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249 lines
8.2 KiB
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249 lines
8.2 KiB
Plaintext
---
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title: "Design a Chat System"
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description: "One-to-one and group chat over WebSockets: message storage, ID ordering, sync across devices, and online presence."
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icon: message-circle
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sidebar:
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label: "Chat System"
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---
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## Introduction
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A **chat system** supports real-time messaging between users. This chapter focuses on designing a chat app that includes:
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- **One-on-One Chat**
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- **Group Chat (max 100 users)**
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- **Online Presence Indicators**
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- **Multiple Device Support**
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- **Push Notifications**
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The system targets **50 million daily active users (DAU)** and stores chat history permanently.
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---
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## Step 1: Understanding the Problem
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### Requirements
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1. **Features:**
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- One-on-one and group chat (max 100 members).
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- Text-based messages (up to 100,000 characters).
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- Online/offline indicators.
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- Support for multiple devices.
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- Push notifications.
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2. **Scale:** Design for 50 million DAU.
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3. **Storage:** Permanent chat history.
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---
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## Step 2: High-Level Design
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### Communication Protocols
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1. **Sender Side:** HTTP for sending messages, leveraging persistent connections for efficiency.
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<Frame caption="Basic Design">
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</Frame>
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2. **Receiver Side:**
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- **Polling:**
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- Client periodically asks the server if there are messages available.
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- Inefficient due to frequent, redundant requests.
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<Frame caption="Polling">
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</Frame>
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- **Long Polling:**
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- Keeps the connection open until messages arrive.
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- Inefficient for inactive users.
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<Frame caption="Long Polling">
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</Frame>
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- **WebSocket:**
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- A bi-directional, persistent connection for real-time communication, chosen for both sending and receiving messages.
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- Uses WebSockets (ws) protocol for sending and recieving messages.
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<Frame caption="Websocket">
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</Frame>
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---
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### Components
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<Frame caption="High Level Architecture">
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</Frame>
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<Frame caption="High Level Architecture">
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</Frame>
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1. **Stateless Services:**
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- Handle signup, login, and user profile management.
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- Integrated with service discovery to recommend the best chat server.
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2. **Stateful Services:**
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- Chat servers maintain persistent WebSocket connections.
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- Responsible for message delivery and synchronization.
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3. **Third-Party Integration:**
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- Push notification services notify users about new messages.
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- Refer Notification System chapter for notifications implementation.
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---
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### Design
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The client maintains a persistent WebSocket connection to a chat server for real-time messaging.
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<Frame caption="High Level Design">
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</Frame>
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- Chat servers facilitate message sending/receiving.
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- Presence servers manage online/offline status.
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- API servers handle everything including user login, signup, change profile, etc.
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- Notification servers send push notifications.
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- Finally, the key-value store is used to store chat history.Key-value stores for the database of the chat history data for following reasons:
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- It allows easy horizontal scaling.
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- KV stores provide very low latency to access data.
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- Relational databases do not handle long tail of data well. When the indexes grow
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large, random access is expensive.
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- KV stores are adopted by other proven reliable chat applications. For example,
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both Facebook messenger and Discord.
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Following are the data models for one-to-one chat and group chat.
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- The primary key is message id, which helps to decide message sequence.
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- For the group chat the composite primary key is (channel_id, message_id).
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- IDs can be generated using a global 64-bit sequence number generator like Snowflake.
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- A better approach is to use local sequence number generator. Local means IDs are only unique within a group.
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- The reason why local IDs work is that maintaining message sequence within one-on-one channel or a group channel is sufficient.
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<Frame caption="One to one chat design">
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</Frame>
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<Frame caption="Group chat design">
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</Frame>
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## Step 3: Design Deep Dive
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### Service Discovery
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<Frame caption="Zookeeper">
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</Frame>
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- The primary role of service discovery is to recommend the best chat server for a client based
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on the criteria like geographical location, server capacity.
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- Uses **Apache Zookeeper** to allocate chat servers based on criteria like geographic location and server capacity.
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- Ensures efficient load distribution and minimizes latency.
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### Messaging Flows
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#### One-on-One Chat
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1. User A sends a message to Chat Server 1.
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2. Chat Server 1 assigns a unique message ID and stores the message in a key-value store.
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3. If User B is online, the message is forwarded to Chat Server 2, maintaining a persistent WebSocket connection.
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4. If User B is offline, a push notification is sent.
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#### Group Chat
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<Frame caption="Group Chat Flow">
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</Frame>
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- Messages are copied to individual inboxes for each recipient in the group.
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- Simplifies synchronization but becomes expensive for larger groups.
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- On the recipient side, a recipient can receive messages from multiple users. Each recipient
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has an inbox (message sync queue) which contains messages from different senders.
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---
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#### Message Synchronization
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Many users have multiple devices. We need to synchronize the message across the devices.
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Each device maintains a variable called cur_max_message_id, which keeps track of the latest
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message ID on the device. Messages that satisfy the following two conditions are considered
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as news messages:
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<Frame caption="Message Synchronization">
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</Frame>
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- The recipient ID is equal to the currently logged-in user ID.
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- Message ID in the key-value store is larger than cur_max_message_id
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---
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### Online Presence
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1. **Heartbeat Mechanism:**
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<Frame caption="Heartbeat Mechanism">
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</Frame>
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- Clients send periodic heartbeats to presence servers to indicate they are online.
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- If no heartbeat is received within a threshold (for eg x = 30), the user is marked offline.
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2. **Fanout Model:**
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<Frame caption="Fanout Presence">
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</Frame>
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- Presence updates are pushed to friends using a publish-subscribe model in which each friend pair maintains a channel.
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- When User A’s online status changes, it publishes the event to three channels, channel A-B, A-C, and A-D.
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- Those three channels are subscribed by User B, C, and D, respectively which get the online status updates.
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- The above design is effective for a small user groups.
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---
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## Additional Considerations
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### Scalability
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- **Horizontal Scaling:** Add servers as user count increases.
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- **Load Balancing:** Distribute traffic evenly across servers.
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- **Caching:** Reduce database load and improve latency.
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### Error Handling
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- **Retry Mechanisms:** Handle message delivery failures with retries and queuing.
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- **Server Failures:** Use service discovery to allocate new servers in case of failures.
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### Future Extensions
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1. **Media Support:** Add handling for photos and videos, including compression and cloud storage.
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2. **End-to-End Encryption:** Ensure message privacy.
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3. **Client-Side Caching:** Reduce data transfer for better performance.
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4. **Improved Load Times:** Use geographically distributed caching networks.
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## Further reading
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- [How Discord stores billions of messages](https://discord.com/blog/how-discord-stores-billions-of-messages)
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- [Flannel: An Application-Level Edge Cache to Make Slack Scale](https://slack.engineering/flannel-an-application-level-edge-cache-to-make-slack-scale/)
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