60-Day LLD & Machine Coding Mastery Roadmap

Day	Problem	Concepts to Learn
1	Parking Lot	Encapsulation, abstraction, interfaces, inheritance vs composition, SRP, Factory, Strategy, class relationships
2	Vending Machine	State machines, State pattern, transitions, invariants, enum vs State objects, validation
3	ATM	Transaction boundaries, State, Chain of Responsibility, error handling, hardware/business logic separation
4	Elevator System	Controllers, scheduling Strategy, state, command queues, extensible algorithms
5	Chess	Polymorphism, OCP, inheritance vs composition, legal moves, Command and undo
6	Splitwise / Expense Sharing	SOLID, dependency injection, Strategy, value objects, rounding, money invariants
7	Library Management	Entities vs value objects, Book vs BookCopy, Repository abstraction, lending lifecycle
8	Car Rental	Factory, Strategy, reservation lifecycle, interface segregation, dependency inversion
9	Notification Framework	Strategy, Factory, Adapter, Observer, provider abstraction, test doubles
10	Food Delivery / Order Management	Aggregate boundaries, composition, state transitions, events, immutable price snapshots
11	Movie Ticket Booking	Seat inventory, holds, pricing Strategy, atomic reservation
12	Meeting Room Booking	Interval overlap, recurrence, availability, concurrency, conflict prevention
13	Doctor Appointment Booking	Scheduling, rescheduling, cancellation, idempotency, reminders
14	Hotel Reservation	Room types vs physical rooms, date ranges, pricing policies, cancellation
15	Flight Booking & Seat Assignment	Flight segments, seat maps, multi-leg bookings, inventory consistency
16	E-commerce Cart & Checkout	Cart vs Order, immutable price snapshots, checkout orchestration, transactions
17	Payment Processing	State, Strategy, Adapter, idempotency keys, duplicate callbacks, refunds, compensation
18	Coupon & Promotion Engine	Strategy, Chain of Responsibility, rule composition, priorities, conflicting discounts
19	Subscription Billing	Billing lifecycle, proration, retries, failed payments, time-based operations
20	Ride Sharing	Matching Strategy, fare calculation, driver assignment, state transitions, events
21	In-memory File System	Composite, tree modeling, recursion, path resolution
22	Organization Directory / File Explorer	Composite, Iterator, recursive traversal, filtering
23	Logger Framework	Chain of Responsibility, Decorator, Strategy, asynchronous logging
24	Configuration Management	Adapter, Observer, configuration precedence, validation
25	Metrics Collection Library	Factory, Template Method, Strategy, thread safety, efficient APIs
26	LRU Cache	HashMap + doubly linked list, O(1) operations, invariants, encapsulation
27	LFU Cache	Frequency buckets, eviction rules, data structure invariants
28	In-memory Key-Value Store with TTL	Expiration, lazy vs eager cleanup, clock abstraction, atomic operations
29	Rate Limiter	Strategy, Token Bucket, Sliding Window, concurrent counters
30	Pub-Sub Message Broker	Observer, subscriptions, acknowledgments, delivery semantics, retries, backpressure
31	Thread Pool	Producer-Consumer, worker lifecycle, bounded queues, shutdown, rejection
32	Thread-safe Blocking Queue	Mutexes, condition variables, wait/notify, bounded buffers, spurious wakeups
33	Read-Write Lock	Mutual exclusion, fairness, starvation, lock semantics
34	Concurrent LRU Cache	Atomic compound operations, critical sections, lock granularity, deadlocks
35	Task Scheduler	Priority queues, delayed jobs, worker pools, retries, cancellation
36	Job Orchestration Engine	DAG dependencies, parallel execution, failure propagation, task states
37	VM Upgrade Scheduler	Bounded concurrency, atomic task claims, leases, retries, health checks, rollback
38	Distributed Lock / Lease Manager	Expiry, ownership tokens, fencing tokens, stale owners, safe release
39	Concurrent Ticket / Inventory Reservation	Race conditions, atomic holds, expiry, double booking, idempotency
40	Parallel Batch Processing Engine	Task decomposition, partitioning, Fork-Join, backpressure, cancellation, partial failure
41	Tic-Tac-Toe	Game engine, board abstraction, polymorphism, validation, extensible rules
42	Snake and Ladder	Board, dice abstraction, turn management, deterministic tests
43	Connect Four	Board state, move validation, win detection, rule encapsulation
44	Card Game / Blackjack	Deck, Hand, Card value objects, polymorphism, lifecycle
45	Task Management / Kanban Board	Entities, state transitions, filtering/sorting Strategy, notifications
46	Shopping Cart with Multiple Pricing Rules	Decorator, Strategy, discount composition, tax, money precision
47	Text Editor with Undo/Redo	Command, Memento, history stacks, snapshots vs reversible operations
48	Spreadsheet Engine	Cell dependencies, Observer, dependency graph, recalculation, cycle detection
49	Search & Filtering Framework	Specification pattern, composable predicates, sorting, pagination
50	Workflow / Rules Engine	State, Strategy, Chain of Responsibility, rule evaluation, configurable workflows
51	API Client / SDK Framework	Adapter, Builder, Decorator, middleware, request/response abstractions
52	Resilient Service Client	Retries, exponential backoff, timeouts, circuit breaker, bulkhead, fallback
53	File Synchronization Engine	Conflict-resolution Strategy, versioning, checksums, incremental updates
54	In-memory Database with Transactions	Unit of Work, Command, rollback, isolation concepts
55	Dependency Injection Container	Object graphs, Factory, constructor injection, lifecycle scopes
56	Extensible Plugin Framework	Plugin contracts, Factory/Registry, Adapter, lifecycle, Open/Closed Principle
57	Event Processing Engine	Routing, filters, ordering, retries, duplicate delivery, idempotent handlers
58	Resource / Connection Pool	Bounded resources, reuse, timeouts, validation, safe return, shutdown

# 60-Day LLD Plan (1 problem/day) — Java

Day	Phase	Problem	Concepts it teaches
1	Foundations	Parking Lot	Requirements gathering, SRP, OCP, Strategy (pricing, spot allocation), Factory, enums vs polymorphism
2	Foundations	Tic-Tac-Toe	Encapsulation, O(1) win detection, generalizing to NxN, player abstraction
3	Foundations	Vending Machine	State pattern, illegal transitions, replacing if/else chains
4	Foundations	Library Management	Entity modeling, composition over inheritance, Repository, Observer (due-date alerts)
5	Foundations	Snake and Ladder	Factory, Strategy (dice), Builder (board config), separating game rules from game loop
6	Foundations	Coffee Machine	Decorator (add-ons), Template Method (brew steps), recipes as data, Builder
7	Foundations	ATM	State, Chain of Responsibility (cash dispensing), Liskov Substitution
8	Foundations	Traffic Signal	State with timers, Observer, scheduling basics
9	Foundations	Logging Framework	Singleton (and pitfalls), Chain of Responsibility, appenders, Factory, thread-safe init
10	Foundations	Notification Service	Strategy, Bridge, Adapter, retries, Interface Segregation, Dependency Inversion
11	Pattern-heavy domains	Splitwise	Strategy (split types), BigDecimal money, debt simplification graph
12	Pattern-heavy domains	Elevator System	State, SCAN/LOOK scheduling, Command, Observer
13	Pattern-heavy domains	Chess	Polymorphism, Template Method, Flyweight, Command (move undo), move validation
14	Pattern-heavy domains	Text Editor	Command, Memento, undo/redo stacks, Flyweight
15	Pattern-heavy domains	Shopping Cart and Pricing	Decorator and Strategy for discounts, Chain of Responsibility, value objects
16	Pattern-heavy domains	Coupon and Promotion Rules Engine	Specification, Composite rules, Interpreter-style rule evaluation
17	Pattern-heavy domains	Movie Ticket Booking	Seat holds with TTL, State, first concurrency hazards (double booking)
18	Pattern-heavy domains	Hotel and Flight Booking	Date-range overlap, inventory modeling, reservation lifecycle
19	Pattern-heavy domains	Car Rental	Builder, pricing strategies, availability search, Specification filters
20	Pattern-heavy domains	Food Delivery	Order lifecycle (State), Observer, Strategy for agent assignment
21	Pattern-heavy domains	Ride Sharing	Strategy (matching, pricing), State, Observer, geo abstractions
22	Pattern-heavy domains	Digital Wallet	Ledger design, immutability, idempotency keys, Command
23	Pattern-heavy domains	Payment Gateway	Adapter, Facade, Strategy routing, retries, idempotency
24	Pattern-heavy domains	Order Matching Engine	Priority queues, price-time priority, Strategy, Observer (trade events)
25	Pattern-heavy domains	Online Auction	Observer, Template Method, concurrent bids, closing rules
26	Frameworks and infra	Social Feed	Observer, Iterator, Strategy (ranking), pagination design
27	Frameworks and infra	Chat Application	Mediator, Observer, State (message status), Composite (groups)
28	Frameworks and infra	Pub-Sub System	Generics, topic registry, subscriber isolation, Observer done right
29	Frameworks and infra	File System	Composite, Iterator, Visitor (size, search), path handling
30	Frameworks and infra	Linux `find`	Specification, fluent API design, Visitor, Composite
31	Frameworks and infra	Spreadsheet	Dependency graph, topological sort, cycle detection, Observer
32	Frameworks and infra	In-Memory KV Store with TTL and Transactions	Nested transactions, Memento or Command, lazy vs active expiry
33	Frameworks and infra	LRU Cache	HashMap + doubly linked list, generics, API design
34	Frameworks and infra	LFU and Pluggable Eviction	Strategy, policy abstraction, O(1) LFU
35	Frameworks and infra	Rate Limiter	Strategy (token bucket, sliding window), Factory, per-key state
36	Frameworks and infra	Middleware Pipeline	Chain of Responsibility, Decorator, Proxy, ordering and short-circuiting
37	Frameworks and infra	Feature Flags and Config Manager	Observer (hot reload), Proxy, Singleton vs DI, immutable snapshots
38	Frameworks and infra	DI Container	Reflection, Registry, Factory, Abstract Factory, Dependency Inversion in depth
39	Frameworks and infra	Workflow and State Machine Engine	State, Strategy, Builder DSL, guards and transitions
40	Frameworks and infra	Document and Report Generator	Builder, Visitor, Template Method, Bridge (PDF/HTML/DOCX), Prototype
41	Concurrency	Producer-Consumer and Blocking Queue	Monitors, wait/notify, ReentrantLock + Condition, bounded buffers
42	Concurrency	FooBar, Zero-Even-Odd and H2O	Thread ordering, Semaphore, CyclicBarrier, CountDownLatch
43	Concurrency	Dining Philosophers	Four deadlock conditions, lock ordering, tryLock, starvation
44	Concurrency	Custom Thread Pool	Worker lifecycle, task queue, rejection policy, graceful shutdown, Future
45	Concurrency	Read-Write Lock, Semaphore and Latch from Scratch	Fairness, reentrancy, how AQS-style synchronizers work
46	Concurrency	Thread-Safe Cache with TTL	ConcurrentHashMap, lock striping, cleanup threads, memory visibility
47	Concurrency	Job Scheduler	Cron and delay scheduling, DelayQueue, priorities, retry with backoff
48	Concurrency	DAG Task Executor	Dependency resolution, parallel execution, CompletableFuture, failure propagation
49	Concurrency	In-Memory Message Queue	Partitions, consumer groups, offsets, concurrent producers/consumers
50	Concurrency	Overselling-Proof Inventory and Ticketing	Pessimistic vs optimistic locking, CAS, versioning, AtomicInteger, LongAdder
51	Concurrency	Bank Transfers	Atomicity, deadlock-free multi-lock acquisition, consistency invariants
52	Concurrency	Parallel Web Crawler and Fork/Join Merge Sort	Work stealing, thread-safe visited sets, parallel streams and pitfalls
53	Concurrency	Circuit Breaker and Retry Library	State, Decorator, sliding-window metrics, atomics
54	Concurrency	Async Pipeline and Mini Actor Framework	CompletableFuture composition, message passing, backpressure, mailboxes
55	Concurrency	Metrics Library	Lock-free counters, histograms, Observer, low-contention design
56	Staff capstones	RBAC and Permissions	Proxy, Composite roles, policy evaluation, clean authorization API
57	Staff capstones	Audit Log and Event Sourcing	Immutable events, Command, replay, CQRS-lite at class level
58	Staff capstones	Mini-Git	Composite, content hashing, commit DAG, Memento, branching
59	Staff capstones	Multi-Tenant SaaS Billing	Many patterns together, money and proration, extensibility, plugin-style pricing plans
60	Staff capstones	Final Mock Day	Timed 45-min mock on an unseen problem (e.g., Calendar/Meeting Scheduler), then redo Parking Lot and compare with Day 1
61	Inventory with Optimistic Locking (standalone)	Version columns, CAS, retry with exponential backoff and jitter, when to pick optimistic vs pessimistic techinterview+1
62	Chunked File Upload	Upload session state machine, chunk manifest, idempotent part IDs, resumable transfers, checksum validation, parallel chunk upload techinterview+2
63	Sync Cursor Design (offline-first sync)	Cursor/version vectors, delta sync, conflict detection, last-write-wins vs merge, tombstones for deletes
64	Collaborative Document Editing with OT	Insert/Delete operations, transform function, server as ordering authority, operation log, snapshotting every N ops techinterview+1
65	Real-Time Presence and Cursor Sync	Ephemeral cursor events over pub/sub, throttling to ~10 Hz, cursor adjustment after remote ops applied techinterview+2
66	CRDT vs OT comparison design	Character-level vs operation-level CRDTs, causal context, convergence guarantees, when to pick CRDT over OT techinterview+1

Day	Problem	Concepts to Learn
1	Parking Lot	Encapsulation, abstraction, interfaces, inheritance vs composition, SRP, Factory, Strategy, class relationships
2	Vending Machine	State machines, State pattern, transitions, invariants, enum vs State objects, validation
3	ATM	Transaction boundaries, State, Chain of Responsibility, error handling, hardware/business logic separation
4	Elevator System	Controllers, scheduling Strategy, state, command queues, extensible algorithms
5	Chess	Polymorphism, OCP, inheritance vs composition, legal moves, Command and undo
6	Splitwise / Expense Sharing	SOLID, dependency injection, Strategy, value objects, rounding, money invariants
7	Library Management	Entities vs value objects, Book vs BookCopy, Repository abstraction, lending lifecycle
8	Car Rental	Factory, Strategy, reservation lifecycle, interface segregation, dependency inversion
9	Notification Framework	Strategy, Factory, Adapter, Observer, provider abstraction, test doubles
10	Food Delivery / Order Management	Aggregate boundaries, composition, state transitions, events, immutable price snapshots
11	Movie Ticket Booking	Seat inventory, holds, pricing Strategy, atomic reservation
12	Meeting Room Booking	Interval overlap, recurrence, availability, concurrency, conflict prevention
13	Doctor Appointment Booking	Scheduling, rescheduling, cancellation, idempotency, reminders
14	Hotel Reservation	Room types vs physical rooms, date ranges, pricing policies, cancellation
15	Flight Booking & Seat Assignment	Flight segments, seat maps, multi-leg bookings, inventory consistency
16	E-commerce Cart & Checkout	Cart vs Order, immutable price snapshots, checkout orchestration, transactions
17	Payment Processing	State, Strategy, Adapter, idempotency keys, duplicate callbacks, refunds, compensation
18	Coupon & Promotion Engine	Strategy, Chain of Responsibility, rule composition, priorities, conflicting discounts
19	Subscription Billing	Billing lifecycle, proration, retries, failed payments, time-based operations
20	Ride Sharing	Matching Strategy, fare calculation, driver assignment, state transitions, events
21	In-memory File System	Composite, tree modeling, recursion, path resolution
22	Organization Directory / File Explorer	Composite, Iterator, recursive traversal, filtering
23	Logger Framework	Chain of Responsibility, Decorator, Strategy, asynchronous logging
24	Configuration Management	Adapter, Observer, configuration precedence, validation
25	Metrics Collection Library	Factory, Template Method, Strategy, thread safety, efficient APIs
26	LRU Cache	HashMap + doubly linked list, O(1) operations, invariants, encapsulation
27	LFU Cache	Frequency buckets, eviction rules, data structure invariants
28	In-memory Key-Value Store with TTL	Expiration, lazy vs eager cleanup, clock abstraction, atomic operations
29	Rate Limiter	Strategy, Token Bucket, Sliding Window, concurrent counters
30	Pub-Sub Message Broker	Observer, subscriptions, acknowledgments, delivery semantics, retries, backpressure
31	Thread Pool	Producer-Consumer, worker lifecycle, bounded queues, shutdown, rejection
32	Thread-safe Blocking Queue	Mutexes, condition variables, wait/notify, bounded buffers, spurious wakeups
33	Read-Write Lock	Mutual exclusion, fairness, starvation, lock semantics
34	Concurrent LRU Cache	Atomic compound operations, critical sections, lock granularity, deadlocks
35	Task Scheduler	Priority queues, delayed jobs, worker pools, retries, cancellation
36	Job Orchestration Engine	DAG dependencies, parallel execution, failure propagation, task states
37	VM Upgrade Scheduler	Bounded concurrency, atomic task claims, leases, retries, health checks, rollback
38	Distributed Lock / Lease Manager	Expiry, ownership tokens, fencing tokens, stale owners, safe release
39	Concurrent Ticket / Inventory Reservation	Race conditions, atomic holds, expiry, double booking, idempotency
40	Parallel Batch Processing Engine	Task decomposition, partitioning, Fork-Join, backpressure, cancellation, partial failure
41	Tic-Tac-Toe	Game engine, board abstraction, polymorphism, validation, extensible rules
42	Snake and Ladder	Board, dice abstraction, turn management, deterministic tests
43	Connect Four	Board state, move validation, win detection, rule encapsulation
44	Card Game / Blackjack	Deck, Hand, Card value objects, polymorphism, lifecycle
45	Task Management / Kanban Board	Entities, state transitions, filtering/sorting Strategy, notifications
46	Shopping Cart with Multiple Pricing Rules	Decorator, Strategy, discount composition, tax, money precision
47	Text Editor with Undo/Redo	Command, Memento, history stacks, snapshots vs reversible operations
48	Spreadsheet Engine	Cell dependencies, Observer, dependency graph, recalculation, cycle detection
49	Search & Filtering Framework	Specification pattern, composable predicates, sorting, pagination
50	Workflow / Rules Engine	State, Strategy, Chain of Responsibility, rule evaluation, configurable workflows
51	API Client / SDK Framework	Adapter, Builder, Decorator, middleware, request/response abstractions
52	Resilient Service Client	Retries, exponential backoff, timeouts, circuit breaker, bulkhead, fallback
53	File Synchronization Engine	Conflict-resolution Strategy, versioning, checksums, incremental updates
54	In-memory Database with Transactions	Unit of Work, Command, rollback, isolation concepts
55	Dependency Injection Container	Object graphs, Factory, constructor injection, lifecycle scopes
56	Extensible Plugin Framework	Plugin contracts, Factory/Registry, Adapter, lifecycle, Open/Closed Principle
57	Event Processing Engine	Routing, filters, ordering, retries, duplicate delivery, idempotent handlers
58	Resource / Connection Pool	Bounded resources, reuse, timeouts, validation, safe return, shutdown

# 60-Day LLD Plan (1 problem/day) — Java

Day	Phase	Problem	Concepts it teaches
1	Foundations	Parking Lot	Requirements gathering, SRP, OCP, Strategy (pricing, spot allocation), Factory, enums vs polymorphism
2	Foundations	Tic-Tac-Toe	Encapsulation, O(1) win detection, generalizing to NxN, player abstraction
3	Foundations	Vending Machine	State pattern, illegal transitions, replacing if/else chains
4	Foundations	Library Management	Entity modeling, composition over inheritance, Repository, Observer (due-date alerts)
5	Foundations	Snake and Ladder	Factory, Strategy (dice), Builder (board config), separating game rules from game loop
6	Foundations	Coffee Machine	Decorator (add-ons), Template Method (brew steps), recipes as data, Builder
7	Foundations	ATM	State, Chain of Responsibility (cash dispensing), Liskov Substitution
8	Foundations	Traffic Signal	State with timers, Observer, scheduling basics
9	Foundations	Logging Framework	Singleton (and pitfalls), Chain of Responsibility, appenders, Factory, thread-safe init
10	Foundations	Notification Service	Strategy, Bridge, Adapter, retries, Interface Segregation, Dependency Inversion
11	Pattern-heavy domains	Splitwise	Strategy (split types), BigDecimal money, debt simplification graph
12	Pattern-heavy domains	Elevator System	State, SCAN/LOOK scheduling, Command, Observer
13	Pattern-heavy domains	Chess	Polymorphism, Template Method, Flyweight, Command (move undo), move validation
14	Pattern-heavy domains	Text Editor	Command, Memento, undo/redo stacks, Flyweight
15	Pattern-heavy domains	Shopping Cart and Pricing	Decorator and Strategy for discounts, Chain of Responsibility, value objects
16	Pattern-heavy domains	Coupon and Promotion Rules Engine	Specification, Composite rules, Interpreter-style rule evaluation
17	Pattern-heavy domains	Movie Ticket Booking	Seat holds with TTL, State, first concurrency hazards (double booking)
18	Pattern-heavy domains	Hotel and Flight Booking	Date-range overlap, inventory modeling, reservation lifecycle
19	Pattern-heavy domains	Car Rental	Builder, pricing strategies, availability search, Specification filters
20	Pattern-heavy domains	Food Delivery	Order lifecycle (State), Observer, Strategy for agent assignment
21	Pattern-heavy domains	Ride Sharing	Strategy (matching, pricing), State, Observer, geo abstractions
22	Pattern-heavy domains	Digital Wallet	Ledger design, immutability, idempotency keys, Command
23	Pattern-heavy domains	Payment Gateway	Adapter, Facade, Strategy routing, retries, idempotency
24	Pattern-heavy domains	Order Matching Engine	Priority queues, price-time priority, Strategy, Observer (trade events)
25	Pattern-heavy domains	Online Auction	Observer, Template Method, concurrent bids, closing rules
26	Frameworks and infra	Social Feed	Observer, Iterator, Strategy (ranking), pagination design
27	Frameworks and infra	Chat Application	Mediator, Observer, State (message status), Composite (groups)
28	Frameworks and infra	Pub-Sub System	Generics, topic registry, subscriber isolation, Observer done right
29	Frameworks and infra	File System	Composite, Iterator, Visitor (size, search), path handling
30	Frameworks and infra	Linux `find`	Specification, fluent API design, Visitor, Composite
31	Frameworks and infra	Spreadsheet	Dependency graph, topological sort, cycle detection, Observer
32	Frameworks and infra	In-Memory KV Store with TTL and Transactions	Nested transactions, Memento or Command, lazy vs active expiry
33	Frameworks and infra	LRU Cache	HashMap + doubly linked list, generics, API design
34	Frameworks and infra	LFU and Pluggable Eviction	Strategy, policy abstraction, O(1) LFU
35	Frameworks and infra	Rate Limiter	Strategy (token bucket, sliding window), Factory, per-key state
36	Frameworks and infra	Middleware Pipeline	Chain of Responsibility, Decorator, Proxy, ordering and short-circuiting
37	Frameworks and infra	Feature Flags and Config Manager	Observer (hot reload), Proxy, Singleton vs DI, immutable snapshots
38	Frameworks and infra	DI Container	Reflection, Registry, Factory, Abstract Factory, Dependency Inversion in depth
39	Frameworks and infra	Workflow and State Machine Engine	State, Strategy, Builder DSL, guards and transitions
40	Frameworks and infra	Document and Report Generator	Builder, Visitor, Template Method, Bridge (PDF/HTML/DOCX), Prototype
41	Concurrency	Producer-Consumer and Blocking Queue	Monitors, wait/notify, ReentrantLock + Condition, bounded buffers
42	Concurrency	FooBar, Zero-Even-Odd and H2O	Thread ordering, Semaphore, CyclicBarrier, CountDownLatch
43	Concurrency	Dining Philosophers	Four deadlock conditions, lock ordering, tryLock, starvation
44	Concurrency	Custom Thread Pool	Worker lifecycle, task queue, rejection policy, graceful shutdown, Future
45	Concurrency	Read-Write Lock, Semaphore and Latch from Scratch	Fairness, reentrancy, how AQS-style synchronizers work
46	Concurrency	Thread-Safe Cache with TTL	ConcurrentHashMap, lock striping, cleanup threads, memory visibility
47	Concurrency	Job Scheduler	Cron and delay scheduling, DelayQueue, priorities, retry with backoff
48	Concurrency	DAG Task Executor	Dependency resolution, parallel execution, CompletableFuture, failure propagation
49	Concurrency	In-Memory Message Queue	Partitions, consumer groups, offsets, concurrent producers/consumers
50	Concurrency	Overselling-Proof Inventory and Ticketing	Pessimistic vs optimistic locking, CAS, versioning, AtomicInteger, LongAdder
51	Concurrency	Bank Transfers	Atomicity, deadlock-free multi-lock acquisition, consistency invariants
52	Concurrency	Parallel Web Crawler and Fork/Join Merge Sort	Work stealing, thread-safe visited sets, parallel streams and pitfalls
53	Concurrency	Circuit Breaker and Retry Library	State, Decorator, sliding-window metrics, atomics
54	Concurrency	Async Pipeline and Mini Actor Framework	CompletableFuture composition, message passing, backpressure, mailboxes
55	Concurrency	Metrics Library	Lock-free counters, histograms, Observer, low-contention design
56	Staff capstones	RBAC and Permissions	Proxy, Composite roles, policy evaluation, clean authorization API
57	Staff capstones	Audit Log and Event Sourcing	Immutable events, Command, replay, CQRS-lite at class level
58	Staff capstones	Mini-Git	Composite, content hashing, commit DAG, Memento, branching
59	Staff capstones	Multi-Tenant SaaS Billing	Many patterns together, money and proration, extensibility, plugin-style pricing plans
60	Staff capstones	Final Mock Day	Timed 45-min mock on an unseen problem (e.g., Calendar/Meeting Scheduler), then redo Parking Lot and compare with Day 1
61	Inventory with Optimistic Locking (standalone)	Version columns, CAS, retry with exponential backoff and jitter, when to pick optimistic vs pessimistic techinterview+1
62	Chunked File Upload	Upload session state machine, chunk manifest, idempotent part IDs, resumable transfers, checksum validation, parallel chunk upload techinterview+2
63	Sync Cursor Design (offline-first sync)	Cursor/version vectors, delta sync, conflict detection, last-write-wins vs merge, tombstones for deletes
64	Collaborative Document Editing with OT	Insert/Delete operations, transform function, server as ordering authority, operation log, snapshotting every N ops techinterview+1
65	Real-Time Presence and Cursor Sync	Ephemeral cursor events over pub/sub, throttling to ~10 Hz, cursor adjustment after remote ops applied techinterview+2
66	CRDT vs OT comparison design	Character-level vs operation-level CRDTs, causal context, convergence guarantees, when to pick CRDT over OT techinterview+1