Paytm Interview Experience – Backend Developer (1.5 Years of Experience)

Round 1: DSA Online Assessment

a. Given an array and a window size k, find the first negative integer in every window. (Sliding Window Technique)

b. Implement an LRU (Least Recently Used) Cache with efficient get and put operations using suitable data structures.

Round 2: Low-Level Design

a. Design a UPI-like system supporting:

Multiple types of wallets (e.g., Paytm Wallet, Fastag, etc.)

A main bank account linked to the user

Required to define proper data models and implement core functionalities like add money, transfer, balance check, and transaction history.

Round 3: System Design

a. Concurrency Control:

Discussed database locking mechanisms (pessimistic vs optimistic)

Explained transaction isolation levels: Read Uncommitted, Read Committed, Repeatable Read, and Serializable with examples.

b. Distributed Caching:

In-depth discussion on Redis – data structures, TTL, eviction policies, and use-cases

Explained caching strategies: write-through, write-behind, and cache-aside.

c. Handling Thread Downtime:

Was asked how I would manage thread failures or downtime under high-load or continuous request scenarios

Discussed approaches like thread pools, retries, circuit breakers, and fallback mechanisms to ensure system resilience.

Verdict: Selected

Although I didn’t join, it was a great experience and a solid learning opportunity.

Shrayansh Jain LLD and HLD videos were incredibly helpful in understanding and preparing for all these topics and real-world scenarios.