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Asynchronous systemsDecoupling producers & consumersAccurate aggregation logicClean extensibility
You publish eventsSubscribers react independentlyDecouplingExtensibilityTestability
Counts are wrongDouble counting happensAggregates reset incorrectly
Multiple event typesMultiple subscribers per eventAsynchronous delivery
subscribe(eventType, handler)publish(event)Each subscriber receives the event exactly oncePublishing must not block
DecouplingAsync executionThread safetyClean handler management
Tracks API usage per customerSupports pricing tiersAggregates monthly usage
recordUsage(customerId)getMonthlyBill(customerId)Tier is fixed per customer
Billing is usage × priceUsage resets every month (simulate via reset method)Accurate counting (no double count)
Clean aggregationTier-based extensibilityCorrect resetsDefensive coding (unknown customers?)
Design internal frameworks (event bus)Handle async behavior confidentlyBuild correct billing logicThink like a backend platform engineer
Payment Rule Engine (rolling windows, rule chaining)Retry Queue with DLQ (backoff, failure handling)
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DAY 6 — Eventing & Usage-Based Systems (Advanced SDE-2)
You’re doing great.
Day 6 is advanced SDE-2 / early SDE-3 territory — interviewers use these problems to test whether you can design internal frameworks, not just features.
Today you’ll work on:
🔴 DAY 6 — Eventing & Usage-Based Systems (Advanced SDE-2)
Concepts Introduced Today (Short & Practical)
1️⃣ Event-Driven Design (In-Memory)
Instead of calling methods directly:
This improves:
2️⃣ Accurate Aggregation
Billing systems fail when:
Interviewers focus on correctness over cleverness.
✅ Problem 1: In-Memory Event Bus (Pub-Sub)
📌 Problem Statement
Design an in-memory event bus that supports:
Functional Requirements
Example
subscribe("ORDER_CREATED", handler1)
subscribe("ORDER_CREATED", handler2)
publish(ORDER_CREATED)
→ handler1 called
→ handler2 called
🧩 Java Interfaces (DO NOT MODIFY)
public interface Event {
String getType();
}
public interface EventHandler {
void handle(Event event);
}
public interface EventBus {
void subscribe(String eventType, EventHandler handler);
void publish(Event event);
}
🧪 Driver Code (Auto Tests + Async Verification)
import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicInteger;
public class EventBusTest {
public static void main(String[] args) throws Exception {
EventBus bus = new InMemoryEventBus();
AtomicInteger counter = new AtomicInteger(0);
bus.subscribe("TEST_EVENT", e -> counter.incrementAndGet());
bus.subscribe("TEST_EVENT", e -> counter.incrementAndGet());
bus.publish(() -> "TEST_EVENT");
// async delivery
Thread.sleep(500);
assert counter.get() == 2;
System.out.println("✅ EventBus tests passed");
}
}
🎯 What Interviewer Evaluates
✅ Problem 2: API Usage-Based Billing System
📌 Problem Statement
Design a billing system that:
Pricing Rules
Tier
Price per call
FREE
₹0
BASIC
₹1
PREMIUM
₹0.5
There are no rows in this table
Functional Requirements
Rules
Example
Customer: C1 (BASIC)
recordUsage() x 3
bill → ₹3
🧩 Java Interfaces (DO NOT MODIFY)
public enum PricingTier {
FREE,
BASIC,
PREMIUM
}
public interface BillingService {
void addCustomer(String customerId, PricingTier tier);
void recordUsage(String customerId);
double getMonthlyBill(String customerId);
void resetMonthlyUsage();
}
🧪 Driver Code (Auto Tests + Edge Cases)
public class BillingTest {
public static void main(String[] args) {
BillingService billing = new InMemoryBillingService();
billing.addCustomer("C1", PricingTier.BASIC);
billing.addCustomer("C2", PricingTier.PREMIUM);
billing.recordUsage("C1");
billing.recordUsage("C1");
billing.recordUsage("C1");
billing.recordUsage("C2");
billing.recordUsage("C2");
assert billing.getMonthlyBill("C1") == 3.0;
assert billing.getMonthlyBill("C2") == 1.0;
billing.resetMonthlyUsage();
assert billing.getMonthlyBill("C1") == 0.0;
System.out.println("✅ Billing tests passed");
}
}
🎯 What Interviewer Evaluates
🧠 Day 6 Takeaways
After today, you should:
This is strong SDE-2+ signal.
⏭️ Final Step
If you say:
“Continue with Day 7”
You’ll do a full interview-simulation day:
Day 7 is intentionally hard — exactly like real interviews.
Say the word 💪
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