Layer 3 · Go deeper · 6 min
Go deeper
Size it, build it, then make the calls an interviewer will push you on.
Build-up · 5 steps
Watch a report read half a transfer
One report, one transfer, two balances. The only thing that changes is when each read takes its snapshot.
Step 1 of 5
Two balances, ₹1,000 in all
Savings and wallet each hold ₹500. Whatever runs alongside it, a correct report must say ₹1,000.
4 interview moments
You’re the candidate
The interviewer pushes on the hard parts. Pick an approach and see how it holds up.
Deep dive 1 of 4
Interviewer
“A UPI-style wallet debit reads the balance, subtracts ₹200 and writes it back. Two debits can race. What do you do?”
Your move. Pick an approach:
Deep dive 2 of 4
Interviewer
“Month-end reconciliation sums every balance while transfers keep committing. The totals must agree. How do you run it?”
Your move. Pick an approach:
Deep dive 3 of 4
Interviewer
“The hospital needs at least one doctor on call. Asha and Ravi try to sign off at the same moment. What do you do?”
Your move. Pick an approach:
Deep dive 4 of 4
Interviewer
“Tatkal opens. Two bookings each count 71 of 72 confirmed berths in coach S4, then insert one. How do you stop a 73rd?”
Your move. Pick an approach:
Build-up · 5 steps
Write skew, and how SERIALIZABLE stops it
Snapshot isolation stops two writers of the same row. Write skew slips through because each transaction writes a different one.
Step 1 of 5
A rule across two rows
The hospital needs at least one doctor on call. Asha and Ravi both are, each in their own row of the roster.