A chat application enables real-time communication between users through text-based messages. It is commonly used in personal messaging, customer support, collaboration tools, and social networking platforms.

In this chapter, we will explore the low-level design of a simple in-memory chat application.

Let's 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:

After gathering the details, we can summarize the key system requirements.

1.1 Functional Requirements

• Support one-on-one and group messaging
• Allow users to view their complete chat history
• Ensure message ordering is preserved, i.e., messages must be delivered in the order they were sent

1.2 Non-Functional Requirements

• Modularity: The system should follow object-oriented design principles with well-defined components.
• Scalability: The system must support many concurrent users and deliver messages in real time with minimal latency.
• Extensibility: The design should be flexible enough to support future features like file sharing, typing indicators, or message reactions
• Maintainability: Code should be clean, testable, and easy to update or extend as requirements evolve.

2. Identifying Core Entities