20 buyers. 5 units. One second.
Same product, same moment. How many orders does the shop accept?
GridUnitsOversold
- Orders
- 0
- Stock
- 5
From ecommerce-backend; an integration test on every commit.
Backend & Android engineer
Darshan Sonawane
I build backends that stay correct when twenty people click at once, and Android apps that handle money the way a payments team would.
- Tests across repos
- 0
- Oversold units
- 0
- Double charges
- 0
- Best line coverage
- 0
(01) — Principle
Pick the one guarantee that matters. Write the test that would catch its failure. Build until it's green. Keep it green.
No oversold unit. No double charge. No unbalanced journal. The numbers on this page are copied from each repository's own README.
(02) — Work
Eight repositories, ranked.
Position by line coverage. Purple is the session best. Open a row for the design decisions.
(03) — Race control
Three incidents. Try to cause them.
Working models of real failures from the repositories. Break one, then switch on the fix. The simulation logic is unit-tested in this site's repo.
Pay ₹499. Lose the radio twice.
The server charges you, then your 4G drops before "Paid" arrives. The app retries. Twice.
From ecommerce-backend and PayFlow.
The ledger can't be edited.
Double-entry and append-only. Try to edit a row. Try to approve your own big transfer. Try to post a journal that doesn't balance.
| Journal | Account | Debit | Credit | Actions |
|---|---|---|---|---|
| Trial balance | ||||
From core-banking-ledger; Postgres triggers reject UPDATE and DELETE on the ledger.
(04) — Driver
I learn by building the hard version of the thing.
The ledger instead of the to-do app. The race test instead of the screenshot. The state machine with all 49 transitions tested instead of a happy path. What I can show is systems that hold up when you push on them, and the tests that prove it.
- Backend
- Java 21, Spring Boot 4, Spring Security, Spring Batch, JPA/Hibernate, Flyway, PostgreSQL, MySQL, Redis, Kafka
- Android
- Kotlin, Jetpack Compose, MVVM, Hilt, Room, WorkManager, CameraX, ML Kit, Keystore, Firebase, Flutter
- Testing
- JUnit 5, Mockito, Testcontainers, Awaitility, ArchUnit, JaCoCo and Kover gates, golden tests, load and chaos simulations
- Delivery
- Docker multi-stage, docker compose, GitHub Actions, JitPack, Actuator, Micrometer/Prometheus
(05) — Contact
Two ways to work together.
A — Freelance
Well-scoped Spring Boot backends or Kotlin/Compose Android apps, with tests, Docker and CI from day one. Also: fixing concurrency, double-charge and data-integrity bugs in systems you already run. You get a repo you own, a README anyone can run, and a short walkthrough video.
B — Full-time
Backend or Android SDE roles on teams that care how systems behave under load. Comfortable owning a service end to end: schema, API, tests, deploy. Resume on request; the repositories above are the real portfolio.