I asked this system design question to 3 guys during a developer interview and none of them gave the answer

6 min readMar 24, 2023

A Common system design question.

The Question

Weeks ago, a candidate came in and we started the interview (senior dev, 3rd round) process. Since he has internet company working experience, a famous ecoms company.

The first question I asked him “what do you do in your team? tell me the details of the current project you are working on, as detailed as possible”.

TBH, This is a pretty standard question. I was expecting a solid very prepared answer.

He replied “Sure our team is dealing with payment, integration with payment gateways, and sync invoices with banks. and since our architecture is microservice, our team is working on payment service. to provide payment service to other teams”.

Ok good.

Then I asked “Since your system is dealing with payment gateway, have you ever faced any issues? like payment went through in the gateway but however failed on your side? or found double invoiced in the bank ?”

He replied, “Well, this sometimes happens, when It happened, we manually fix it”.

“How do you manually fix”? I asked again.

“Trigger some script to sync the data or status,” he said.

“Do you mean manually roll back the transaction either your side or the gateway or any…




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