Microsoft L65 Interview Experience

Summary

I interviewed for a Senior Backend / Ads / Distributed Systems role at Microsoft, navigating 5 rounds that thoroughly assessed my skills in coding, system design, and behavioral aspects, including deep dives into advertising, payment systems, and analytics.

How it went

Interview Experience – Senior Backend / Ads / Distributed Systems (5 Rounds)

🔹 Round 1 – Coding + Ads Fundamentals

Focus Areas:

Project deep dive
3–4 behavioral questions
Complete Ads ecosystem explanation
- Advertiser → DSP → Ad Exchange → SSP → Publisher
- Real-time bidding (RTB) flow
- Ranking & auction basics
End-to-end ad serving lifecycle

Coding Question:

Map of Map encoding problem
Required graph search
Implemented, executed, and discussed optimizations

🔹 Round 2 – Ads HLD + Search + ML

1️⃣ Design HLD for Microsoft Bing Ads

Designed high-level architecture similar to Microsoft Bing Ads.

Covered:

Campaign setup & management
Budget pacing
Ranking logic & quality score
Auction mechanism
Publisher integration via SSP
End-to-end ads delivery flow

2️⃣ Search API Design

Problem:
Design an API for: “Plumbers near Kondapur”

Discussion Points:

Data ingestion pipeline
Indexing (inverted index + geo-spatial indexing)
Semantic matching
Hybrid retrieval (BM25 + embeddings)
Ranking signals & personalization

3️⃣ ML & LLM Discussion

Random Forest vs SVM
RAG (Retrieval-Augmented Generation) with LLMs
Trade-offs: classical ML vs LLM-based systems
Agentic frameworks for automation in ads optimization

🔹 Round 3 – Payment System Design (Razorpay-like)

Designed a payment gateway system similar to Razorpay.

Covered:

Payment Flow

App → Gateway → Bank
OTP handling
3DS flow
Callback/webhook handling

Reliability & Safety

Idempotency keys
Retry mechanisms
Handling bank timeouts
Double debit protection

System Concerns

DDoS protection
Fraud detection
Rate limiting
Observability

Integration & UX

Integration across app, gateway, and bank APIs
Localization & language support
UI rendering based on user country/currency

🔹 Round 4 – Hiring Manager + Bar Raiser (Onsite)

System Design: Website Activity Analytics

Requirement:

Track clicks/actions/users
18-month lookback
5-minute granularity
Filter by language, country, and other attributes

Discussion Points:

Event-based schema (fact + dimension tables)
SQL vs Time-Series DB
Partitioning strategy & pruning
Logged-in vs logged-out user tracking
Data pipelines & SLAs

Advanced Topics

Bloom Filters
Count-Min Sketch
Hashing in Redis
Sampling & standard deviation use cases
Transformer architecture basics
Effect of Temperature, Top-K, Top-P on LLM outputs
AI tool adoption metrics & measuring agentic AI impact

🔹 Round 5 – (LLD) - Onsite

Coding on Whiteboard

LRU Cache (write code on board)

System & Behavioral Discussion

Eviction strategies
Caching trade-offs
Reason for leaving
Leadership & conflict handling
Notice period expectations

Questions asked (6)

01
Map of Map Encoding Problem
Data Structures & Algorithms
I was given a map of map encoding problem that required graph search. I had to implement the solution, execute it, and discuss optimizations.
02
Design High-Level Architecture for Microsoft Bing Ads
System Design
I was asked to design a high-level architecture similar to Microsoft Bing Ads. This included campaign setup & management, budget pacing, ranking logic & quality score, auction mechanism, publisher integration via SSP, and the end-to-end ads delivery flow.
03
Design Search API for Local Services
System Design
I needed to design an API for the search query: 'Plumbers near Kondapur'. Discussion points included data ingestion pipeline, indexing (inverted index + geo-spatial indexing), semantic matching, hybrid retrieval (BM25 + embeddings), ranking signals & personalization.
04
Design a Payment Gateway System (Razorpay-like)
System Design
I designed a payment gateway system similar to Razorpay. This involved detailing the payment flow (App → Gateway → Bank, OTP handling, 3DS flow, Callback/webhook handling), reliability & safety considerations (Idempotency keys, Retry mechanisms, Handling bank timeouts, Double debit protection), system concerns (DDoS protection, Fraud detection, Rate limiting, Observability), and integration & UX aspects (Integration across app, gateway, and bank APIs, Localization & language support, UI rendering based on user country/currency).
05
Design Website Activity Analytics System
System Design
The requirement was to design a system to track clicks/actions/users with an 18-month lookback and 5-minute granularity, allowing filtering by language, country, and other attributes. Discussion points included event-based schema (fact + dimension tables), SQL vs Time-Series DB, partitioning strategy & pruning, logged-in vs logged-out user tracking, and data pipelines & SLAs.
06
Implement LRU Cache
Data Structures & Algorithms
I was asked to implement an LRU (Least Recently Used) Cache on the whiteboard.