High-Level Architecture

A messaging platform is not a single application.

It consists of multiple specialized services working together.

Components

Client

The mobile or desktop application.

Responsible for:

• Sending messages
• Receiving events
• Local storage
• Encryption
• UI updates

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WebSocket Gateway

Maintains millions of persistent client connections.

Instead of opening a new HTTP connection for every message, clients maintain one long-lived WebSocket connection.

This enables instant message delivery.

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Messaging Service

Responsible for:

• Validating requests
• Creating Message IDs
• Storing encrypted payloads for durable delivery
• Publishing delivery events

Think of this as the brain of the messaging platform.

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Message Queue

Instead of delivering every message synchronously, the messaging service publishes events to a queue.

Common technologies include:

• Kafka
• RabbitMQ
• Pulsar

This allows delivery to happen asynchronously without slowing down the sender.

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Delivery Service

Responsible for locating recipients and pushing events to their connected devices.

If a recipient is offline, delivery can be retried later.

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Presence Service

Tracks whether users are:

• Online
• Offline
• Typing
• Recently Active

This service powers the online indicator and typing indicator.

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Pending Message Store

Holds encrypted messages safely until they can be delivered (and synced to the recipient's devices).

This is primarily store-and-forward, not a long-term plaintext chat history database. Once devices have received the message, server-side storage for that payload is no longer needed for chat history — the durable copy lives on clients.

Large messaging systems typically replicate this pending data across multiple servers for durability and availability while delivery is in progress.

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