---
title: "Designing a Chat Application"
description: "Explore the architecture of chat apps like WhatsApp and Messenger."
image: "https://assets.bytebytego.com/diagrams/0134-chat-app.jpeg"
createdAt: "2024-03-10"
draft: false
categories:
  - how-it-works
tags:
  - architecture
  - messaging
---

![](https://assets.bytebytego.com/diagrams/0134-chat-app.jpeg)

The diagram below shows a design for a simplified 1-to-1 chat application.

## User Login Flow

*   Step 1: Alice logs in to the chat application and establishes a web socket connection with the server side.

*   Steps 2-4: The presence service receives Alice's notification, updates her presence, and notifies Alice's friends about her presence.

## Messaging Flow

*   Steps 1-2: Alice sends a chat message to Bob. The chat message is routed to Chat Service A.

*   Steps 3-4: The chat message is sent to the sequencing service, which generates a unique ID, and is persisted in the message store.

*   Step 5: The chat message is sent to the message sync queue to sync to Bob’s chat service.

*   Step 6: Before forwarding the messaging, the message sync service checks Bob’s presence:

    *   If Bob is online, the chat message is sent to chat service B.

    *   If Bob is offline, the message is sent to the push server and pushed to Bob’s device.

*   Steps 7-8: If Bob is online, the chat message is pushed to Bob via the web socket.