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Design In-Process Pub-Sub

Topics, subscribers, and delivery guarantees in one JVM.

Advanced15 min read

Read these first

Why this matters

  • publish(topic, message) delivers to all subscribers
  • Scope — what "Topic registry" covers inside In-Process Pub-Sub and what it does not
  • Scope — what "Delivery" covers inside In-Process Pub-Sub and what it does not
  • Scope — what "Error handling" covers inside In-Process Pub-Sub and what it does not
  • In-memory systems test data structures + thread safety together.
SubjectnotifyObservers()
Observer A
Observer B
Observer C
Subject notifies subscribers when state changes — decouples publishers from listeners.

CampusOS context

CampusOS is the fictional smart-campus platform used across this track. Treat this module as a real product slice:

Context

  • Belongs to CampusOS — same naming and design vocabulary as parking, library, and booking modules
  • Scope is one JVM — persistence is in-memory or behind a repository interface
  • Goal is a class diagram + core Java that an interviewer can follow in 35–45 minutes
  • Extension paragraph at the end links to System Design when scale exceeds one process

Requirements snapshot

Functional requirements

  • publish(topic, message) delivers to all subscribers
  • subscribe(topic, handler) registers callback
  • unsubscribe removes handler
  • Topics are string names; multiple subscribers per topic
  • Sync vs async delivery policy

Non-functional requirements

  • Single JVM, in-process design for this LLD article
  • Core flows testable with in-memory repositories
  • Extension points for new rules without editing central god classes
  • Clear public API on facade/service layer
  • CampusOS extension links to System Design when scale demands distribution

Overview

This CampusOS module designs In-Process Pub-Sub as a production-ready in-process service. Read requirements point-by-point before drawing classes.

Requirements

Points to cover

  • publish(topic, message) delivers to all subscribers
  • subscribe(topic, handler) registers callback
  • unsubscribe removes handler
  • Topics are string names; multiple subscribers per topic
  • Sync vs async delivery policy

Topic registry

Points to cover

  • Scope — what "Topic registry" covers inside In-Process Pub-Sub and what it does not
  • Classes — name 2–4 classes/interfaces responsible for this slice
  • Flow — step-by-step: trigger → validation → state change → side effect
  • Edge cases — invalid input, concurrent access, empty state, timeout
  • Pattern — Strategy, State, Observer, or Facade if it fits naturally
  • Test hook — one scenario an interviewer will ask you to walk through

Delivery

Points to cover

  • Scope — what "Delivery" covers inside In-Process Pub-Sub and what it does not
  • Classes — name 2–4 classes/interfaces responsible for this slice
  • Flow — step-by-step: trigger → validation → state change → side effect
  • Edge cases — invalid input, concurrent access, empty state, timeout
  • Pattern — Strategy, State, Observer, or Facade if it fits naturally
  • Test hook — one scenario an interviewer will ask you to walk through

Error handling

Points to cover

  • Scope — what "Error handling" covers inside In-Process Pub-Sub and what it does not
  • Classes — name 2–4 classes/interfaces responsible for this slice
  • Flow — step-by-step: trigger → validation → state change → side effect
  • Edge cases — invalid input, concurrent access, empty state, timeout
  • Pattern — Strategy, State, Observer, or Facade if it fits naturally
  • Test hook — one scenario an interviewer will ask you to walk through
Java
// CampusOS — InprocessPubsub facade (keep thin)
public final class InprocessPubsubService {
    private final InprocessPubsubRepository repository;

    public InprocessPubsubService(InprocessPubsubRepository repository) {
        this.repository = repository;
    }

    /** Orchestrate: validate → domain rules → persist → return result */
    public Result handle(Request request) {
        request.validate();
        var domain = DomainModel.from(request);
        domain.applyRules();
        return repository.save(domain);
    }
}

Quick recall

Everything you need if you only revisit this box.

  1. Requirements — review this slice before mock interviews.
  2. Topic registry — review this slice before mock interviews.
  3. Delivery — review this slice before mock interviews.
  4. Error handling — review this slice before mock interviews.

CampusOS extension

When this outgrows one JVM

  • Shard inventory or state by campus ID or geographic region
  • Push notifications, payments, and search to dedicated services (System Design track)
  • Use message queues for async side effects (email, analytics)
  • The class diagram you drew here becomes one box on the HLD architecture diagram

Test yourself

Answer these before moving on — recall is what makes it stick.