Design Pub Sub System
What is a Pub-Sub System?
A Publish-Subscribe (Pub-Sub) system is a messaging pattern where publishers send messages to topics without knowing who will receive them, and subscribers receive messages by subscribing to those topics.
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This decouples senders and receivers, making it easier to build scalable and flexible systems. Popular examples include Kafka, Redis Pub/Sub, and Google Cloud Pub/Sub.
In this chapter, we will explore the low-level design of a simple in-memory pub-sub system.
Lets start by clarifying the requirements:
1. Clarifying Requirements
Before starting the design, it's important to ask thoughtful questions to uncover hidden assumptions and better define the scope of the system.
Here is an example of how a conversation between the candidate and the interviewer might unfold:
Candidate: Should the system support multiple publishers and subscribers for a single topic?
Interviewer: Yes, each topic can have multiple publishers and multiple subscribers. For this design, the publisher can simply be the client / demo using it.
Candidate: Should message delivery be synchronous or asynchronous?
Interviewer: It should be asynchronous. When a publisher sends a message, it should not wait for subscribers to consume it.
Candidate: Do we need to guarantee message ordering for subscribers?
Interviewer: Yes, messages within a topic should be delivered to each subscriber in the order they were published.
Candidate: Should we support different delivery semantics like exactly-once or at-least-once delivery?
Interviewer: For this design, let’s keep it simple and go with a "fire-and-forget" model. That means best-effort delivery without retries, acknowledgments, or delivery guarantees.
Candidate: Can subscribers unsubscribe at any point during runtime?
Interviewer: Yes, subscribers should be able to dynamically subscribe and unsubscribe from topics at any time.
After gathering the details, we can summarize the key system requirements.
1.1 Functional Requirements
- Support creation and management of multiple topics
- Allow multiple publishers to publish messages to a topic
- Allow multiple subscribers to subscribe to one or more topics
- Deliver messages to all active subscribers of a topic in the order they were published
- Ensure non-blocking, asynchronous message delivery
- Follow a "fire-and-forget" delivery model: no retries, or acknowledgments
1.2 Non-Functional Requirements
- Modularity: The system should follow object-oriented principles with clear separation of responsibilities
- Scalability: The system should efficiently support many concurrent publishers and subscribers
- Extensibility: The design should be flexible enough to support future enhancements such as message persistence, retries, or delivery guarantees
- Reliability: While exact delivery guarantees are not required, message ordering and dispatching must remain consistent and predictable within each topic
After the requirements are clear, lets identify the core entities/objects we will have in our system.