Should I continue using them, or better just posting the "results" in the way: "these are replications that guarantee no data loss"