Requirements of a Chat Application

Messaging is the backbone of modern communication. Whether it’s sharing a quick voice note with a friend, coordinating group plans, or sending a document to a colleague, users expect their chat apps to respond instantly, work reliably across changing network conditions, and preserve their privacy. On mobile, this expectation becomes a design challenge. Limited memory, background restrictions, and network variability must all be accounted for, without letting any of it show to the user.

This lesson introduces the design problem for building a robust, high-performing mobile chat application. From handling message delivery in unreliable conditions to managing encryption and real-time presence, we will outline the problem space. We will then define the key functional, and nonfunctional requirements that shape how we approach this System Design.

Understanding the design problem

Chat systems differ from content feeds or browsing experiences in one crucial way: interaction is active, not passive. There is urgency. When a user hits send, they expect their message to be delivered right away. When someone else starts typing, they expect to see that indicator blink within milliseconds. And when the app transitions to the background, it should quietly queue messages, receive pushes, and pick up exactly where it left off.

Let’s ground this in a real-world scenario.

This scenario surfaces two foundational goals:

• The mobile app must immediately reflect user interactions, manage feedback like delivery receipts or errors, and synchronize state across devices with minimal latency.

• The mobile app must tolerate connectivity changes, persist message state across foreground/background transitions, and operate within OS-imposed limitations while preserving functionality and battery efficiency.

Let’s understand the requirements of the mobile chat application.

Requirements

To meet these foundational expectations, we need to establish clear functional and nonfunctional requirements that guide the design of a dependable and responsive chat experience on mobile.

Functional requirements

The functional requirements, focusing on the functionalities offered by the mobile app, are outlined below.

• Instant messaging: The mobile app should allow users to compose and send messages in real time for one-to-one and communication or a group chat.

• Multimedia sharing: The mobile app should allow users to share images, videos, voice notes, or documents.

• Status indicators: The mobile app should dynamically display user statuses (online or offline), real-time message progress indicators (sending, delivered, read), and a typing indicator when a user begins composing a message.

• Real-time updates: The mobile app should notify users about new messages when the app is in the background or closed/terminated by the OS.

• Message reactions and replies: Users should be able to react to messages, reply to specific texts, or forward messages to other users.

After deciding on the functional requirements, let’s now explore how the mobile app should cater to providing a seamless user experience through nonfunctional requirements.

Nonfunctional requirements

The nonfunctional requirements of the chat app are underlined below.

• Low latency: The mobile chat app should deliver a near-instantaneous message experience by ensuring low-latency operations for sending, receiving, and loading conversations.

• Performance: The chat application must remain responsive, even when handling numerous messages in a single chat.

• Offline messaging: The mobile chat app should ensure consistent and reliable message delivery under varying network conditions, including retries, duplicate prevention, and error recovery.

• Battery optimization: The mobile chat app should minimize battery consumption during background operations, message synchronization, and push notification handling.

• Security and privacy: The mobile chat app should protect all user messages and metadata using secure communication channels and local data protection mechanisms.

• Accessibility: The mobile chat app should support inclusive design by meeting accessibility standards for visual, auditory, and motor impairments.

With these requirements defined, we now turn to the critical design challenges that make building a chat application for mobile specifically complex. This is where real-time expectations must coexist with limited resources, fluctuating connectivity, and diverse user behaviors.

Challenges in designing mobile chat application

Here are the most important questions to ask when approaching chat System Design on mobile.

How will we design chat application?

This chapter is organized into a number of lessons which are mentioned below.

• Design considerations for chat application mobile system: We’ll explore the core decisions around app architecture, real-time communication, offline handling, and managing mobile constraints like app life cycle and network variability.

• Mobile System Design and API design of chat application: Based on design considerations, we’ll look into the mobile chat application’s System Design and define key APIs, data models, and back-end interactions that support messaging, media sharing, user presence, and sync across devices.

• Evaluations of nonfunctional requirements of chat application: We’ll assess performance, reliability, battery efficiency, and security concerns. We’ll also explore how to make informed trade-offs when building a scalable mobile chat application.

Next, we’ll begin with the design considerations that define the backbone of a mobile messaging experience.