Design Reddit | System Design (original) (raw)

Last Updated : 8 Apr, 2026

Designing Reddit involves handling massive user-generated content, enabling discussions, and delivering personalized feeds to keep users engaged. It requires scalable systems to manage posts, comments, and voting while maintaining performance and reliability.

Reddit is an American social media platform and online community where registered users can submit content, such as text posts, links, images, and videos. Other users can then vote on and discuss these posts, creating a dynamic and interactive environment. Reddit is a widely used platform that has had a significant impact on online discussions and content sharing.

1. Requirements Gathering

This step focuses on understanding the problem clearly and identifying what the system is expected to achieve

1. Functional Requirements

These define the core features and functionalities that the system must support.

2. Non-Functional Requirements

These define the system’s quality attributes such as performance, scalability, usability, and reliability.

2. Capacity Estimation

To estimate the scale of the system and to get the idea about the storage requirements, we have to make some assumptions about the data.

1. Traffic Estimation

This step involves estimating the number of users, requests, and data generated in the system to help design scalability and infrastructure.

Daily Active Users (DAU): 100,000
Average API requests per user: 100 requests/day
Total Daily API Requests: 100,000 * 100 : 10,000,000 requests/day
**Daily new posts: 10,000
**Daily new comments: 500,000

2. Storage Estimation

This step involves estimating the amount of data the system will store over time, including user data, content, and metadata, to ensure scalable storage design

Average post size: 500 KB
Average comments per post: 50
Total Daily Storage: (10,000 * 500 KB) + (500,000 * 500 KB) = 2500 GB/day
Total Monthly Storage: (2500*30) GB = 75 TB
Assuming we store data for 5 years:
**Total Storage for 5 Years: 2500 GB/day * (365 * 5) = 4,562,500 GB (approximately 4.56 PB)

3. Bandwidth Estimation

This step involves estimating the amount of data transferred between clients and servers over the network to ensure smooth performance and low latency.

Bandwidth for API Requests

Average request size: 5 KB (considering headers and payload)
Total Daily Bandwidth for API Requests: 10,000,000 * 5 KB = 50 GB/day
Total Bandwidth for 5 Years: (50 GB/day )* (365 * 5 ) = 91.25 TB

Bandwidth for Content Delivery

Average video size: 20 MB
Daily video views: 50,000
Total Daily Bandwidth for Video Streaming: 20 MB * 50,000 = 1 TB/day
**Total Bandwidth for 5 Years: 1 TB/day * 365 * 5 = 1.825 PB

These revised estimates provide an overview of the server capacity required in terms of traffic, storage, and bandwidth for a Reddit-like platform while storing data over a span of 5 years.

3. Uses Case Diagram

This diagram represents the interaction between users (actors) and the system, showing the various functionalities the system provides.

use_case_diagram_3

Use Case Diagram

Below is the explanation of the components of the diagram above:

4. Low Level Design(LLD)

This stage focuses on designing detailed class structures, relationships, methods, and interactions to implement the system effectively.

Low-Level-Design-of-Reddit

The low level components are:

5. High Level Design(HLD)

The design is read intensive as more users will fetch the conte nts than the users who will actually upload the contents. At a high level, our system will need to handle two core flows:

High-Level-Design-of-Reddit

1. Uploading the contents

Uploading the Contents: Users first authenticate using authentication services to ensure secure access. After successful authentication, they upload content through post services such as text, images, or videos. The uploaded data is then processed and stored in the database. This ensures persistence and availability for future retrieval and interactions.

2. Streaming the contents

Streaming the Contents: Users authenticate through authentication services to access the platform securely. Feed services generate personalized feeds for each user using data from the database. These feeds are then pushed to the CDN for faster and scalable delivery. Finally, users fetch their feeds from the CDN, ensuring low latency and smooth performance.

3. Client Interaction

Users access the platform via various clients, including web browsers, mobile apps, and desktop applications. These clients communicate with the backend services through APIs to perform actions like posting content, interacting with posts, and accessing user-specific feeds.

4. Load Balancer

Incoming user requests are distributed across multiple backend servers using a load balancer. This ensures even distribution of traffic and prevents any single server from becoming overwhelmed.

5. API Servers

API servers receive requests from clients and route them to the appropriate microservices or backend components. They handle authentication, manage user sessions, and direct requests to services like post creation, comment handling, or user profile management.

6. Post Services

Responsible for creating, editing, and managing posts. Includes functionalities for uploading images, videos, texts and adding comments, voting, and content moderation.

7. Authentication Services

Manages user accounts, authentication, and profile settings.

8. Feed Services

Provides personalized feeds based on user preferences and interactions.

9. CDN (Content Delivery Network)

Stores and delivers static content like images, videos, and other media to users globally, ensuring faster load times and reduced server load.

6. Microservices Used

This section outlines the key services responsible for handling different functionalities of the system in a scalable and modular way.

Microservices-Used-for-Reddit

1. Load Balancer

It is responsible for distributing incoming traffic efficiently across multiple servers or resources. It acts as a traffic manager, ensuring that no single server gets overwhelmed by handling all user requests, thereby optimizing the platform's performance, reliability, and responsiveness.

2. Post Services

The post services manage user requests to upload diverse content types such as images, text, or links. Upon receiving a user's submission, they forward the content to the moderation services for assessment. Upon receiving positive feedback from moderation, the post services proceed to publish the content.

3. Subreddit Services

The Subreddit services oversee the creation and administration of subreddits, holding authority over their data. Users interact with these services to subscribe or unsubscribe from subreddits and set varying levels of access. Additionally, they facilitate user notifications regarding subreddit activities, such as new post uploads, by leveraging requests sent to the fanout services.

4. Fanout Services

Fanout Services primarily handle the distribution of new posts to users' feeds based on their subscriptions or follows. Two models govern their operation:

Let us explain this service using an example:

**Celebrity Problem: The "celebrity problem" arises when a user amasses a significant following, leading to scalability and performance challenges within the platform. Addressing this involves employing a hybrid approach:

5. Upvote/Downvote Services

When a user submits an upvote or downvote on a post or comment, this service handles the request. It accesses the database to retrieve the current count of upvotes and downvotes associated with the specific post or comment. Based on the user's action, it modifies these counts accordingly. For better understanding of the working of Upvote/Downvote services, you can refer to this article

6. Recommendation Services

The Recommendation Services access all user metadata from the database. Using machine learning models, they predict the types of posts users might prefer and then push them to users' feeds. The model must adhere to specific criteria: fairness—ensuring no post is favored without reason, scalability to handle a large number of posts, and low latency in predicting user interests.

We can update our algorithm through two methods.

7. Messaging Services

Messaging Services facilitate user connections and message exchanges. The users will be connected through WebSocket. We opt for WebSocket connections due to several advantages:

8. Notification Services

These services handle the delivery of real-time notifications to users, alerting them about various activities within the platform. They encompass a wide range of notifications, including new post alerts, comments on subscribed threads, direct messages, mentions, or interactions such as likes or shares on their content.

**Function of Notification System:

The comment services within the platform facilitate user engagement by allowing users to engage in discussions, provide feedback, and interact with posts. These services handle the creation, editing, and deletion of comments associated with posts. They ensure that comments are linked to the appropriate posts and manage the threading or hierarchical structure of discussions.

Database Design

This section focuses on structuring data models, tables, and relationships to ensure efficient storage, retrieval, and scalability of the system.

Database-Design-for-Reddit-2223In the above diagram, we have discussed about the database design:

1. Users

User `

{ userID (Primary Key) username email password(Hash) other user-related fields (e.g., Profile Info, Preferences) }

`

2. Posts

Posts `

{ postID (Primary Key) userID (Foreign Key) title content (Text, Links, Media) type (Text, Link, Image, Video) time_stamp upvotes downvotes other post-related fields }

`

{ commentID (Primary Key) postID (Foreign Key) userID (Foreign Key) parentCommentID (For nested comments) content timeStamp upvotes downvotes other comment-related fields }

`

4. Subreddits

Subreddits `

{ subredditsID (Primary Key) name description createdAt other community-related fields }

`

5. User_Subscriptions

Subscription `

{ subscriptionID (Primary Key) userID (Foreign Key) communityID (Foreign Key) createdAt }

`

6. User_Interactions

User_Interaction `

{ interactionID (Primary Key) userID (Foreign Key) targetID (PostID/CommentID) interactionType (Upvote/Downvote/Comment) timestamp other interaction-related fields }

`

Choosing the Right Database

The database acts as the core storage layer for user-generated content such as posts, comments, media, and interactions like upvotes and downvotes. To ensure high availability and reliability, data is replicated and sharded across multiple database instances.

**Relational Databases for Structured Data: Relational databases like PostgreSQL or MySQL are used to store structured data such as user profiles, posts, comments, and community information. They help maintain strong relationships between entities like Users, Posts, Comments, and Communities using well-defined schemas.

**NoSQL Databases for Flexible Data: NoSQL databases like MongoDB or Cassandra are used to handle unstructured or semi-structured data such as media files and dynamic content. They provide flexibility in data modeling, horizontal scalability, and faster read/write performance for large-scale systems.

API used for communicating with the servers

RESTful APIs (Representational State Transfer) are an ideal choice for the Reddit system design due to their simplicity, flexibility, and compatibility with various client applications. Reddit, being a large-scale platform, benefits from RESTful APIs' statelessness, allowing for scalability and reduced server load. These APIs enable straightforward communication between clients and servers, offering a uniform interface for accessing and manipulating resources like posts, comments, and user profiles.

1. User Registration

Register `

Endpoint: 'POST /api/users/register'

Request For Body

{ "username": "example_user", "email": "user@example.com", "password": "examplePassword123" }

`

2. User Login

Login `

Endpoint: 'POST /api/users/login'

Request For Body

{ "username": "example_user", "password": "examplePassword123" }

`

3. User Profile

User Profile `

Endpoint: 'GET /api/users/{userID}/profile'

`

Returns user profile information.

4. Update User Profile

UpdateUserProfile `

Endpoint: 'PUT /api/users/{userID}/profile’

Request for Body

{ "bio": "New bio description", "preferences": { "theme": "dark", "notifications": true } }

`

5. Create Post

Create `

Endpoint: 'POST /api/posts/create'

Request for Body

{ "title": "Title of the post", "content": "Text, link, or media content", "type": "text/link/media" }

Comment

Endpoint: 'POST /api/posts/{postID}/comment'

Request Body

{ "content": "Comment text" }

`

**Upvote Post

Upvote `

Endpoint: 'POST /api/posts/{postID}/upvote'

`

**Downvote

DownVote `

Endpoint: 'POST /api/posts/{postID}/downvote'

`

7. Subscriptions & Feeds:

Follow Subreddit

follow `

Endpoint: 'POST /api/subreddits/follow'

Request for Body

{ "subreddit": "subreddit_name" }

`

User Feed

Feed `

Endpoint: 'GET /api/users/{userID}/feed'

`

Retrieves personalized feed based on subscriptions and user interactions.

Further Optimizations

The system can undergo additional optimization to enhance its performance and scalability.