Nishant

Retail banking REST API: accounts, deposits, withdrawals and transfers.

• Locked every account a request touches with SELECT … FOR UPDATE in id order, so racing transfers queue instead of losing updates or deadlocking; a test fires 10 parallel withdrawals of 30.00 at 100.00 and asserts exactly 3 succeed. code test
• Built idempotent payments: an Idempotency-Key replays the original result on retry, a reused key with a new body returns 422, and a unique index settles racing duplicates. code
• Money as BigInt minor units in a double-entry ledger whose credits minus debits must equal total balances, with CHECK constraints as a last line of defence. code code
• Refresh tokens stored only as SHA-256 hashes; replaying a rotated token revokes the whole token family. code

Reconciles bank statements against ledger records without silently signing off on differences.

• Matching that refuses to guess: tied or conflicting candidates stay unresolved for a human; money is exact integer minor units per currency. code
• Independent review (a run's creator cannot approve it) and a hash-linked audit log with chain verification, missing-tail detection and SQL immutability triggers; 150+ tests in CI. code code

Evidence-driven incident response API.

• Closing an incident needs two non-creator approvals of the current SHA-256 evidence manifest; new evidence makes earlier approvals stale. code code
• Optimistic concurrency: each write claims the next revision and a conflicting writer's whole batch rolls back; SQL triggers keep audit events append-only. code code

Local-first dataset contracts and drift reports for CSV, JSONL and Parquet.

• Drift checks report “incomparable” instead of a false “stable” when the data cannot support a verdict; bounded ingestion, fingerprinted reports, 100+ tests. code

Hospital operations backend with payments and role-based access.

• Razorpay payments verified by HMAC-SHA256 signature; TOTP two-factor auth, a Redis token blacklist and role-based access. code