June 14, 2021

Understanding the Architecture & System Design of a Chat Application

Cosette Cressler

Understanding the Architecture & System Design of a Chat Application

Chat applications and messaging apps are surging in popularity. The reason for this is simple—people love to chat. It’s the preferred method of communication in a multitude of different scenarios from collaborating with a colleague to checking in on a loved one.

Chat and messaging applications help foster a sense of community and connection that other forms of communication can’t reproduce. 

Understanding the system design & architecture of a real-time chat application can be an intimidating task. Fortunately, it's a task that we know well. In this article, we're going to break down chat app architecture so that you can identify the best way to add chat to your website or app. 

Components of Chat Application Infrastructure

While the system design of a chat app is unique in how it deals with the idiosyncratic business needs, you can always break it down to two major components: the chat client and the chat server.

messaging app system design

Chat Client 

The chat client is what the user experiences. A desktop, web or smartphone chat application, the chat client is responsible for interacting with the operating system (i.e. your computer, browser or smartphone).

Interactions include sending push notifications, displaying data to the user and storing messages and files. When you type a message and hit send, the chat client transmits that message to the other major component: the chat server. 

Chat Server

The chat server is just that, a server (or usually many many servers) that hosts all the software, frameworks and databases necessary for the chat app to operate. This server, or pool of servers, is responsible for securely receiving a message, identifying the correct recipient, queuing the message and then forwarding the message to the recipient's chat client.

The chat server’s resources can include a REST API, a WebSocket server, an AWS instance for media storage, etc.

Chat Rest API

A Chat REST API is most often used to facilitate the functionality of the chat app outside of messaging. For example, authentication, profile settings and notification settings can all be managed through a REST API. All these functionalities are built into CometChat’s SDKs and Chat APIs. This article talks about Chat APIs, their features and how they work in more detail.

WebSocket Server

A WebSocket server and client library are useful components for a chat app. Typical communication between a client and server is done using HTTP and requires that the client makes a request for data from the server. The server itself can’t push data to the client without the client first making a request.

In a chat app, this quickly leads to inefficiencies since the client would have to poll the server every second for new messages. A WebSocket is a persistent connection between client and server that provides a bidirectional communication pathway.

That means, the server can send data to the client without first getting a request. Real-time chat is the perfect use case for WebSockets. This article talks about websockets in more detail

Media Storage

Of course, almost every mobile or web app requires some kind of data storage. Chat apps require both data storage and media storage. Profiles, messages and media files need to be accessible at all times.

Your choice of database, whether it’s a relational database or a NoSQL database (like MongoDB), relies heavily on your use cases, what’s important to your users, and what your development team is familiar with.

You can even use a mix of both, using a reliable and robust relational database for generic data like profile settings and using a NoSQL database for the messages. Since key-value stores allow for easier horizontal scaling and low latency to access data, NoSQL databases like Cassandra are perfect for storing messages.

While every chat app has a chat client and server, the size and functionality of those components will vary depending on the app’s intended use. The components can scale from an MVP chat app with basic capabilities to a full-blown, mature chat app like WhatsApp that has over two billion active users.

Chat Application Architecture Diagram

chat architecture diagram

Features and Functionality of a Chat App

As mentioned above, the implementation and functionality of those components will vary depending on the app’s intended use. So, let’s cover the core features of a chat app before diving into the deep end of functionality.

  • User Registration and Authentication: If you want users, you have to give people a way to use your chat app. Pretty straightforward. The most common way to register and authenticate users is with their email or phone number. 
  • Individual and group messaging: The core feature of a chat app is the ability to send and receive messages. Both individual (peer to peer) messaging and group messaging are essential aspects of a good chat app.
  • Push notifications: You don’t want your users to have to open your app every few minutes to check for new messages. That would create a terrible user experience, especially on mobile. Push notifications to notify users of a new incoming message or reaction have become a must-have feature.
  • Sharing of media files: Gone are the days of plain text messages. (Remember T9? My, how far we’ve come.) Gifs, emojis, audio, images and video files are the new standard. So your app will need local media storage along with cloud storage for fast retrieval.
  • User settings & profile customization: Customization lets your users express their individuality and creativity. Whether it’s the ability to change their font, background color, nickname or something else—it’s important to allow users some flexibility to personalize their experience.

Now that we know what the basic features of a chat app are, let's take a look at the actual frameworks and technology stacks that facilitate these features.

When you’re building a chat app you’ll be faced with an abundance of choices. What programming languages are you going to use?  What chat protocol? What kind of database? Do you want to use an existing chat framework? It can be very intimidating.

We recommend starting with the technologies that you’re most familiar with and going from there.

Messaging App Programming Languages

The backend (chat server) and the frontend (chat client) will most likely be written in different programming languages depending on your messaging app system design. This is because if you’re creating a mobile app, both iOS and Android have their own programming languages—Swift and Kotlin, respectively.

There is one exception, which is if you’re using React Native and Node.js. In this case you’ll use JavaScript as the core programming language. React Native can compile into both an iOS and Android app which allows you to use the same codebase to create a cross-platform application.

Common Backend Languages for Chat Apps

  • Erlang: A less popular coding language but its emphasis on concurrency, scalability and reliability make it a good choice for chat apps. WhatsApp is built with Erlang and even Facebook used it for a while.
  • Scala: Another more obscure language but it is used by Twitter—probably due to its scalability and multi-threaded functionality.
  • PHP: A great choice for web applications, especially those built on top of Wordpress.
  • Java: An oldie but a goodie. This popular language is the go-to for many developers.
  • JavaScript: Node.js makes server-side coding with JavaScript a breeze and it’s a bonus that the frontend and backend can be written in the same language.

While these are some great choices for backend chat applications, keep in mind that your backend programming language choice really only makes a difference when you’re operating at a large scale. You can use almost any server-side programming language to achieve MVP status for your chat application.

For your front-end programming language you have less options overall. 

Common Front-end Languages for Chat Apps

  • JavaScript: React and React Native can be used to create cross-platform applications that work on web, iOS and Android.
  • Swift: This is the iOS programming language to create mobile apps for iPhones. It’s the best choice if you want to provide the most native iOS feel to your application.
  • Kotlin/Java: This is the Android programming language and it’s the go-to choice if you want a native experience for Android users. 

Please note, both Swift and Kotlin require additional codebases if you want to create a cross-platform chat app.

Messaging App Frameworks

There are plenty of existing frameworks and tools that are specifically designed to bring chat functionality to new and existing apps. Deciding which frameworks to include in your chat app should not be taken lightly. It’s a decision that can add or subtract hundreds of developer hours.

Chat Protocols

One of the most important choices to make is deciding on a chat protocol—the backend protocol tasked with transmitting messaging. While you can create your own protocol from scratch, you may want to consider using one of the many open-source, highly tested and scalable protocols that are already available.

Options include Extensible Messaging and Presence Protocol (XMPP) or Message Queue Telemetry Transport (MQTT). XMPP is the protocol used by WhatsApp. It has a ton of documentation around its use and implementation. MQTT is a bit newer, but is quickly becoming a favorite within the developer community. There’s also WebRTC, a newer and very promising protocol that is being worked on everyday.

Chat SDKs

One of the fastest ways to get a chat app up and running is to integrate an existing chat SDK (software development kit) into your application. These SDKs can tackle many aspects of building a chat app. Plus, they require minimal customization for basic use cases. 

CometChat’s SDKs are designed to help you quickly build a reliable & full-featured chat experience into any mobile or web app. Plus, CometChat is available for web, iOS and Android—making cross-platform a breeze.

Technology Stacks

Let's take a look at some comprehensive technology stacks to build cross-platform chat apps:

  • React Native + Javascript + Firebase (a backend-as-a-service platform provided by Google) + Expo (cross platform development) + CometChat Pro
  • Node.js + Javascript + Expo (cross platform development) + CometChat Pro
  • Ionic (Cross platform development) + Javascript/Typescript + PhoneGap/Cordova + CometChat Pro

Take a Peek Under the Hood of 2 Popular Messaging Apps

Curious what technologies the most popular and heavily-used chat apps use?

Let’s take a look at the system design of WhatsApp, arguably the most popular chat app. As a backend programming language it utilizes Erlang, which they most likely chose for its concurrency and reliability. WhatsApp uses the XMPP protocol via an Ejabberd server, another choice fueled by reliability. Mnesia DB is the core database which also happens to be the default Erlang database. All this is built on the FreeBSD operating system. Get an in-depth look at building a WhatsApp clone here.

WeChat, another popular chat app, has a pretty similar architecture as WhatsApp. It utilizes Erlang as a programming language and also relies on an Ejabberd server. However, unlike WhatsApp, it uses the HTTPS protocol rather than XMPP to securely transmit messages. It also happens to use a different database—EnMicroMsg DB and SQLCipher.

Next Steps

We hope we’ve answered all your questions about designing a chat application architecture and building a messaging app. If not, checkout this complete guide to building a video chat application. You can also shoot a message to one of our in-app chat experts for a 1-on-1 meet or simply sign up to our dashboard if you’re ready to start building your chat app for free.

About the Author

Cosette Cressler is a passionate content marketer specializing in SaaS, technology, careers, productivity, entrepreneurship and self-development. She helps grow businesses of all sizes by creating consistent, digestible content that captures attention and drives action.

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