What Does It Take to Build a Scalable Chat Application?

A scalable chat app should offer dependable message delivery, secure data storage, connection handling, synchronisation, and infrastructure that can scale as the number of users and messages increases.

I always suggest creating a minimum viable product (MVP) that works well for a small number of users. Extensive engineering for millions of users on day one is rarely necessary. However, the core components should be designed to grow without requiring a complete application redesign.

To achieve scalability, first determine what the chat app needs to support, including real-time functionality, authentication, message routing, and message persistence.

Here is how to ensure the app remains reliable as it scales to thousands of users.

Identify What Your Chat App Needs to Support

The requirements of a private messaging app differ from those of a workplace messaging service, marketplace chat app, or large community platform. Consider the following features.

a. One-to-One vs Group Conversations

Messaging between one sender and one receiver is relatively straightforward. Group conversations require additional functionality, including membership management, access permissions, notifications, and message distribution. Large groups also generate more traffic because a single message may need to reach many users.

b. Real-Time Delivery, Presence and Read Status

Users expect messages to be sent and delivered promptly. They also expect typing indicators, online status, read receipts, and delivery confirmations. These features generate continuous streams of real-time events that the system must handle reliably.

c. Message History, Media and Notifications

Users may share images, documents, videos, and voice messages. They also expect older conversations to load quickly and notifications to arrive when the app is closed. Media files require additional storage because they are generally much larger than text messages.

d. Estimate User Activity

Registered user counts alone cannot determine the infrastructure requirements. Estimate concurrent connections, messages per second, group sizes, media traffic, and peak usage. These figures provide a better indication of the workload the system must support.

Choose the Right Real-Time Communication Model

A robust chat communication model should deliver updates as soon as events occur.

a. Persistent Connections Are Crucial for Chat

Persistent connections maintain an open communication channel between the client and server. This allows the server to forward new messages immediately instead of waiting for the application to request updates repeatedly.

b. WebSockets vs Other Real-Time Technologies

WebSockets are popular because they support bidirectional communication over a long-lived connection. However, they only address the transport layer. The application also needs a messaging layer to handle storage, authentication, message ordering, retrieval, recovery, synchronisation, and retries after connection failures. This functionality can be developed in-house or provided through a messaging API or SDK, depending on the team’s resources and infrastructure requirements.

c. Differentiate Between Standard Requests and Live Events

Not every operation requires a real-time connection. Authentication, profiles, configuration, and older message history can be handled through REST APIs. The real-time channel can handle messages, presence information, and other live events.

Create the Chat Backend Around the Message Flow

a. Users Connect to the Chat Service

Users authenticate before establishing a real-time connection. Connection servers track active sessions. As the number of connected users increases, these sessions must be distributed across multiple servers.

b. Forward Messages to the Appropriate Group

When a user sends a message, the system must identify the conversation and the intended recipients. When multiple servers are involved, message queues, brokers, or publish-subscribe systems can distribute events between them.

c. Store Messages and Conversation History

Messages should remain available according to the application’s retention policy, even when a connection is lost. A chat platform should not automatically delete messages and files after 90 or 180 days unless that behaviour is part of its intended retention policy.

d. Manage Presence and Connection State

Presence indicates whether users are online or offline. Frequent disconnections and reconnections can generate substantial traffic at scale, so presence management must be treated as a separate real-time workload.

Significant engineering work goes into integrating these essential chat features. Teams that want to reduce development time can use a customisable chat API and SDK such as MirrorFly to integrate messaging capabilities into an existing application.

Make Message Delivery Reliable at Scale

Fast message delivery matters only when users can trust the messaging system. Reliability depends on several important capabilities.

a. Handle Ordering, Retries and Duplicates

If an application does not receive delivery confirmation, it may retry sending a message. Unique message IDs and idempotent processing help prevent duplicate messages from appearing.

b. Reconnect After a Connection Loss

If a connection breaks during transmission, the client should reconnect and retrieve missed messages. The sent, delivered, and read states help users understand the status of each message.

c. Provide Delivery Confirmation

The sender should receive confirmation that the message has been successfully received by the intended recipient’s messaging system.

Scale the Chat System as User Activity Grows

As the number of users increases, scaling decisions should be based on actual bottlenecks rather than simply moving to a larger server.

a. Support More Concurrent Connections

Persistent connections remain open for extended periods. As traffic grows, these connections can be distributed across multiple servers. Horizontal scaling increases capacity by adding servers as needed.

b. Distribute Messages Across Multiple Servers

When multiple servers are running, they need a shared mechanism for communicating messaging events. Queues, brokers, and publish-subscribe systems can distribute these events without relying on a single server.

c. Store Chat Messages and Media Files Separately

Media files should be stored separately from message records so that large files do not slow down access to conversation data. This separation also makes it easier to scale storage independently.

d. Plan for Group Message Fan-Out

A single message sent to a group may generate multiple deliveries, notifications, read receipts, and presence events. Capacity planning becomes increasingly important as group sizes grow.

Keep Conversations Available Across Devices and Network Conditions

Users may log in from multiple devices, close the app, or lose network connectivity. The messaging system should preserve conversation state and restore it when users reconnect.

a. Deliver Offline Messages After Reconnection

When recipients are offline, messages should be retained for later delivery. Once they reconnect, the system can deliver missed messages. A synchronisation cursor can help track which messages a client has already received.

b. Keep Multiple Devices in Sync

If a user reads a message on one device, the same read status should appear on their other devices. Message history, unread counts, and read states should remain consistent across devices.

c. Use Push Notifications for Offline Users

Push notifications alert users to events when they are not actively using the app. However, notifications are not a substitute for message storage. Conversations must remain stored and synchronised within the messaging system.

Build for Failure, Security and Availability

A reliable engineering team plans for connection drops, server restarts, and unstable networks. The system should recover from these problems while maintaining a good user experience.

Security is equally important. Authentication must verify users, while permissions determine who can access each conversation. Message data should be encrypted and protected through appropriate access controls.

The system also needs monitoring, failover mechanisms, recovery procedures, and backups.

The goal is not to eliminate every failure. It is to recover without losing conversations or unnecessarily disrupting the service.

Test the Chat Application Before Scaling It

Validate scalability assumptions before exposing the application to real users. Load tests can simulate large numbers of users and messages, while network tests can assess performance under low bandwidth or intermittent connectivity.

Monitor latency, throughput, failed deliveries, reconnection time, and server utilisation. These measurements can reveal bottlenecks in databases, message queues, connection servers, and notification systems.

Identify When Your Architecture Needs to Scale Further

Scaling decisions should be based on actual usage. Track active connections, message volumes, latency, delivery failures, queue delays, storage consumption, and server utilisation.

Different components, including servers, media storage, and databases, may reach their limits at different times. Scaling these components independently helps maintain reliable service as demand increases.

Conclusion

Building a scalable chat application requires careful planning. A reliable system needs persistent connections, durable storage, efficient message routing, offline recovery, multi-device synchronisation, security, and thorough testing.

The best architecture does not attempt to solve every future problem on day one. Instead, it separates the application’s core responsibilities so that each component can scale when actual usage demands it.