LLD Problems

Java solutions to classic Low-Level Design (LLD) interview problems. Each problem lives in its own package under src/, with a problem.md describing the requirements and design as originally sketched out, a runnable Simulation/Main class, and (where a review pass has happened) an sde3-lld-review*.md documenting bugs found and fixed at an SDE3 bar.

Problems

Package	Problem	Key patterns
src/elevatordesign	Multi-elevator control system: hall/car calls, scheduling, maintenance/emergency modes	Command, Strategy (scheduling + car selection), Observer, per-elevator locking
src/loggingsystem	Logging framework with multiple levels and output destinations	Chain of handlers, Strategy (filters), Singleton
src/socialmedia	Social platform: profiles, follow graph, posts (text/media), likes, comments, DMs, feed	Facade, Factory, Strategy, Singleton

Each package's problem.md is the design brief (functional/non-functional requirements, core entities, chosen patterns) written before implementation.

Running

No build tool — plain javac/java against the src/ root. From the repo root:

# Elevator system
javac -d out $(find src/elevatordesign -name "*.java")
java -cp out elevatordesign.Simulation

# Logging system
javac -d out $(find src/loggingsystem -name "*.java")
java -cp out loggingsystem.Main

# Social media platform
javac -d out $(find src/socialmedia -name "*.java")
java -cp out socialmedia.Simulation

Or open the project in IntelliJ IDEA (mechinecode.iml is the module file) and run any Simulation/Main class directly.

Social media platform — architecture

socialmedia.service.SocialMediaManager is a Facade singleton in front of six independently-singletoned subsystems:

• ProfileService — profile CRUD.
• FollowService — follow graph (followingMap/followerMap), including cascade cleanup on profile delete.
• service.post — PostFactory (Factory pattern) routes to TextPostService/MediaPostService by PostType, both extending the shared PostService for storage/ownership logic.
• LikeService / CommentService — per-post likes and comments, with duplicate-like and duplicate-comment guards and ownership-checked removal.
• MessageService — direct messages, indexed both per-user and per-conversation.
• FeedService — Strategy pattern (FeedStrategy); currently RuntimeFeedStrategy generates a feed on read by pulling posts from everyone a user follows, rather than fanning out on write (the tradeoff is discussed in problem.md).

Deleting a post or a profile cascades: deletePost also removes that post's comments and likes; deleteProfile removes the user's own posts (which cascades their comments/likes), any comments/likes they left on other posts, their follow edges in both directions, and their conversations, before removing the profile itself.

See src/socialmedia/sde3-lld-review.md for the review history — bugs found (duplicate likes, missing ownership checks, orphaned data on delete, mutable value objects used as map keys) and how each was fixed and verified.

Elevator system — notes

See src/elevatordesign/sde3-lld-review-and-thread-safety.md for a detailed walkthrough of the concurrency bugs found in the original single-threaded design (lost requests from a non-thread-safe queue, a state-corrupting race between completeArrival() and addRequest(), and stale reads with no happens-before edge) and the two fixes considered: a coarse per-elevator lock (implemented) versus an actor-per-elevator model built on the existing Command pattern (proposed as the more "SDE3-flavored" answer).