Perfect.This is where everything you’ve learned finally comes together into a REAL banking-grade design.Today we’ll cover:Ledger-based system design — how banks actually store and protect moneyNo fluff. This is core financial system architecture.? LEDGER-BASED SYSTEM DESIGN1️⃣ The Biggest 

Excellent.Day 7 is one of the most practical days — this is where we eliminate an entire class of production bugs by changing how we write SQL.Today’s focus:Avoid Read → Modify → Write bugs using ATOMIC SQL UPDATES? DAY 7 — ATOMIC SQL UPDATES1️⃣ The Core Problem (MOST COMMON BUG)❌ Re

Perfect.Day 5 is one of the MOST important days — this is where many “it works in testing but fails in production” bugs come from.I’ll explain Isolation Levels in a way you can feel what the database is doing.TRANSACTION ISOLATION LEVELSIsolation = how much of other transactions your transac

I was learning about deadlock and found that when multiple threads locks the same row in that situation, it creates deadlock. So I asked chatgpt that what if 500 managers are working on 100 bank accounts simultaneously, will it cause deadlock and how to avoid it? Chatgpt replied:Short answer first

Deadlocks: When Safety Itself Freezes the System“Deadlocks happen not because code is unsafe, but because it is too safe in the wrong order.”Yesterday you learned how to lock correctly.Today you learn what happens when locks fight each other.1️⃣ What Is a Deadlock (Plain English)A deadlock h

✅ 1️⃣ What Day 2 Was AboutDay 2 topic: IdempotencyHow to make sure the same request executed multiple times does NOT corrupt dataThis sits between Day 1 (why systems fail) and Day 3 (race conditions).✅ 2️⃣ What Was Covered in Day 2 (Explicitly)✔ Meaning of “method called twice”User

Excellent question.This shows you’re thinking at the exact point where many engineers level up.Short answer first, then the real answer:NO — you don’t always have to run a direct UPDATE.YES — you must ensure the read–check–write is ATOMIC.There are two correct ways to avoid unsafe “rea

This is a very important foundational question.If you truly understand “read shared data”, race conditions will start making sense everywhere.I’ll explain it from first principles, with plain language + real examples + mental model.1️⃣ What Does “Shared Data” Mean?Shared data is any da

Race Conditions & Concurrency“If two correct actions happen at the same time and the result is wrong — you have a race condition.”Today we move from duplicates (Day 2) to simultaneous execution, which is the #1 cause of “works in test, breaks in production”.1️⃣ What Is a Race Condi

Great — this is the right moment to learn idempotency, because it is one of the pillars of “code that doesn’t break”.I’ll explain it in three layers:Theory (mental model)Database-level design (most important)Spring Boot practical patternsNo shortcuts, no magic — just solid engineering.1�

This is a very important question.Most production bugs exist because developers don’t truly understand this.Let’s break it down slowly, clearly, and with real-world examples.1️⃣ What “Method Can Be Called Twice” REALLY MeansIt does NOT mean:You wrote method() twice in code ❌It DOES mea

? Day 1 — Why Software Systems Fail (and How to Think Like an Unbreakable Engineer)“Systems don’t fail because of bugs.They fail because engineers didn’t imagine the failure.”Today is about changing how you think, not writing code yet.If you get Day 1 right, the rest becomes natural.1️�