60-Day Plan

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Day1
60-day SSE plan
Parking Lot System • URL Shortener • Sliding Window • Tech: Design Pattern
Open day
Algorithm
DSA
Easy–Medium
Sliding Window — Minimum Window Substring (LC 76)
Find the smallest substring of s that contains all characters of t.
Example
Input: s="ADOBECODEBANC", t="ABC" → Output: "BANC"
Where it shows up
Token scanning, query parsing, log/event windowing.
References
LeetCode
NeetCode Sliding Window
GFG Sliding Window
Low-level design
LLD
Parking Lot System
Requirements
Multiple floors, spot types (Car/Bike/Truck).
Park, unpark, ticketing, fee calculation.
Expectations
Provide UML class diagram (ParkingLot, Floor, Spot, Vehicle, Ticket, Payment).
Implement core classes and simulate park/unpark; discuss extendability.
Where it shows up
Real parking garages in malls/airports.
References
Grokking OOD
UML Basics
High-level design
HLD
URL Shortener
Requirements
Shorten URLs, redirect keys, optional custom alias, basic stats.
Scale to millions of reads/day; low latency redirects.
Expectations
APIs, DB schema, key generation, cache-aside, sharding strategy.
Trade-offs: SQL vs NoSQL, consistency, TTL, analytics pipeline.
Where it shows up
Bitly/Twitter links, marketing campaigns, QR codes.
References
System Design Primer
Educative Grokking SD
Today's deep-dive
Tech
Design Pattern
Implement Strategy + Observer around a pricing/notification domain. Unit test swapping strategies without touching clients.
Where it shows up
Dynamic pricing engines, notification systems, and plug-and-play business logic where strategies or observers change at runtime.
References
Refactoring Guru
Head First Design Patterns Notes
Day2
60-day SSE plan
Rate Limiter (Token Bucket) • WhatsApp-like Chat • Binary Search on Answer • Tech: Advanced Java
Open day
Algorithm
DSA
Easy–Medium
Binary Search on Answer — Capacity to Ship Packages (LC 1011)
Minimize ship capacity so all packages ship within D days using feasibility check.
Example
Input: weights=[1,2,3,1,1], D=4 → Output: 3
Where it shows up
Capacity planning, throughput sizing, SLO budgeting.
References
LeetCode
Binary Search Patterns
Low-level design
LLD
Rate Limiter (Token Bucket)
Requirements
Per-user limits with rate & capacity.
Thread-safe tryAcquire(); refill over time.
Expectations
UML for TokenBucket & RateLimiterService.
Concurrency-safe Java implementation + tests.
Where it shows up
API gateways, throttling abusive clients.
References
Token Bucket
Java Concurrency
High-level design
HLD
WhatsApp-like Chat
Requirements
1:1 + group messaging, delivery/read receipts, offline sync.
Real-time delivery via WebSockets; scalable fan-out.
Expectations
APIs, WebSocket fan-out, message broker, storage model.
Multi-region strategy, exactly-once UX via idempotency keys.
Where it shows up
Consumer chat apps, support chat in products.
References
Gaurav Sen Chat SD
Today's deep-dive
Tech
Advanced Java
Explore JVM memory (heap, metaspace), class loading, JIT; write a micro-benchmark of String vs StringBuilder concatenation.
Where it shows up
Performance tuning in enterprise Java apps, optimizing backend services, and understanding memory/CPU trade-offs in production systems.
References
JVM Spec (overview)
JMH Intro
Day3
60-day SSE plan
Splitwise • Twitter Feed / Timeline • Graph BFS/DFS • Tech: Concurrency & Multithreading
Open day
Algorithm
DSA
Easy–Medium
Graph BFS — Number of Islands (LC 200)
Count connected components in a grid via BFS/DFS.
Example
Input: grid with 1s and 0s → Output: number of islands
Where it shows up
Cluster detection, network partitioning, region counting.
References
LeetCode
GFG BFS/DFS
Low-level design
LLD
Splitwise
Requirements
Create groups, add expenses, split & settle balances.
Expectations
UML for Group, Expense, Split, BalanceSheet.
Implement core services; show simplified settle-up.
Where it shows up
Bill-splitting consumer apps, travel groups.
References
OOD Examples
High-level design
HLD
Twitter Feed / Timeline
Requirements
Post, follow, generate home timeline with pagination and basic ranking.
Expectations
Fan-out on write vs read; caches; cold-start handling.
Explain hybrid approach for celebs vs normals.
Where it shows up
Social feeds, enterprise activity streams.
References
System Design Primer
Today's deep-dive
Tech
Concurrency & Multithreading
Build a bounded thread pool demo; compare synchronized vs ReentrantLock; show visibility with volatile.
Where it shows up
Building scalable chat/messaging platforms, real-time collaboration tools, and distributed systems requiring concurrency control.
References
Java Concurrency in Practice (notes)
java.util.concurrent
Day4
60-day SSE plan
Logger Service • E-commerce Order Service • Graph Topological Sort • Tech: Solr
Open day
Algorithm
DSA
Easy–Medium
Graph Topological Sort — Course Schedule (LC 207)
Detect if you can finish all courses given prerequisites via Kahn's or DFS.
Example
Input: prerequisites edges → Output: true/false
Where it shows up
Dependency resolution, job scheduling, build pipelines.
References
LeetCode
Topological Sort (Kahn)
Low-level design
LLD
Logger Service
Requirements
Append logs concurrently; rotate file on size/time.
Expectations
Thread-safe singleton; async queue & writer thread.
Demonstrate rotation strategy.
Where it shows up
Service logging, audit trails, observability.
References
Java Logging
High-level design
HLD
E-commerce Order Service
Requirements
Create/manage orders; inventory reserve; payment auth; status events.
Expectations
Saga orchestration, outbox pattern, idempotency design.
APIs, schema, eventing & retries.
Where it shows up
Retail marketplaces, online stores.
References
Outbox Pattern
Today's deep-dive
Tech
Solr
Run Solr in Docker; create a core; index sample JSON; test analyzers & facets.
Where it shows up
Search engines, e-commerce product search, log analytics, and any system requiring fast, flexible text search and faceting.
References
Solr Guide
Solr Docker
Day5
60-day SSE plan
Elevator System • Payment Gateway • Heap / Priority Queue • Tech: Cassandra
Open day
Algorithm
DSA
Easy–Medium
Heap / Priority Queue — Merge k Sorted Lists (LC 23)
Merge k sorted linked lists using a min-heap.
Example
Input: lists = [[1,4,5],[1,3,4],[2,6]] → Output: [1,1,2,3,4,4,5,6]
Where it shows up
Log mergers, multi-way merge in external sorting, streaming.
References
LeetCode
Heaps in Java
Low-level design
LLD
Elevator System
Requirements
Multiple elevators; up/down requests; scheduling.
Expectations
UML for Elevator, Request, Controller; strategy for scheduling.
Simulate requests & movement.
Where it shows up
High-rise building operations, dispatch systems.
References
Elevator LLD Guides
High-level design
HLD
Payment Gateway
Requirements
Payment intents, confirm/capture, refunds; idempotency; webhooks.
Expectations
PSP adapters, circuit breakers, retries with backoff.
Audit logs; PCI isolation considerations.
Where it shows up
Fintech, marketplaces, subscriptions.
References
Idempotency Keys
Today's deep-dive
Tech
Cassandra
Model a table for user_events with proper partition key; write queries; test CL=QUORUM reads/writes.
Where it shows up
Distributed databases, event sourcing, and high-availability systems where partition tolerance and tunable consistency are critical.
References
Cassandra Docs
Data Modeling
Day6
60-day SSE plan
Cache (LRU) • Uber-style Ride Hailing • Two Pointers • Tech: Kafka
Open day
Algorithm
DSA
Easy–Medium
Two Pointers — Trapping Rain Water (LC 42)
Compute trapped water using two-pointer approach.
Example
Input: [0,1,0,2,1,0,1,3,2,1,2,1] → Output: 6
Where it shows up
Resource aggregation, elevation maps, histogram problems.
References
LeetCode
Two Pointers
Low-level design
LLD
Cache (LRU)
Requirements
Put/get with capacity; LRU eviction policy.
Expectations
Classes for LRUCache using LinkedHashMap or custom DLL+HashMap.
Complexity guarantees O(1) operations.
Where it shows up
Databases, OS page cache, web caching layers.
References
LRU Patterns
High-level design
HLD
Uber-style Ride Hailing
Requirements
Driver/rider match by proximity; live location; trip lifecycle; pricing.
Expectations
Geo-indexing (geohash), matching service, WS gateway, storage model.
Regional partitions, degradation strategies.
Where it shows up
Mobility platforms, logistics dispatch.
References
Geohash Basics
Today's deep-dive
Tech
Kafka
Write a producer & consumer; demonstrate consumer groups and rebalancing; handle offsets safely.
Where it shows up
Event-driven architectures, real-time analytics, and data pipelines in organizations processing large-scale streaming data.
References
Kafka Docs
Confluent Patterns
Day7
60-day SSE plan
Tic Tac Toe • Distributed Cache (Redis-like) • Union-Find • Tech: SQL Queries Practice
Open day
Algorithm
DSA
Easy–Medium
Union-Find — Redundant Connection (LC 684)
Find the extra edge that creates a cycle in an undirected graph.
Example
Input: edges forming a cycle → Output: the extra edge
Where it shows up
Cycle detection, connectivity in networks, Kruskal pre-checks.
References
LeetCode
Disjoint Set (DSU)
Low-level design
LLD
Tic Tac Toe
Requirements
Design 3x3 game; win/draw detection; replay.
Expectations
Classes for Board, Player, Game; strategy for winner check.
Unit tests for edge cases.
Where it shows up
Turn-based game engines, board game modeling.
References
OOD Games
High-level design
HLD
Distributed Cache (Redis-like)
Requirements
SET/GET/DEL, TTL; sharding; replication; eviction policies.
Expectations
Consistent hashing, leader election, cache-aside/write-through.
Failure handling & split-brain prevention.
Where it shows up
Backend acceleration, DB offload, low latency reads.
References
Redis Cluster
Today's deep-dive
Tech
SQL Queries Practice
Practice joins, window functions, indexing & EXPLAIN on sample schema; optimize a slow query.
Where it shows up
API development for SaaS products, public web services, and internal microservices requiring robust, scalable, and maintainable interfaces.
References
SQLZoo
Use The Index, Luke
Day8
60-day SSE plan
Notification Service • Real-time Analytics Dashboard • DP (Strings) • Tech: Docker
Open day
Algorithm
DSA
Easy–Medium
Dynamic Programming — Longest Common Subsequence (LC 1143)
Find LCS length for two strings using DP.
Example
Input: text1="abcde", text2="ace" → Output: 3
Where it shows up
Diff tools, DNA sequence alignment, versioning.
References
LeetCode
GFG LCS
Low-level design
LLD
Notification Service
Requirements
Send email/SMS; templates; retries; rate limit.
Expectations
UML for Template, Request, Channel; retry/backoff design.
Mock provider integration tests.
Where it shows up
E-commerce, fintech transactional messaging.
References
Retry/Backoff
High-level design
HLD
Real-time Analytics Dashboard
Requirements
Ingest events at scale; compute aggregates; serve queries in seconds.
Expectations
Event ingestion, stream processing, materialized views, TSDB/NoSQL.
Exactly-once/at-least-once semantics; reprocessing strategy.
Where it shows up
Product analytics, ops dashboards, monitoring.
References
Kafka Streams 101
Today's deep-dive
Tech
Docker
Create multi-stage build for a Spring Boot app; add HEALTHCHECK; test graceful shutdown.
Where it shows up
Containerized deployments, CI/CD pipelines, and scalable infrastructure management in cloud-native environments.
References
Dockerfile Best Practices
Day9
60-day SSE plan
ATM System • Video Streaming • Backtracking • Tech: Kubernetes
Open day
Algorithm
DSA
Easy–Medium
Backtracking — Permutations (LC 46)
Generate all permutations of an array.
Example
Input: [1,2,3] → Output: all permutations
Where it shows up
State space search, scheduling, puzzle solvers.
References
LeetCode
Backtracking Patterns
Low-level design
LLD
ATM System
Requirements
Authenticate, withdraw, deposit, balance; ATM state & cash bins.
Expectations
UML for Account, Card, ATM, Transaction; state pattern for ATM.
Simulate basic flows with validations.
Where it shows up
Bank ATMs; kiosk-based transactional systems.
References
OOD Banking
High-level design
HLD
Video Streaming
Requirements
Upload → transcode → store renditions → stream via HLS/DASH.
Expectations
Worker pipeline, object storage, CDN, player ABR logic basics.
Metadata & search index; cost controls.
Where it shows up
YouTube/OTT, education platforms.
References
HLS Spec
Today's deep-dive
Tech
Kubernetes
Deploy app with Deployment + Service; add readiness/liveness probes; rolling update.
Where it shows up
Microservices-based architectures, distributed teams, and organizations needing modular, independently deployable services.
References
K8s Docs
Day10
60-day SSE plan
Chess Game • Dropbox/Google Drive-like Storage • Trees (BFS) • Tech: Microservices
Open day
Algorithm
DSA
Easy–Medium
Trees — Binary Tree Level Order Traversal (LC 102)
Return level-order traversal using BFS.
Example
Input: binary tree → Output: list of levels
Where it shows up
Hierarchy traversals, org charts, breadth-wise processing.
References
LeetCode
GFG Tree Traversals
Low-level design
LLD
Chess Game
Requirements
Represent board, moves, turns; check/checkmate validation.
Expectations
UML for Board, Piece hierarchy, Move, Game; strategy for legal moves.
Basic engine for move validation.
Where it shows up
Game engines, rules validation systems.
References
Chess Design
High-level design
HLD
Dropbox/Google Drive-like Storage
Requirements
Chunked uploads, versioning, deduplication, sharing links.
Expectations
Content-addressable storage; metadata vs blob separation; sync engine.
Conflict resolution approach.
Where it shows up
Consumer & enterprise file sync/share.
References
Content-addressable Storage
Today's deep-dive
Tech
Microservices
Draw service boundaries for a small e-commerce; define contracts, versioning, and observability (metrics, logs, traces).
Where it shows up
Automated testing, continuous integration, and quality assurance in software development lifecycles.
References
Microservices.io
Day11
60-day SSE plan
Parking Lot System • URL Shortener • Sliding Window • Tech: REST APIs
Open day
Algorithm
DSA
Medium
Sliding Window — Minimum Window Substring (LC 76)
Find the smallest substring of s that contains all characters of t.
Example
Input: s="ADOBECODEBANC", t="ABC" → Output: "BANC"
Where it shows up
Token scanning, query parsing, log/event windowing.
References
LeetCode
NeetCode Sliding Window
GFG Sliding Window
Low-level design
LLD
Parking Lot System
Requirements
Multiple floors, spot types (Car/Bike/Truck).
Park, unpark, ticketing, fee calculation.
Expectations
Provide UML class diagram (ParkingLot, Floor, Spot, Vehicle, Ticket, Payment).
Implement core classes and simulate park/unpark; discuss extendability.
Where it shows up
Real parking garages in malls/airports.
References
Grokking OOD
UML Basics
High-level design
HLD
URL Shortener
Requirements
Shorten URLs, redirect keys, optional custom alias, basic stats.
Scale to millions of reads/day; low latency redirects.
Expectations
APIs, DB schema, key generation, cache-aside, sharding strategy.
Trade-offs: SQL vs NoSQL, consistency, TTL, analytics pipeline.
Where it shows up
Bitly/Twitter links, marketing campaigns, QR codes.
References
System Design Primer
Educative Grokking SD
Today's deep-dive
Tech
REST APIs
Define REST guidelines; build a small API with pagination, filtering, error envelopes, idempotent PUT/PATCH.
Where it shows up
API development for SaaS products, public web services, and internal microservices requiring robust, scalable, and maintainable interfaces.
References
Microsoft REST Guidelines
Day12
60-day SSE plan
Rate Limiter (Token Bucket) • WhatsApp-like Chat • Binary Search on Answer • Tech: Spring Boot
Open day
Algorithm
DSA
Medium
Binary Search on Answer — Capacity to Ship Packages (LC 1011)
Minimize ship capacity so all packages ship within D days using feasibility check.
Example
Input: weights=[1,2,3,1,1], D=4 → Output: 3
Where it shows up
Capacity planning, throughput sizing, SLO budgeting.
References
LeetCode
Binary Search Patterns
Low-level design
LLD
Rate Limiter (Token Bucket)
Requirements
Per-user limits with rate & capacity.
Thread-safe tryAcquire(); refill over time.
Expectations
UML for TokenBucket & RateLimiterService.
Concurrency-safe Java implementation + tests.
Where it shows up
API gateways, throttling abusive clients.
References
Token Bucket
Java Concurrency
High-level design
HLD
WhatsApp-like Chat
Requirements
1:1 + group messaging, delivery/read receipts, offline sync.
Real-time delivery via WebSockets; scalable fan-out.
Expectations
APIs, WebSocket fan-out, message broker, storage model.
Multi-region strategy, exactly-once UX via idempotency keys.
Where it shows up
Consumer chat apps, support chat in products.
References
Gaurav Sen Chat SD
Today's deep-dive
Tech
Spring Boot
Create Orders API with validation and @ControllerAdvice; profiles dev/test; write slice tests.
Where it shows up
Building production-grade Java microservices, rapid prototyping, and enterprise backend development with robust configuration and testing support.
References
Baeldung Spring Guides
Spring Docs
Day13
60-day SSE plan
Splitwise • Twitter Feed / Timeline • Graph BFS/DFS • Tech: Design Pattern
Open day
Algorithm
DSA
Medium
Graph BFS — Number of Islands (LC 200)
Count connected components in a grid via BFS/DFS.
Example
Input: grid with 1s and 0s → Output: number of islands
Where it shows up
Cluster detection, network partitioning, region counting.
References
LeetCode
GFG BFS/DFS
Low-level design
LLD
Splitwise
Requirements
Create groups, add expenses, split & settle balances.
Expectations
UML for Group, Expense, Split, BalanceSheet.
Implement core services; show simplified settle-up.
Where it shows up
Bill-splitting consumer apps, travel groups.
References
OOD Examples
High-level design
HLD
Twitter Feed / Timeline
Requirements
Post, follow, generate home timeline with pagination and basic ranking.
Expectations
Fan-out on write vs read; caches; cold-start handling.
Explain hybrid approach for celebs vs normals.
Where it shows up
Social feeds, enterprise activity streams.
References
System Design Primer
Today's deep-dive
Tech
Design Pattern
Implement Strategy + Observer around a pricing/notification domain. Unit test swapping strategies without touching clients.
Where it shows up
Dynamic pricing engines, notification systems, and plug-and-play business logic where strategies or observers change at runtime.
References
Refactoring Guru
Head First Design Patterns Notes
Day14
60-day SSE plan
Logger Service • E-commerce Order Service • Graph Topological Sort • Tech: Advanced Java
Open day
Algorithm
DSA
Medium
Graph Topological Sort — Course Schedule (LC 207)
Detect if you can finish all courses given prerequisites via Kahn's or DFS.
Example
Input: prerequisites edges → Output: true/false
Where it shows up
Dependency resolution, job scheduling, build pipelines.
References
LeetCode
Topological Sort (Kahn)
Low-level design
LLD
Logger Service
Requirements
Append logs concurrently; rotate file on size/time.
Expectations
Thread-safe singleton; async queue & writer thread.
Demonstrate rotation strategy.
Where it shows up
Service logging, audit trails, observability.
References
Java Logging
High-level design
HLD
E-commerce Order Service
Requirements
Create/manage orders; inventory reserve; payment auth; status events.
Expectations
Saga orchestration, outbox pattern, idempotency design.
APIs, schema, eventing & retries.
Where it shows up
Retail marketplaces, online stores.
References
Outbox Pattern
Today's deep-dive
Tech
Advanced Java
Explore JVM memory (heap, metaspace), class loading, JIT; write a micro-benchmark of String vs StringBuilder concatenation.
Where it shows up
Performance tuning in enterprise Java apps, optimizing backend services, and understanding memory/CPU trade-offs in production systems.
References
JVM Spec (overview)
JMH Intro
Day15
60-day SSE plan
Elevator System • Payment Gateway • Heap / Priority Queue • Tech: Concurrency & Multithreading
Open day
Algorithm
DSA
Medium
Heap / Priority Queue — Merge k Sorted Lists (LC 23)
Merge k sorted linked lists using a min-heap.
Example
Input: lists = [[1,4,5],[1,3,4],[2,6]] → Output: [1,1,2,3,4,4,5,6]
Where it shows up
Log mergers, multi-way merge in external sorting, streaming.
References
LeetCode
Heaps in Java
Low-level design
LLD
Elevator System
Requirements
Multiple elevators; up/down requests; scheduling.
Expectations
UML for Elevator, Request, Controller; strategy for scheduling.
Simulate requests & movement.
Where it shows up
High-rise building operations, dispatch systems.
References
Elevator LLD Guides
High-level design
HLD
Payment Gateway
Requirements
Payment intents, confirm/capture, refunds; idempotency; webhooks.
Expectations
PSP adapters, circuit breakers, retries with backoff.
Audit logs; PCI isolation considerations.
Where it shows up
Fintech, marketplaces, subscriptions.
References
Idempotency Keys
Today's deep-dive
Tech
Concurrency & Multithreading
Build a bounded thread pool demo; compare synchronized vs ReentrantLock; show visibility with volatile.
Where it shows up
Building scalable chat/messaging platforms, real-time collaboration tools, and distributed systems requiring concurrency control.
References
Java Concurrency in Practice (notes)
java.util.concurrent
Day16
60-day SSE plan
Cache (LRU) • Uber-style Ride Hailing • Two Pointers • Tech: Solr
Open day
Algorithm
DSA
Medium
Two Pointers — Trapping Rain Water (LC 42)
Compute trapped water using two-pointer approach.
Example
Input: [0,1,0,2,1,0,1,3,2,1,2,1] → Output: 6
Where it shows up
Resource aggregation, elevation maps, histogram problems.
References
LeetCode
Two Pointers
Low-level design
LLD
Cache (LRU)
Requirements
Put/get with capacity; LRU eviction policy.
Expectations
Classes for LRUCache using LinkedHashMap or custom DLL+HashMap.
Complexity guarantees O(1) operations.
Where it shows up
Databases, OS page cache, web caching layers.
References
LRU Patterns
High-level design
HLD
Uber-style Ride Hailing
Requirements
Driver/rider match by proximity; live location; trip lifecycle; pricing.
Expectations
Geo-indexing (geohash), matching service, WS gateway, storage model.
Regional partitions, degradation strategies.
Where it shows up
Mobility platforms, logistics dispatch.
References
Geohash Basics
Today's deep-dive
Tech
Solr
Run Solr in Docker; create a core; index sample JSON; test analyzers & facets.
Where it shows up
Search engines, e-commerce product search, log analytics, and any system requiring fast, flexible text search and faceting.
References
Solr Guide
Solr Docker
Day17
60-day SSE plan
Tic Tac Toe • Distributed Cache (Redis-like) • Union-Find • Tech: Cassandra
Open day
Algorithm
DSA
Medium
Union-Find — Redundant Connection (LC 684)
Find the extra edge that creates a cycle in an undirected graph.
Example
Input: edges forming a cycle → Output: the extra edge
Where it shows up
Cycle detection, connectivity in networks, Kruskal pre-checks.
References
LeetCode
Disjoint Set (DSU)
Low-level design
LLD
Tic Tac Toe
Requirements
Design 3x3 game; win/draw detection; replay.
Expectations
Classes for Board, Player, Game; strategy for winner check.
Unit tests for edge cases.
Where it shows up
Turn-based game engines, board game modeling.
References
OOD Games
High-level design
HLD
Distributed Cache (Redis-like)
Requirements
SET/GET/DEL, TTL; sharding; replication; eviction policies.
Expectations
Consistent hashing, leader election, cache-aside/write-through.
Failure handling & split-brain prevention.
Where it shows up
Backend acceleration, DB offload, low latency reads.
References
Redis Cluster
Today's deep-dive
Tech
Cassandra
Model a table for user_events with proper partition key; write queries; test CL=QUORUM reads/writes.
Where it shows up
Distributed databases, event sourcing, and high-availability systems where partition tolerance and tunable consistency are critical.
References
Cassandra Docs
Data Modeling
Day18
60-day SSE plan
Notification Service • Real-time Analytics Dashboard • DP (Strings) • Tech: Kafka
Open day
Algorithm
DSA
Medium
Dynamic Programming — Longest Common Subsequence (LC 1143)
Find LCS length for two strings using DP.
Example
Input: text1="abcde", text2="ace" → Output: 3
Where it shows up
Diff tools, DNA sequence alignment, versioning.
References
LeetCode
GFG LCS
Low-level design
LLD
Notification Service
Requirements
Send email/SMS; templates; retries; rate limit.
Expectations
UML for Template, Request, Channel; retry/backoff design.
Mock provider integration tests.
Where it shows up
E-commerce, fintech transactional messaging.
References
Retry/Backoff
High-level design
HLD
Real-time Analytics Dashboard
Requirements
Ingest events at scale; compute aggregates; serve queries in seconds.
Expectations
Event ingestion, stream processing, materialized views, TSDB/NoSQL.
Exactly-once/at-least-once semantics; reprocessing strategy.
Where it shows up
Product analytics, ops dashboards, monitoring.
References
Kafka Streams 101
Today's deep-dive
Tech
Kafka
Write a producer & consumer; demonstrate consumer groups and rebalancing; handle offsets safely.
Where it shows up
Event-driven architectures, real-time analytics, and data pipelines in organizations processing large-scale streaming data.
References
Kafka Docs
Confluent Patterns
Day19
60-day SSE plan
ATM System • Video Streaming • Backtracking • Tech: SQL Queries Practice
Open day
Algorithm
DSA
Medium
Backtracking — Permutations (LC 46)
Generate all permutations of an array.
Example
Input: [1,2,3] → Output: all permutations
Where it shows up
State space search, scheduling, puzzle solvers.
References
LeetCode
Backtracking Patterns
Low-level design
LLD
ATM System
Requirements
Authenticate, withdraw, deposit, balance; ATM state & cash bins.
Expectations
UML for Account, Card, ATM, Transaction; state pattern for ATM.
Simulate basic flows with validations.
Where it shows up
Bank ATMs; kiosk-based transactional systems.
References
OOD Banking
High-level design
HLD
Video Streaming
Requirements
Upload → transcode → store renditions → stream via HLS/DASH.
Expectations
Worker pipeline, object storage, CDN, player ABR logic basics.
Metadata & search index; cost controls.
Where it shows up
YouTube/OTT, education platforms.
References
HLS Spec
Today's deep-dive
Tech
SQL Queries Practice
Practice joins, window functions, indexing & EXPLAIN on sample schema; optimize a slow query.
Where it shows up
Data analysis, reporting, and performance optimization in business intelligence, analytics, and backend engineering roles.
References
SQLZoo
Use The Index, Luke
Day20
60-day SSE plan
Chess Game • Dropbox/Google Drive-like Storage • Trees (BFS) • Tech: Docker
Open day
Algorithm
DSA
Medium
Trees — Binary Tree Level Order Traversal (LC 102)
Return level-order traversal using BFS.
Example
Input: binary tree → Output: list of levels
Where it shows up
Hierarchy traversals, org charts, breadth-wise processing.
References
LeetCode
GFG Tree Traversals
Low-level design
LLD
Chess Game
Requirements
Represent board, moves, turns; check/checkmate validation.
Expectations
UML for Board, Piece hierarchy, Move, Game; strategy for legal moves.
Basic engine for move validation.
Where it shows up
Game engines, rules validation systems.
References
Chess Design
High-level design
HLD
Dropbox/Google Drive-like Storage
Requirements
Chunked uploads, versioning, deduplication, sharing links.
Expectations
Content-addressable storage; metadata vs blob separation; sync engine.
Conflict resolution approach.
Where it shows up
Consumer & enterprise file sync/share.
References
Content-addressable Storage
Today's deep-dive
Tech
Docker
Create multi-stage build for a Spring Boot app; add HEALTHCHECK; test graceful shutdown.
Where it shows up
Containerized deployments, CI/CD pipelines, and scalable infrastructure management in cloud-native environments.
References
Dockerfile Best Practices
Day21
60-day SSE plan
Parking Lot System • URL Shortener • Sliding Window • Tech: Kubernetes
Open day
Algorithm
DSA
Medium
Sliding Window — Minimum Window Substring (LC 76)
Find the smallest substring of s that contains all characters of t.
Example
Input: s="ADOBECODEBANC", t="ABC" → Output: "BANC"
Where it shows up
Token scanning, query parsing, log/event windowing.
References
LeetCode
NeetCode Sliding Window
GFG Sliding Window
Low-level design
LLD
Parking Lot System
Requirements
Multiple floors, spot types (Car/Bike/Truck).
Park, unpark, ticketing, fee calculation.
Expectations
Provide UML class diagram (ParkingLot, Floor, Spot, Vehicle, Ticket, Payment).
Implement core classes and simulate park/unpark; discuss extendability.
Where it shows up
Real parking garages in malls/airports.
References
Grokking OOD
UML Basics
High-level design
HLD
URL Shortener
Requirements
Shorten URLs, redirect keys, optional custom alias, basic stats.
Scale to millions of reads/day; low latency redirects.
Expectations
APIs, DB schema, key generation, cache-aside, sharding strategy.
Trade-offs: SQL vs NoSQL, consistency, TTL, analytics pipeline.
Where it shows up
Bitly/Twitter links, marketing campaigns, QR codes.
References
System Design Primer
Educative Grokking SD
Today's deep-dive
Tech
Kubernetes
Deploy app with Deployment + Service; add readiness/liveness probes; rolling update.
Where it shows up
Microservices-based architectures, distributed teams, and organizations needing modular, independently deployable services.
References
K8s Docs
Day22
60-day SSE plan
Rate Limiter (Token Bucket) • WhatsApp-like Chat • Binary Search on Answer • Tech: Microservices
Open day
Algorithm
DSA
Medium
Binary Search on Answer — Capacity to Ship Packages (LC 1011)
Minimize ship capacity so all packages ship within D days using feasibility check.
Example
Input: weights=[1,2,3,1,1], D=4 → Output: 3
Where it shows up
Capacity planning, throughput sizing, SLO budgeting.
References
LeetCode
Binary Search Patterns
Low-level design
LLD
Rate Limiter (Token Bucket)
Requirements
Per-user limits with rate & capacity.
Thread-safe tryAcquire(); refill over time.
Expectations
UML for TokenBucket & RateLimiterService.
Concurrency-safe Java implementation + tests.
Where it shows up
API gateways, throttling abusive clients.
References
Token Bucket
Java Concurrency
High-level design
HLD
WhatsApp-like Chat
Requirements
1:1 + group messaging, delivery/read receipts, offline sync.
Real-time delivery via WebSockets; scalable fan-out.
Expectations
APIs, WebSocket fan-out, message broker, storage model.
Multi-region strategy, exactly-once UX via idempotency keys.
Where it shows up
Consumer chat apps, support chat in products.
References
Gaurav Sen Chat SD
Today's deep-dive
Tech
Microservices
Draw service boundaries for a small e-commerce; define contracts, versioning, and observability (metrics, logs, traces).
Where it shows up
Automated testing, continuous integration, and quality assurance in software development lifecycles.
References
Microservices.io
Day23
60-day SSE plan
Splitwise • Twitter Feed / Timeline • Graph BFS/DFS • Tech: REST APIs
Open day
Algorithm
DSA
Medium
Graph BFS — Number of Islands (LC 200)
Count connected components in a grid via BFS/DFS.
Example
Input: grid with 1s and 0s → Output: number of islands
Where it shows up
Cluster detection, network partitioning, region counting.
References
LeetCode
GFG BFS/DFS
Low-level design
LLD
Splitwise
Requirements
Create groups, add expenses, split & settle balances.
Expectations
UML for Group, Expense, Split, BalanceSheet.
Implement core services; show simplified settle-up.
Where it shows up
Bill-splitting consumer apps, travel groups.
References
OOD Examples
High-level design
HLD
Twitter Feed / Timeline
Requirements
Post, follow, generate home timeline with pagination and basic ranking.
Expectations
Fan-out on write vs read; caches; cold-start handling.
Explain hybrid approach for celebs vs normals.
Where it shows up
Social feeds, enterprise activity streams.
References
System Design Primer
Today's deep-dive
Tech
REST APIs
Define REST guidelines; build a small API with pagination, filtering, error envelopes, idempotent PUT/PATCH.
Where it shows up
Enterprise search, e-commerce product search, log analytics, and content management systems requiring fast, full-text search and faceted navigation.
References
Microsoft REST Guidelines
Day24
60-day SSE plan
Logger Service • E-commerce Order Service • Graph Topological Sort • Tech: Spring Boot
Open day
Algorithm
DSA
Medium
Graph Topological Sort — Course Schedule (LC 207)
Detect if you can finish all courses given prerequisites via Kahn's or DFS.
Example
Input: prerequisites edges → Output: true/false
Where it shows up
Dependency resolution, job scheduling, build pipelines.
References
LeetCode
Topological Sort (Kahn)
Low-level design
LLD
Logger Service
Requirements
Append logs concurrently; rotate file on size/time.
Expectations
Thread-safe singleton; async queue & writer thread.
Demonstrate rotation strategy.
Where it shows up
Service logging, audit trails, observability.
References
Java Logging
High-level design
HLD
E-commerce Order Service
Requirements
Create/manage orders; inventory reserve; payment auth; status events.
Expectations
Saga orchestration, outbox pattern, idempotency design.
APIs, schema, eventing & retries.
Where it shows up
Retail marketplaces, online stores.
References
Outbox Pattern
Today's deep-dive
Tech
Spring Boot
Create Orders API with validation and @ControllerAdvice; profiles dev/test; write slice tests.
Where it shows up
Distributed databases for high-write, high-availability use cases such as IoT, recommendation engines, and time-series data in large-scale applications.
References
Baeldung Spring Guides
Spring Docs
Day25
60-day SSE plan
Elevator System • Payment Gateway • Heap / Priority Queue • Tech: Design Pattern
Open day
Algorithm
DSA
Medium
Heap / Priority Queue — Merge k Sorted Lists (LC 23)
Merge k sorted linked lists using a min-heap.
Example
Input: lists = [[1,4,5],[1,3,4],[2,6]] → Output: [1,1,2,3,4,4,5,6]
Where it shows up
Log mergers, multi-way merge in external sorting, streaming.
References
LeetCode
Heaps in Java
Low-level design
LLD
Elevator System
Requirements
Multiple elevators; up/down requests; scheduling.
Expectations
UML for Elevator, Request, Controller; strategy for scheduling.
Simulate requests & movement.
Where it shows up
High-rise building operations, dispatch systems.
References
Elevator LLD Guides
High-level design
HLD
Payment Gateway
Requirements
Payment intents, confirm/capture, refunds; idempotency; webhooks.
Expectations
PSP adapters, circuit breakers, retries with backoff.
Audit logs; PCI isolation considerations.
Where it shows up
Fintech, marketplaces, subscriptions.
References
Idempotency Keys
Today's deep-dive
Tech
Design Pattern
Implement Strategy + Observer around a pricing/notification domain. Unit test swapping strategies without touching clients.
Where it shows up
Event streaming, log aggregation, real-time analytics, and decoupled microservices communication in scalable distributed systems.
References
Refactoring Guru
Head First Design Patterns Notes
Day26
60-day SSE plan
Cache (LRU) • Uber-style Ride Hailing • Two Pointers • Tech: Advanced Java
Open day
Algorithm
DSA
Medium
Two Pointers — Trapping Rain Water (LC 42)
Compute trapped water using two-pointer approach.
Example
Input: [0,1,0,2,1,0,1,3,2,1,2,1] → Output: 6
Where it shows up
Resource aggregation, elevation maps, histogram problems.
References
LeetCode
Two Pointers
Low-level design
LLD
Cache (LRU)
Requirements
Put/get with capacity; LRU eviction policy.
Expectations
Classes for LRUCache using LinkedHashMap or custom DLL+HashMap.
Complexity guarantees O(1) operations.
Where it shows up
Databases, OS page cache, web caching layers.
References
LRU Patterns
High-level design
HLD
Uber-style Ride Hailing
Requirements
Driver/rider match by proximity; live location; trip lifecycle; pricing.
Expectations
Geo-indexing (geohash), matching service, WS gateway, storage model.
Regional partitions, degradation strategies.
Where it shows up
Mobility platforms, logistics dispatch.
References
Geohash Basics
Today's deep-dive
Tech
Advanced Java
Explore JVM memory (heap, metaspace), class loading, JIT; write a micro-benchmark of String vs StringBuilder concatenation.
Where it shows up
Performance tuning in enterprise Java apps, optimizing backend services, and understanding memory/CPU trade-offs in production systems.
References
JVM Spec (overview)
JMH Intro
Day27
60-day SSE plan
Tic Tac Toe • Distributed Cache (Redis-like) • Union-Find • Tech: Concurrency & Multithreading
Open day
Algorithm
DSA
Medium
Union-Find — Redundant Connection (LC 684)
Find the extra edge that creates a cycle in an undirected graph.
Example
Input: edges forming a cycle → Output: the extra edge
Where it shows up
Cycle detection, connectivity in networks, Kruskal pre-checks.
References
LeetCode
Disjoint Set (DSU)
Low-level design
LLD
Tic Tac Toe
Requirements
Design 3x3 game; win/draw detection; replay.
Expectations
Classes for Board, Player, Game; strategy for winner check.
Unit tests for edge cases.
Where it shows up
Turn-based game engines, board game modeling.
References
OOD Games
High-level design
HLD
Distributed Cache (Redis-like)
Requirements
SET/GET/DEL, TTL; sharding; replication; eviction policies.
Expectations
Consistent hashing, leader election, cache-aside/write-through.
Failure handling & split-brain prevention.
Where it shows up
Backend acceleration, DB offload, low latency reads.
References
Redis Cluster
Today's deep-dive
Tech
Concurrency & Multithreading
Build a bounded thread pool demo; compare synchronized vs ReentrantLock; show visibility with volatile.
Where it shows up
Building scalable chat/messaging platforms, real-time collaboration tools, and distributed systems requiring concurrency control.
References
Java Concurrency in Practice (notes)
java.util.concurrent
Day28
60-day SSE plan
Notification Service • Real-time Analytics Dashboard • DP (Strings) • Tech: Solr
Open day
Algorithm
DSA
Medium
Dynamic Programming — Longest Common Subsequence (LC 1143)
Find LCS length for two strings using DP.
Example
Input: text1="abcde", text2="ace" → Output: 3
Where it shows up
Diff tools, DNA sequence alignment, versioning.
References
LeetCode
GFG LCS
Low-level design
LLD
Notification Service
Requirements
Send email/SMS; templates; retries; rate limit.
Expectations
UML for Template, Request, Channel; retry/backoff design.
Mock provider integration tests.
Where it shows up
E-commerce, fintech transactional messaging.
References
Retry/Backoff
High-level design
HLD
Real-time Analytics Dashboard
Requirements
Ingest events at scale; compute aggregates; serve queries in seconds.
Expectations
Event ingestion, stream processing, materialized views, TSDB/NoSQL.
Exactly-once/at-least-once semantics; reprocessing strategy.
Where it shows up
Product analytics, ops dashboards, monitoring.
References
Kafka Streams 101
Today's deep-dive
Tech
Solr
Run Solr in Docker; create a core; index sample JSON; test analyzers & facets.
Where it shows up
Hands-on SQL optimization, analytics, and reporting for business intelligence, data warehousing, and application performance tuning.
References
Solr Guide
Solr Docker
Day29
60-day SSE plan
ATM System • Video Streaming • Backtracking • Tech: Cassandra
Open day
Algorithm
DSA
Medium
Backtracking — Permutations (LC 46)
Generate all permutations of an array.
Example
Input: [1,2,3] → Output: all permutations
Where it shows up
State space search, scheduling, puzzle solvers.
References
LeetCode
Backtracking Patterns
Low-level design
LLD
ATM System
Requirements
Authenticate, withdraw, deposit, balance; ATM state & cash bins.
Expectations
UML for Account, Card, ATM, Transaction; state pattern for ATM.
Simulate basic flows with validations.
Where it shows up
Bank ATMs; kiosk-based transactional systems.
References
OOD Banking
High-level design
HLD
Video Streaming
Requirements
Upload → transcode → store renditions → stream via HLS/DASH.
Expectations
Worker pipeline, object storage, CDN, player ABR logic basics.
Metadata & search index; cost controls.
Where it shows up
YouTube/OTT, education platforms.
References
HLS Spec
Today's deep-dive
Tech
Cassandra
Model a table for user_events with proper partition key; write queries; test CL=QUORUM reads/writes.
Where it shows up
Containerized deployments, CI/CD pipelines, and scalable infrastructure management in cloud-native environments.
References
Cassandra Docs
Data Modeling
Day30
60-day SSE plan
Chess Game • Dropbox/Google Drive-like Storage • Trees (BFS) • Tech: Kafka
Open day
Algorithm
DSA
Medium
Trees — Binary Tree Level Order Traversal (LC 102)
Return level-order traversal using BFS.
Example
Input: binary tree → Output: list of levels
Where it shows up
Hierarchy traversals, org charts, breadth-wise processing.
References
LeetCode
GFG Tree Traversals
Low-level design
LLD
Chess Game
Requirements
Represent board, moves, turns; check/checkmate validation.
Expectations
UML for Board, Piece hierarchy, Move, Game; strategy for legal moves.
Basic engine for move validation.
Where it shows up
Game engines, rules validation systems.
References
Chess Design
High-level design
HLD
Dropbox/Google Drive-like Storage
Requirements
Chunked uploads, versioning, deduplication, sharing links.
Expectations
Content-addressable storage; metadata vs blob separation; sync engine.
Conflict resolution approach.
Where it shows up
Consumer & enterprise file sync/share.
References
Content-addressable Storage
Today's deep-dive
Tech
Kafka
Write a producer & consumer; demonstrate consumer groups and rebalancing; handle offsets safely.
Where it shows up
Orchestrating containerized workloads, automating deployments, scaling, and managing microservices in production environments.
References
Kafka Docs
Confluent Patterns
Day31
60-day SSE plan
Parking Lot System • URL Shortener • Sliding Window • Tech: SQL Queries Practice
Open day
Algorithm
DSA
Medium
Sliding Window — Minimum Window Substring (LC 76)
Find the smallest substring of s that contains all characters of t.
Example
Input: s="ADOBECODEBANC", t="ABC" → Output: "BANC"
Where it shows up
Token scanning, query parsing, log/event windowing.
References
LeetCode
NeetCode Sliding Window
GFG Sliding Window
Low-level design
LLD
Parking Lot System
Requirements
Multiple floors, spot types (Car/Bike/Truck).
Park, unpark, ticketing, fee calculation.
Expectations
Provide UML class diagram (ParkingLot, Floor, Spot, Vehicle, Ticket, Payment).
Implement core classes and simulate park/unpark; discuss extendability.
Where it shows up
Real parking garages in malls/airports.
References
Grokking OOD
UML Basics
High-level design
HLD
URL Shortener
Requirements
Shorten URLs, redirect keys, optional custom alias, basic stats.
Scale to millions of reads/day; low latency redirects.
Expectations
APIs, DB schema, key generation, cache-aside, sharding strategy.
Trade-offs: SQL vs NoSQL, consistency, TTL, analytics pipeline.
Where it shows up
Bitly/Twitter links, marketing campaigns, QR codes.
References
System Design Primer
Educative Grokking SD
Today's deep-dive
Tech
SQL Queries Practice
Practice joins, window functions, indexing & EXPLAIN on sample schema; optimize a slow query.
Where it shows up
Performance tuning in enterprise Java apps, optimizing backend services, and understanding memory/CPU trade-offs in production systems.
References
SQLZoo
Use The Index, Luke
Day32
60-day SSE plan
Rate Limiter (Token Bucket) • WhatsApp-like Chat • Binary Search on Answer • Tech: Docker
Open day
Algorithm
DSA
Medium
Binary Search on Answer — Capacity to Ship Packages (LC 1011)
Minimize ship capacity so all packages ship within D days using feasibility check.
Example
Input: weights=[1,2,3,1,1], D=4 → Output: 3
Where it shows up
Capacity planning, throughput sizing, SLO budgeting.
References
LeetCode
Binary Search Patterns
Low-level design
LLD
Rate Limiter (Token Bucket)
Requirements
Per-user limits with rate & capacity.
Thread-safe tryAcquire(); refill over time.
Expectations
UML for TokenBucket & RateLimiterService.
Concurrency-safe Java implementation + tests.
Where it shows up
API gateways, throttling abusive clients.
References
Token Bucket
Java Concurrency
High-level design
HLD
WhatsApp-like Chat
Requirements
1:1 + group messaging, delivery/read receipts, offline sync.
Real-time delivery via WebSockets; scalable fan-out.
Expectations
APIs, WebSocket fan-out, message broker, storage model.
Multi-region strategy, exactly-once UX via idempotency keys.
Where it shows up
Consumer chat apps, support chat in products.
References
Gaurav Sen Chat SD
Today's deep-dive
Tech
Docker
Create multi-stage build for a Spring Boot app; add HEALTHCHECK; test graceful shutdown.
Where it shows up
Building scalable chat/messaging platforms, real-time collaboration tools, and distributed systems requiring concurrency control.
References
Dockerfile Best Practices
Day33
60-day SSE plan
Splitwise • Twitter Feed / Timeline • Graph BFS/DFS • Tech: Kubernetes
Open day
Algorithm
DSA
Medium
Graph BFS — Number of Islands (LC 200)
Count connected components in a grid via BFS/DFS.
Example
Input: grid with 1s and 0s → Output: number of islands
Where it shows up
Cluster detection, network partitioning, region counting.
References
LeetCode
GFG BFS/DFS
Low-level design
LLD
Splitwise
Requirements
Create groups, add expenses, split & settle balances.
Expectations
UML for Group, Expense, Split, BalanceSheet.
Implement core services; show simplified settle-up.
Where it shows up
Bill-splitting consumer apps, travel groups.
References
OOD Examples
High-level design
HLD
Twitter Feed / Timeline
Requirements
Post, follow, generate home timeline with pagination and basic ranking.
Expectations
Fan-out on write vs read; caches; cold-start handling.
Explain hybrid approach for celebs vs normals.
Where it shows up
Social feeds, enterprise activity streams.
References
System Design Primer
Today's deep-dive
Tech
Kubernetes
Deploy app with Deployment + Service; add readiness/liveness probes; rolling update.
Where it shows up
API development for SaaS products, public web services, and internal microservices requiring robust, scalable, and maintainable interfaces.
References
K8s Docs
Day34
60-day SSE plan
Logger Service • E-commerce Order Service • Graph Topological Sort • Tech: Microservices
Open day
Algorithm
DSA
Medium
Graph Topological Sort — Course Schedule (LC 207)
Detect if you can finish all courses given prerequisites via Kahn's or DFS.
Example
Input: prerequisites edges → Output: true/false
Where it shows up
Dependency resolution, job scheduling, build pipelines.
References
LeetCode
Topological Sort (Kahn)
Low-level design
LLD
Logger Service
Requirements
Append logs concurrently; rotate file on size/time.
Expectations
Thread-safe singleton; async queue & writer thread.
Demonstrate rotation strategy.
Where it shows up
Service logging, audit trails, observability.
References
Java Logging
High-level design
HLD
E-commerce Order Service
Requirements
Create/manage orders; inventory reserve; payment auth; status events.
Expectations
Saga orchestration, outbox pattern, idempotency design.
APIs, schema, eventing & retries.
Where it shows up
Retail marketplaces, online stores.
References
Outbox Pattern
Today's deep-dive
Tech
Microservices
Draw service boundaries for a small e-commerce; define contracts, versioning, and observability (metrics, logs, traces).
Where it shows up
Automated testing, continuous integration, and quality assurance in software development lifecycles.
References
Microservices.io
Day35
60-day SSE plan
Elevator System • Payment Gateway • Heap / Priority Queue • Tech: REST APIs
Open day
Algorithm
DSA
Medium
Heap / Priority Queue — Merge k Sorted Lists (LC 23)
Merge k sorted linked lists using a min-heap.
Example
Input: lists = [[1,4,5],[1,3,4],[2,6]] → Output: [1,1,2,3,4,4,5,6]
Where it shows up
Log mergers, multi-way merge in external sorting, streaming.
References
LeetCode
Heaps in Java
Low-level design
LLD
Elevator System
Requirements
Multiple elevators; up/down requests; scheduling.
Expectations
UML for Elevator, Request, Controller; strategy for scheduling.
Simulate requests & movement.
Where it shows up
High-rise building operations, dispatch systems.
References
Elevator LLD Guides
High-level design
HLD
Payment Gateway
Requirements
Payment intents, confirm/capture, refunds; idempotency; webhooks.
Expectations
PSP adapters, circuit breakers, retries with backoff.
Audit logs; PCI isolation considerations.
Where it shows up
Fintech, marketplaces, subscriptions.
References
Idempotency Keys
Today's deep-dive
Tech
REST APIs
Define REST guidelines; build a small API with pagination, filtering, error envelopes, idempotent PUT/PATCH.
Where it shows up
API development for SaaS products, public web services, and internal microservices requiring robust, scalable, and maintainable interfaces.
References
Microsoft REST Guidelines
Day36
60-day SSE plan
Cache (LRU) • Uber-style Ride Hailing • Two Pointers • Tech: Spring Boot
Open day
Algorithm
DSA
Medium
Two Pointers — Trapping Rain Water (LC 42)
Compute trapped water using two-pointer approach.
Example
Input: [0,1,0,2,1,0,1,3,2,1,2,1] → Output: 6
Where it shows up
Resource aggregation, elevation maps, histogram problems.
References
LeetCode
Two Pointers
Low-level design
LLD
Cache (LRU)
Requirements
Put/get with capacity; LRU eviction policy.
Expectations
Classes for LRUCache using LinkedHashMap or custom DLL+HashMap.
Complexity guarantees O(1) operations.
Where it shows up
Databases, OS page cache, web caching layers.
References
LRU Patterns
High-level design
HLD
Uber-style Ride Hailing
Requirements
Driver/rider match by proximity; live location; trip lifecycle; pricing.
Expectations
Geo-indexing (geohash), matching service, WS gateway, storage model.
Regional partitions, degradation strategies.
Where it shows up
Mobility platforms, logistics dispatch.
References
Geohash Basics
Today's deep-dive
Tech
Spring Boot
Create Orders API with validation and @ControllerAdvice; profiles dev/test; write slice tests.
Where it shows up
Building production-grade Java microservices, rapid prototyping, and enterprise backend development with robust configuration and testing support.
References
Baeldung Spring Guides
Spring Docs
Day37
60-day SSE plan
Tic Tac Toe • Distributed Cache (Redis-like) • Union-Find • Tech: Design Pattern
Open day
Algorithm
DSA
Medium
Union-Find — Redundant Connection (LC 684)
Find the extra edge that creates a cycle in an undirected graph.
Example
Input: edges forming a cycle → Output: the extra edge
Where it shows up
Cycle detection, connectivity in networks, Kruskal pre-checks.
References
LeetCode
Disjoint Set (DSU)
Low-level design
LLD
Tic Tac Toe
Requirements
Design 3x3 game; win/draw detection; replay.
Expectations
Classes for Board, Player, Game; strategy for winner check.
Unit tests for edge cases.
Where it shows up
Turn-based game engines, board game modeling.
References
OOD Games
High-level design
HLD
Distributed Cache (Redis-like)
Requirements
SET/GET/DEL, TTL; sharding; replication; eviction policies.
Expectations
Consistent hashing, leader election, cache-aside/write-through.
Failure handling & split-brain prevention.
Where it shows up
Backend acceleration, DB offload, low latency reads.
References
Redis Cluster
Today's deep-dive
Tech
Design Pattern
Implement Strategy + Observer around a pricing/notification domain. Unit test swapping strategies without touching clients.
Where it shows up
Automated testing, continuous integration, and quality assurance in software development lifecycles.
References
Refactoring Guru
Head First Design Patterns Notes
Day38
60-day SSE plan
Notification Service • Real-time Analytics Dashboard • DP (Strings) • Tech: Advanced Java
Open day
Algorithm
DSA
Medium
Dynamic Programming — Longest Common Subsequence (LC 1143)
Find LCS length for two strings using DP.
Example
Input: text1="abcde", text2="ace" → Output: 3
Where it shows up
Diff tools, DNA sequence alignment, versioning.
References
LeetCode
GFG LCS
Low-level design
LLD
Notification Service
Requirements
Send email/SMS; templates; retries; rate limit.
Expectations
UML for Template, Request, Channel; retry/backoff design.
Mock provider integration tests.
Where it shows up
E-commerce, fintech transactional messaging.
References
Retry/Backoff
High-level design
HLD
Real-time Analytics Dashboard
Requirements
Ingest events at scale; compute aggregates; serve queries in seconds.
Expectations
Event ingestion, stream processing, materialized views, TSDB/NoSQL.
Exactly-once/at-least-once semantics; reprocessing strategy.
Where it shows up
Product analytics, ops dashboards, monitoring.
References
Kafka Streams 101
Today's deep-dive
Tech
Advanced Java
Explore JVM memory (heap, metaspace), class loading, JIT; write a micro-benchmark of String vs StringBuilder concatenation.
Where it shows up
—
References
JVM Spec (overview)
JMH Intro
Day39
60-day SSE plan
ATM System • Video Streaming • Backtracking • Tech: Concurrency & Multithreading
Open day
Algorithm
DSA
Medium
Backtracking — Permutations (LC 46)
Generate all permutations of an array.
Example
Input: [1,2,3] → Output: all permutations
Where it shows up
State space search, scheduling, puzzle solvers.
References
LeetCode
Backtracking Patterns
Low-level design
LLD
ATM System
Requirements
Authenticate, withdraw, deposit, balance; ATM state & cash bins.
Expectations
UML for Account, Card, ATM, Transaction; state pattern for ATM.
Simulate basic flows with validations.
Where it shows up
Bank ATMs; kiosk-based transactional systems.
References
OOD Banking
High-level design
HLD
Video Streaming
Requirements
Upload → transcode → store renditions → stream via HLS/DASH.
Expectations
Worker pipeline, object storage, CDN, player ABR logic basics.
Metadata & search index; cost controls.
Where it shows up
YouTube/OTT, education platforms.
References
HLS Spec
Today's deep-dive
Tech
Concurrency & Multithreading
Build a bounded thread pool demo; compare synchronized vs ReentrantLock; show visibility with volatile.
Where it shows up
—
References
Java Concurrency in Practice (notes)
java.util.concurrent
Day40
60-day SSE plan
Chess Game • Dropbox/Google Drive-like Storage • Trees (BFS) • Tech: Solr
Open day
Algorithm
DSA
Medium
Trees — Binary Tree Level Order Traversal (LC 102)
Return level-order traversal using BFS.
Example
Input: binary tree → Output: list of levels
Where it shows up
Hierarchy traversals, org charts, breadth-wise processing.
References
LeetCode
GFG Tree Traversals
Low-level design
LLD
Chess Game
Requirements
Represent board, moves, turns; check/checkmate validation.
Expectations
UML for Board, Piece hierarchy, Move, Game; strategy for legal moves.
Basic engine for move validation.
Where it shows up
Game engines, rules validation systems.
References
Chess Design
High-level design
HLD
Dropbox/Google Drive-like Storage
Requirements
Chunked uploads, versioning, deduplication, sharing links.
Expectations
Content-addressable storage; metadata vs blob separation; sync engine.
Conflict resolution approach.
Where it shows up
Consumer & enterprise file sync/share.
References
Content-addressable Storage
Today's deep-dive
Tech
Solr
Run Solr in Docker; create a core; index sample JSON; test analyzers & facets.
Where it shows up
—
References
Solr Guide
Solr Docker
Day41
60-day SSE plan
Parking Lot System • URL Shortener • Sliding Window • Tech: Cassandra
Open day
Algorithm
DSA
Medium
Sliding Window — Minimum Window Substring (LC 76)
Find the smallest substring of s that contains all characters of t.
Example
Input: s="ADOBECODEBANC", t="ABC" → Output: "BANC"
Where it shows up
Token scanning, query parsing, log/event windowing.
References
LeetCode
NeetCode Sliding Window
GFG Sliding Window
Low-level design
LLD
Parking Lot System
Requirements
Multiple floors, spot types (Car/Bike/Truck).
Park, unpark, ticketing, fee calculation.
Expectations
Provide UML class diagram (ParkingLot, Floor, Spot, Vehicle, Ticket, Payment).
Implement core classes and simulate park/unpark; discuss extendability.
Where it shows up
Real parking garages in malls/airports.
References
Grokking OOD
UML Basics
High-level design
HLD
URL Shortener
Requirements
Shorten URLs, redirect keys, optional custom alias, basic stats.
Scale to millions of reads/day; low latency redirects.
Expectations
APIs, DB schema, key generation, cache-aside, sharding strategy.
Trade-offs: SQL vs NoSQL, consistency, TTL, analytics pipeline.
Where it shows up
Bitly/Twitter links, marketing campaigns, QR codes.
References
System Design Primer
Educative Grokking SD
Today's deep-dive
Tech
Cassandra
Model a table for user_events with proper partition key; write queries; test CL=QUORUM reads/writes.
Where it shows up
—
References
Cassandra Docs
Data Modeling
Day42
60-day SSE plan
Rate Limiter (Token Bucket) • WhatsApp-like Chat • Binary Search on Answer • Tech: Kafka
Open day
Algorithm
DSA
Medium
Binary Search on Answer — Capacity to Ship Packages (LC 1011)
Minimize ship capacity so all packages ship within D days using feasibility check.
Example
Input: weights=[1,2,3,1,1], D=4 → Output: 3
Where it shows up
Capacity planning, throughput sizing, SLO budgeting.
References
LeetCode
Binary Search Patterns
Low-level design
LLD
Rate Limiter (Token Bucket)
Requirements
Per-user limits with rate & capacity.
Thread-safe tryAcquire(); refill over time.
Expectations
UML for TokenBucket & RateLimiterService.
Concurrency-safe Java implementation + tests.
Where it shows up
API gateways, throttling abusive clients.
References
Token Bucket
Java Concurrency
High-level design
HLD
WhatsApp-like Chat
Requirements
1:1 + group messaging, delivery/read receipts, offline sync.
Real-time delivery via WebSockets; scalable fan-out.
Expectations
APIs, WebSocket fan-out, message broker, storage model.
Multi-region strategy, exactly-once UX via idempotency keys.
Where it shows up
Consumer chat apps, support chat in products.
References
Gaurav Sen Chat SD
Today's deep-dive
Tech
Kafka
Write a producer & consumer; demonstrate consumer groups and rebalancing; handle offsets safely.
Where it shows up
—
References
Kafka Docs
Confluent Patterns
Day43
60-day SSE plan
Splitwise • Twitter Feed / Timeline • Graph BFS/DFS • Tech: SQL Queries Practice
Open day
Algorithm
DSA
Medium
Graph BFS — Number of Islands (LC 200)
Count connected components in a grid via BFS/DFS.
Example
Input: grid with 1s and 0s → Output: number of islands
Where it shows up
Cluster detection, network partitioning, region counting.
References
LeetCode
GFG BFS/DFS
Low-level design
LLD
Splitwise
Requirements
Create groups, add expenses, split & settle balances.
Expectations
UML for Group, Expense, Split, BalanceSheet.
Implement core services; show simplified settle-up.
Where it shows up
Bill-splitting consumer apps, travel groups.
References
OOD Examples
High-level design
HLD
Twitter Feed / Timeline
Requirements
Post, follow, generate home timeline with pagination and basic ranking.
Expectations
Fan-out on write vs read; caches; cold-start handling.
Explain hybrid approach for celebs vs normals.
Where it shows up
Social feeds, enterprise activity streams.
References
System Design Primer
Today's deep-dive
Tech
SQL Queries Practice
Practice joins, window functions, indexing & EXPLAIN on sample schema; optimize a slow query.
Where it shows up
—
References
SQLZoo
Use The Index, Luke
Day44
60-day SSE plan
Logger Service • E-commerce Order Service • Graph Topological Sort • Tech: Docker
Open day
Algorithm
DSA
Medium
Graph Topological Sort — Course Schedule (LC 207)
Detect if you can finish all courses given prerequisites via Kahn's or DFS.
Example
Input: prerequisites edges → Output: true/false
Where it shows up
Dependency resolution, job scheduling, build pipelines.
References
LeetCode
Topological Sort (Kahn)
Low-level design
LLD
Logger Service
Requirements
Append logs concurrently; rotate file on size/time.
Expectations
Thread-safe singleton; async queue & writer thread.
Demonstrate rotation strategy.
Where it shows up
Service logging, audit trails, observability.
References
Java Logging
High-level design
HLD
E-commerce Order Service
Requirements
Create/manage orders; inventory reserve; payment auth; status events.
Expectations
Saga orchestration, outbox pattern, idempotency design.
APIs, schema, eventing & retries.
Where it shows up
Retail marketplaces, online stores.
References
Outbox Pattern
Today's deep-dive
Tech
Docker
Create multi-stage build for a Spring Boot app; add HEALTHCHECK; test graceful shutdown.
Where it shows up
—
References
Dockerfile Best Practices
Day45
60-day SSE plan
Elevator System • Payment Gateway • Heap / Priority Queue • Tech: Kubernetes
Open day
Algorithm
DSA
Medium
Heap / Priority Queue — Merge k Sorted Lists (LC 23)
Merge k sorted linked lists using a min-heap.
Example
Input: lists = [[1,4,5],[1,3,4],[2,6]] → Output: [1,1,2,3,4,4,5,6]
Where it shows up
Log mergers, multi-way merge in external sorting, streaming.
References
LeetCode
Heaps in Java
Low-level design
LLD
Elevator System
Requirements
Multiple elevators; up/down requests; scheduling.
Expectations
UML for Elevator, Request, Controller; strategy for scheduling.
Simulate requests & movement.
Where it shows up
High-rise building operations, dispatch systems.
References
Elevator LLD Guides
High-level design
HLD
Payment Gateway
Requirements
Payment intents, confirm/capture, refunds; idempotency; webhooks.
Expectations
PSP adapters, circuit breakers, retries with backoff.
Audit logs; PCI isolation considerations.
Where it shows up
Fintech, marketplaces, subscriptions.
References
Idempotency Keys
Today's deep-dive
Tech
Kubernetes
Deploy app with Deployment + Service; add readiness/liveness probes; rolling update.
Where it shows up
—
References
K8s Docs
Day46
60-day SSE plan
Cache (LRU) • Uber-style Ride Hailing • Two Pointers • Tech: Microservices
Open day
Algorithm
DSA
Hard
Two Pointers — Trapping Rain Water (LC 42)
Compute trapped water using two-pointer approach.
Example
Input: [0,1,0,2,1,0,1,3,2,1,2,1] → Output: 6
Where it shows up
Resource aggregation, elevation maps, histogram problems.
References
LeetCode
Two Pointers
Low-level design
LLD
Cache (LRU)
Requirements
Put/get with capacity; LRU eviction policy.
Expectations
Classes for LRUCache using LinkedHashMap or custom DLL+HashMap.
Complexity guarantees O(1) operations.
Where it shows up
Databases, OS page cache, web caching layers.
References
LRU Patterns
High-level design
HLD
Uber-style Ride Hailing
Requirements
Driver/rider match by proximity; live location; trip lifecycle; pricing.
Expectations
Geo-indexing (geohash), matching service, WS gateway, storage model.
Regional partitions, degradation strategies.
Where it shows up
Mobility platforms, logistics dispatch.
References
Geohash Basics
Today's deep-dive
Tech
Microservices
Draw service boundaries for a small e-commerce; define contracts, versioning, and observability (metrics, logs, traces).
Where it shows up
—
References
Microservices.io
Day47
60-day SSE plan
Tic Tac Toe • Distributed Cache (Redis-like) • Union-Find • Tech: REST APIs
Open day
Algorithm
DSA
Hard
Union-Find — Redundant Connection (LC 684)
Find the extra edge that creates a cycle in an undirected graph.
Example
Input: edges forming a cycle → Output: the extra edge
Where it shows up
Cycle detection, connectivity in networks, Kruskal pre-checks.
References
LeetCode
Disjoint Set (DSU)
Low-level design
LLD
Tic Tac Toe
Requirements
Design 3x3 game; win/draw detection; replay.
Expectations
Classes for Board, Player, Game; strategy for winner check.
Unit tests for edge cases.
Where it shows up
Turn-based game engines, board game modeling.
References
OOD Games
High-level design
HLD
Distributed Cache (Redis-like)
Requirements
SET/GET/DEL, TTL; sharding; replication; eviction policies.
Expectations
Consistent hashing, leader election, cache-aside/write-through.
Failure handling & split-brain prevention.
Where it shows up
Backend acceleration, DB offload, low latency reads.
References
Redis Cluster
Today's deep-dive
Tech
REST APIs
Define REST guidelines; build a small API with pagination, filtering, error envelopes, idempotent PUT/PATCH.
Where it shows up
—
References
Microsoft REST Guidelines
Day48
60-day SSE plan
Notification Service • Real-time Analytics Dashboard • DP (Strings) • Tech: Spring Boot
Open day
Algorithm
DSA
Hard
Dynamic Programming — Longest Common Subsequence (LC 1143)
Find LCS length for two strings using DP.
Example
Input: text1="abcde", text2="ace" → Output: 3
Where it shows up
Diff tools, DNA sequence alignment, versioning.
References
LeetCode
GFG LCS
Low-level design
LLD
Notification Service
Requirements
Send email/SMS; templates; retries; rate limit.
Expectations
UML for Template, Request, Channel; retry/backoff design.
Mock provider integration tests.
Where it shows up
E-commerce, fintech transactional messaging.
References
Retry/Backoff
High-level design
HLD
Real-time Analytics Dashboard
Requirements
Ingest events at scale; compute aggregates; serve queries in seconds.
Expectations
Event ingestion, stream processing, materialized views, TSDB/NoSQL.
Exactly-once/at-least-once semantics; reprocessing strategy.
Where it shows up
Product analytics, ops dashboards, monitoring.
References
Kafka Streams 101
Today's deep-dive
Tech
Spring Boot
Create Orders API with validation and @ControllerAdvice; profiles dev/test; write slice tests.
Where it shows up
—
References
Baeldung Spring Guides
Spring Docs
Day49
60-day SSE plan
ATM System • Video Streaming • Backtracking • Tech: Design Pattern
Open day
Algorithm
DSA
Hard
Backtracking — Permutations (LC 46)
Generate all permutations of an array.
Example
Input: [1,2,3] → Output: all permutations
Where it shows up
State space search, scheduling, puzzle solvers.
References
LeetCode
Backtracking Patterns
Low-level design
LLD
ATM System
Requirements
Authenticate, withdraw, deposit, balance; ATM state & cash bins.
Expectations
UML for Account, Card, ATM, Transaction; state pattern for ATM.
Simulate basic flows with validations.
Where it shows up
Bank ATMs; kiosk-based transactional systems.
References
OOD Banking
High-level design
HLD
Video Streaming
Requirements
Upload → transcode → store renditions → stream via HLS/DASH.
Expectations
Worker pipeline, object storage, CDN, player ABR logic basics.
Metadata & search index; cost controls.
Where it shows up
YouTube/OTT, education platforms.
References
HLS Spec
Today's deep-dive
Tech
Design Pattern
Implement Strategy + Observer around a pricing/notification domain. Unit test swapping strategies without touching clients.
Where it shows up
—
References
Refactoring Guru
Head First Design Patterns Notes
Day50
60-day SSE plan
Chess Game • Dropbox/Google Drive-like Storage • Trees (BFS) • Tech: Advanced Java
Open day
Algorithm
DSA
Hard
Trees — Binary Tree Level Order Traversal (LC 102)
Return level-order traversal using BFS.
Example
Input: binary tree → Output: list of levels
Where it shows up
Hierarchy traversals, org charts, breadth-wise processing.
References
LeetCode
GFG Tree Traversals
Low-level design
LLD
Chess Game
Requirements
Represent board, moves, turns; check/checkmate validation.
Expectations
UML for Board, Piece hierarchy, Move, Game; strategy for legal moves.
Basic engine for move validation.
Where it shows up
Game engines, rules validation systems.
References
Chess Design
High-level design
HLD
Dropbox/Google Drive-like Storage
Requirements
Chunked uploads, versioning, deduplication, sharing links.
Expectations
Content-addressable storage; metadata vs blob separation; sync engine.
Conflict resolution approach.
Where it shows up
Consumer & enterprise file sync/share.
References
Content-addressable Storage
Today's deep-dive
Tech
Advanced Java
Explore JVM memory (heap, metaspace), class loading, JIT; write a micro-benchmark of String vs StringBuilder concatenation.
Where it shows up
—
References
JVM Spec (overview)
JMH Intro
Day51
60-day SSE plan
Parking Lot System • URL Shortener • Sliding Window • Tech: Concurrency & Multithreading
Open day
Algorithm
DSA
Hard
Sliding Window — Minimum Window Substring (LC 76)
Find the smallest substring of s that contains all characters of t.
Example
Input: s="ADOBECODEBANC", t="ABC" → Output: "BANC"
Where it shows up
Token scanning, query parsing, log/event windowing.
References
LeetCode
NeetCode Sliding Window
GFG Sliding Window
Low-level design
LLD
Parking Lot System
Requirements
Multiple floors, spot types (Car/Bike/Truck).
Park, unpark, ticketing, fee calculation.
Expectations
Provide UML class diagram (ParkingLot, Floor, Spot, Vehicle, Ticket, Payment).
Implement core classes and simulate park/unpark; discuss extendability.
Where it shows up
Real parking garages in malls/airports.
References
Grokking OOD
UML Basics
High-level design
HLD
URL Shortener
Requirements
Shorten URLs, redirect keys, optional custom alias, basic stats.
Scale to millions of reads/day; low latency redirects.
Expectations
APIs, DB schema, key generation, cache-aside, sharding strategy.
Trade-offs: SQL vs NoSQL, consistency, TTL, analytics pipeline.
Where it shows up
Bitly/Twitter links, marketing campaigns, QR codes.
References
System Design Primer
Educative Grokking SD
Today's deep-dive
Tech
Concurrency & Multithreading
Build a bounded thread pool demo; compare synchronized vs ReentrantLock; show visibility with volatile.
Where it shows up
—
References
Java Concurrency in Practice (notes)
java.util.concurrent
Day52
60-day SSE plan
Rate Limiter (Token Bucket) • WhatsApp-like Chat • Binary Search on Answer • Tech: Solr
Open day
Algorithm
DSA
Hard
Binary Search on Answer — Capacity to Ship Packages (LC 1011)
Minimize ship capacity so all packages ship within D days using feasibility check.
Example
Input: weights=[1,2,3,1,1], D=4 → Output: 3
Where it shows up
Capacity planning, throughput sizing, SLO budgeting.
References
LeetCode
Binary Search Patterns
Low-level design
LLD
Rate Limiter (Token Bucket)
Requirements
Per-user limits with rate & capacity.
Thread-safe tryAcquire(); refill over time.
Expectations
UML for TokenBucket & RateLimiterService.
Concurrency-safe Java implementation + tests.
Where it shows up
API gateways, throttling abusive clients.
References
Token Bucket
Java Concurrency
High-level design
HLD
WhatsApp-like Chat
Requirements
1:1 + group messaging, delivery/read receipts, offline sync.
Real-time delivery via WebSockets; scalable fan-out.
Expectations
APIs, WebSocket fan-out, message broker, storage model.
Multi-region strategy, exactly-once UX via idempotency keys.
Where it shows up
Consumer chat apps, support chat in products.
References
Gaurav Sen Chat SD
Today's deep-dive
Tech
Solr
Run Solr in Docker; create a core; index sample JSON; test analyzers & facets.
Where it shows up
—
References
Solr Guide
Solr Docker
Day53
60-day SSE plan
Splitwise • Twitter Feed / Timeline • Graph BFS/DFS • Tech: Cassandra
Open day
Algorithm
DSA
Hard
Graph BFS — Number of Islands (LC 200)
Count connected components in a grid via BFS/DFS.
Example
Input: grid with 1s and 0s → Output: number of islands
Where it shows up
Cluster detection, network partitioning, region counting.
References
LeetCode
GFG BFS/DFS
Low-level design
LLD
Splitwise
Requirements
Create groups, add expenses, split & settle balances.
Expectations
UML for Group, Expense, Split, BalanceSheet.
Implement core services; show simplified settle-up.
Where it shows up
Bill-splitting consumer apps, travel groups.
References
OOD Examples
High-level design
HLD
Twitter Feed / Timeline
Requirements
Post, follow, generate home timeline with pagination and basic ranking.
Expectations
Fan-out on write vs read; caches; cold-start handling.
Explain hybrid approach for celebs vs normals.
Where it shows up
Social feeds, enterprise activity streams.
References
System Design Primer
Today's deep-dive
Tech
Cassandra
Model a table for user_events with proper partition key; write queries; test CL=QUORUM reads/writes.
Where it shows up
—
References
Cassandra Docs
Data Modeling
Day54
60-day SSE plan
Logger Service • E-commerce Order Service • Graph Topological Sort • Tech: Kafka
Open day
Algorithm
DSA
Hard
Graph Topological Sort — Course Schedule (LC 207)
Detect if you can finish all courses given prerequisites via Kahn's or DFS.
Example
Input: prerequisites edges → Output: true/false
Where it shows up
Dependency resolution, job scheduling, build pipelines.
References
LeetCode
Topological Sort (Kahn)
Low-level design
LLD
Logger Service
Requirements
Append logs concurrently; rotate file on size/time.
Expectations
Thread-safe singleton; async queue & writer thread.
Demonstrate rotation strategy.
Where it shows up
Service logging, audit trails, observability.
References
Java Logging
High-level design
HLD
E-commerce Order Service
Requirements
Create/manage orders; inventory reserve; payment auth; status events.
Expectations
Saga orchestration, outbox pattern, idempotency design.
APIs, schema, eventing & retries.
Where it shows up
Retail marketplaces, online stores.
References
Outbox Pattern
Today's deep-dive
Tech
Kafka
Write a producer & consumer; demonstrate consumer groups and rebalancing; handle offsets safely.
Where it shows up
—
References
Kafka Docs
Confluent Patterns
Day55
60-day SSE plan
Elevator System • Payment Gateway • Heap / Priority Queue • Tech: SQL Queries Practice
Open day
Algorithm
DSA
Hard
Heap / Priority Queue — Merge k Sorted Lists (LC 23)
Merge k sorted linked lists using a min-heap.
Example
Input: lists = [[1,4,5],[1,3,4],[2,6]] → Output: [1,1,2,3,4,4,5,6]
Where it shows up
Log mergers, multi-way merge in external sorting, streaming.
References
LeetCode
Heaps in Java
Low-level design
LLD
Elevator System
Requirements
Multiple elevators; up/down requests; scheduling.
Expectations
UML for Elevator, Request, Controller; strategy for scheduling.
Simulate requests & movement.
Where it shows up
High-rise building operations, dispatch systems.
References
Elevator LLD Guides
High-level design
HLD
Payment Gateway
Requirements
Payment intents, confirm/capture, refunds; idempotency; webhooks.
Expectations
PSP adapters, circuit breakers, retries with backoff.
Audit logs; PCI isolation considerations.
Where it shows up
Fintech, marketplaces, subscriptions.
References
Idempotency Keys
Today's deep-dive
Tech
SQL Queries Practice
Practice joins, window functions, indexing & EXPLAIN on sample schema; optimize a slow query.
Where it shows up
—
References
SQLZoo
Use The Index, Luke
Day56
60-day SSE plan
Cache (LRU) • Uber-style Ride Hailing • Two Pointers • Tech: Docker
Open day
Algorithm
DSA
Hard
Two Pointers — Trapping Rain Water (LC 42)
Compute trapped water using two-pointer approach.
Example
Input: [0,1,0,2,1,0,1,3,2,1,2,1] → Output: 6
Where it shows up
Resource aggregation, elevation maps, histogram problems.
References
LeetCode
Two Pointers
Low-level design
LLD
Cache (LRU)
Requirements
Put/get with capacity; LRU eviction policy.
Expectations
Classes for LRUCache using LinkedHashMap or custom DLL+HashMap.
Complexity guarantees O(1) operations.
Where it shows up
Databases, OS page cache, web caching layers.
References
LRU Patterns
High-level design
HLD
Uber-style Ride Hailing
Requirements
Driver/rider match by proximity; live location; trip lifecycle; pricing.
Expectations
Geo-indexing (geohash), matching service, WS gateway, storage model.
Regional partitions, degradation strategies.
Where it shows up
Mobility platforms, logistics dispatch.
References
Geohash Basics
Today's deep-dive
Tech
Docker
Create multi-stage build for a Spring Boot app; add HEALTHCHECK; test graceful shutdown.
Where it shows up
—
References
Dockerfile Best Practices
Day57
60-day SSE plan
Tic Tac Toe • Distributed Cache (Redis-like) • Union-Find • Tech: Kubernetes
Open day
Algorithm
DSA
Hard
Union-Find — Redundant Connection (LC 684)
Find the extra edge that creates a cycle in an undirected graph.
Example
Input: edges forming a cycle → Output: the extra edge
Where it shows up
Cycle detection, connectivity in networks, Kruskal pre-checks.
References
LeetCode
Disjoint Set (DSU)
Low-level design
LLD
Tic Tac Toe
Requirements
Design 3x3 game; win/draw detection; replay.
Expectations
Classes for Board, Player, Game; strategy for winner check.
Unit tests for edge cases.
Where it shows up
Turn-based game engines, board game modeling.
References
OOD Games
High-level design
HLD
Distributed Cache (Redis-like)
Requirements
SET/GET/DEL, TTL; sharding; replication; eviction policies.
Expectations
Consistent hashing, leader election, cache-aside/write-through.
Failure handling & split-brain prevention.
Where it shows up
Backend acceleration, DB offload, low latency reads.
References
Redis Cluster
Today's deep-dive
Tech
Kubernetes
Deploy app with Deployment + Service; add readiness/liveness probes; rolling update.
Where it shows up
—
References
K8s Docs
Day58
60-day SSE plan
Notification Service • Real-time Analytics Dashboard • DP (Strings) • Tech: Microservices
Open day
Algorithm
DSA
Hard
Dynamic Programming — Longest Common Subsequence (LC 1143)
Find LCS length for two strings using DP.
Example
Input: text1="abcde", text2="ace" → Output: 3
Where it shows up
Diff tools, DNA sequence alignment, versioning.
References
LeetCode
GFG LCS
Low-level design
LLD
Notification Service
Requirements
Send email/SMS; templates; retries; rate limit.
Expectations
UML for Template, Request, Channel; retry/backoff design.
Mock provider integration tests.
Where it shows up
E-commerce, fintech transactional messaging.
References
Retry/Backoff
High-level design
HLD
Real-time Analytics Dashboard
Requirements
Ingest events at scale; compute aggregates; serve queries in seconds.
Expectations
Event ingestion, stream processing, materialized views, TSDB/NoSQL.
Exactly-once/at-least-once semantics; reprocessing strategy.
Where it shows up
Product analytics, ops dashboards, monitoring.
References
Kafka Streams 101
Today's deep-dive
Tech
Microservices
Draw service boundaries for a small e-commerce; define contracts, versioning, and observability (metrics, logs, traces).
Where it shows up
—
References
Microservices.io
Day59
60-day SSE plan
ATM System • Video Streaming • Backtracking • Tech: REST APIs
Open day
Algorithm
DSA
Hard
Backtracking — Permutations (LC 46)
Generate all permutations of an array.
Example
Input: [1,2,3] → Output: all permutations
Where it shows up
State space search, scheduling, puzzle solvers.
References
LeetCode
Backtracking Patterns
Low-level design
LLD
ATM System
Requirements
Authenticate, withdraw, deposit, balance; ATM state & cash bins.
Expectations
UML for Account, Card, ATM, Transaction; state pattern for ATM.
Simulate basic flows with validations.
Where it shows up
Bank ATMs; kiosk-based transactional systems.
References
OOD Banking
High-level design
HLD
Video Streaming
Requirements
Upload → transcode → store renditions → stream via HLS/DASH.
Expectations
Worker pipeline, object storage, CDN, player ABR logic basics.
Metadata & search index; cost controls.
Where it shows up
YouTube/OTT, education platforms.
References
HLS Spec
Today's deep-dive
Tech
REST APIs
Define REST guidelines; build a small API with pagination, filtering, error envelopes, idempotent PUT/PATCH.
Where it shows up
—
References
Microsoft REST Guidelines
Day60
60-day SSE plan
Chess Game • Dropbox/Google Drive-like Storage • Trees (BFS) • Tech: Spring Boot
Open day
Algorithm
DSA
Hard
Trees — Binary Tree Level Order Traversal (LC 102)
Return level-order traversal using BFS.
Example
Input: binary tree → Output: list of levels
Where it shows up
Hierarchy traversals, org charts, breadth-wise processing.
References
LeetCode
GFG Tree Traversals
Low-level design
LLD
Chess Game
Requirements
Represent board, moves, turns; check/checkmate validation.
Expectations
UML for Board, Piece hierarchy, Move, Game; strategy for legal moves.
Basic engine for move validation.
Where it shows up
Game engines, rules validation systems.
References
Chess Design
High-level design
HLD
Dropbox/Google Drive-like Storage
Requirements
Chunked uploads, versioning, deduplication, sharing links.
Expectations
Content-addressable storage; metadata vs blob separation; sync engine.
Conflict resolution approach.
Where it shows up
Consumer & enterprise file sync/share.
References
Content-addressable Storage
Today's deep-dive
Tech
Spring Boot
Create Orders API with validation and @ControllerAdvice; profiles dev/test; write slice tests.
Where it shows up
—
References
Baeldung Spring Guides
Spring Docs

Every day, on one page

Sliding Window — Minimum Window Substring (LC 76)

Parking Lot System

URL Shortener

Design Pattern

Binary Search on Answer — Capacity to Ship Packages (LC 1011)

Rate Limiter (Token Bucket)

WhatsApp-like Chat

Advanced Java

Graph BFS — Number of Islands (LC 200)

Splitwise

Twitter Feed / Timeline

Concurrency & Multithreading

Graph Topological Sort — Course Schedule (LC 207)

Logger Service

E-commerce Order Service

Solr

Heap / Priority Queue — Merge k Sorted Lists (LC 23)

Elevator System

Payment Gateway

Cassandra

Two Pointers — Trapping Rain Water (LC 42)

Cache (LRU)

Uber-style Ride Hailing

Kafka

Union-Find — Redundant Connection (LC 684)

Tic Tac Toe

Distributed Cache (Redis-like)

SQL Queries Practice

Dynamic Programming — Longest Common Subsequence (LC 1143)

Notification Service

Real-time Analytics Dashboard

Docker

Backtracking — Permutations (LC 46)

ATM System

Video Streaming

Kubernetes

Trees — Binary Tree Level Order Traversal (LC 102)

Chess Game

Dropbox/Google Drive-like Storage

Microservices

Sliding Window — Minimum Window Substring (LC 76)

Parking Lot System

URL Shortener

REST APIs

Binary Search on Answer — Capacity to Ship Packages (LC 1011)

Rate Limiter (Token Bucket)

WhatsApp-like Chat

Spring Boot

Graph BFS — Number of Islands (LC 200)

Splitwise

Twitter Feed / Timeline

Design Pattern

Graph Topological Sort — Course Schedule (LC 207)

Logger Service

E-commerce Order Service

Advanced Java

Heap / Priority Queue — Merge k Sorted Lists (LC 23)

Elevator System

Payment Gateway

Concurrency & Multithreading

Two Pointers — Trapping Rain Water (LC 42)

Cache (LRU)

Uber-style Ride Hailing

Solr

Union-Find — Redundant Connection (LC 684)

Tic Tac Toe

Distributed Cache (Redis-like)

Cassandra

Dynamic Programming — Longest Common Subsequence (LC 1143)

Notification Service

Real-time Analytics Dashboard

Kafka

Backtracking — Permutations (LC 46)

ATM System

Video Streaming

SQL Queries Practice

Trees — Binary Tree Level Order Traversal (LC 102)

Chess Game

Dropbox/Google Drive-like Storage