Zeni Study

Pages
- Learn Python In 7 Days
  Day 1 : Variable + if/else
  Day 2: Loops
  Day 3 : Array + String
  Day 4 — Functions
  Day 5 — Dictionaries - Hashmap
  Day 6 — Files & Errors
  Day 7 — Mini Project & Programmer Thinking
- Machine Coding:
  Day 1 — Bank account + Wallet :
  Day 2 : RateLimiter + seat booking
  Day 3 : Library Management + Classroom Attendance System
  Day 4 : Bank account and notification system.
  Day 5 : Key-value and payment processor.
  Day 6 : Discount engine + Seat booking( in depth)
  Day 7: Parking lot System :
- Machine coding extended:
  Day 7.5 — Parking Lot (Constraint-Heavy Variant)
  Day 8 : Order + Payment System
  Day 8 — Extension Round (Mid-Interview Change)
  Day 9 — Ticket Booking with Expiry (Simulated Time)
  Day 9 — Extension Round (Seat Lock Ownership + Forced Unlock)
- SQL + DB Thinking course
  Day 1 — Tables, Rows & Thinking in Data
  Day 2 — Relationships & Foreign Keys (Thinking in Connections)
  Day 3 — Filtering, Aggregation & Answering Data Questions
  Day 4 — JOINs (Combining Tables the Right Way)
  Day 5 — Subqueries, EXISTS & NOT EXISTS (Thinking in Layers)
  Day 6 — Indexes, Constraints & How Databases Think
  Day 7 — Full SQL Interview Simulation (SDE-1 Ready)
  Extended Day 1 — Core Business Systems (Deep Practice)
  Extended Day 2 — Booking & Platform Systems (High-Depth Practice)
  Extended Day 3 — Content & Learning Platforms (Capstone Practice)
- LLD: Java
  DAY 1 — Core Foundations (SDE-2 Level)
  DAY 2 — State, Time & Domain Correctness
  DAY 3 — Extensibility & Failure Handling (SDE-2 Core Signal)
  DAY 4 — Concurrency, Scheduling & Consistency (SDE-2 Critical)
  DAY 5 — Workflow Design & Config-Driven Systems (SDE-2+)
  DAY 6 — Eventing & Usage-Based Systems (Advanced SDE-2)
  DAY 7 — Interview Simulation (Hard, Production-Grade)
  Hidden Edge Cases Interviewers Expect (Day-wise)
  Detailed Testing file:
- Assignment:
  If else Practice Set

Zeni Study

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DAY 4 — Concurrency, Scheduling & Consistency (SDE-2 Critical)

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DAY 4 — Concurrency, Scheduling & Consistency (SDE-2 Critical)

Excellent. Day 4 is where many SDE-2 candidates break — because now concurrency + correctness are tested together.

Interviewers expect:

You know Java concurrency primitives

You protect invariants under race conditions

You don’t over-engineer, but you also don’t hand-wave

You’ll feel this day is harder than previous ones — that’s intentional.

⁠

🔵 DAY 4 — Concurrency, Scheduling & Consistency (SDE-2 Critical)

⁠

Concepts Introduced Today (Short + Interview-Relevant)

1️⃣ Concurrency ≠ Multithreading

Concurrency is about:

Correctness under race

Atomicity

Visibility

A solution that “works in single thread” is ❌ today.

⁠

2️⃣ Time + Threads = Tricky

Schedulers, delays, cancellation — interviewers check:

Do tasks execute at the right time?

Do canceled tasks really stop?

Does the system leak threads?

⁠

3️⃣ Consistency Over Performance

Overselling inventory or executing a task twice is instant rejection.

⁠

✅ Problem 1: Concurrent Task Scheduler

⁠

📌 Problem Statement

Design an in-memory task scheduler that:

Executes tasks at a future timestamp

Supports cancellation

Is thread-safe

⁠

Functional Requirements

schedule(taskId, runnable, executeAtMillis)

cancel(taskId)

Task should run once

Cancelled tasks must never execute

⁠

Example

schedule("t1", task, now + 2 sec)

cancel("t1")

→ task never runs

⁠

🧩 Java Interfaces (DO NOT MODIFY)

public interface TaskScheduler {

void schedule(String taskId, Runnable task, long executeAtMillis);

boolean cancel(String taskId);

}

⁠

🧪 Driver Code (Auto Tests + Edge Cases)

import java.util.concurrent.*;

import java.util.concurrent.atomic.AtomicInteger;

public class TaskSchedulerTest {

public static void main(String[] args) throws Exception {

TaskScheduler scheduler = new InMemoryTaskScheduler();

AtomicInteger counter = new AtomicInteger(0);

scheduler.schedule("task1", counter::incrementAndGet,

System.currentTimeMillis() + 1000);

Thread.sleep(1500);

assert counter.get() == 1;

// Cancellation test

scheduler.schedule("task2", counter::incrementAndGet,

System.currentTimeMillis() + 1000);

boolean cancelled = scheduler.cancel("task2");

assert cancelled;

Thread.sleep(1500);

assert counter.get() == 1;

// Cancel non-existent

assert !scheduler.cancel("missing");

System.out.println("✅ TaskScheduler tests passed");

}

⁠

🎯 What Interviewer Evaluates

Correct scheduling

Thread safety

No double execution

Clean shutdown strategy (if asked)

⁠

✅ Problem 2: Inventory Management System (Oversell Prevention)

⁠

📌 Problem Statement

Design an inventory system that supports:

Adding stock

Reserving stock

Releasing stock

Preventing oversell under concurrency

⁠

Functional Requirements

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