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Design Reddit | System Design

Last Updated : 14 Feb, 2024
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Designing Reddit is not just about sharing posts and comments; it’s about managing a bustling community, ensuring everyone’s voice is heard, and delivering personalized content to keep users engaged. Let’s dive into the system design of Reddit to handle a huge amount of user-generated content while making sure everyone gets their slice of the conversation.


What is Reddit?

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.

Requirements for Reddit System Design

Functional Requirements for Reddit System Design:


  • User Authentication and Management:
    • User registration, login, and profile management.
    • Ability to follow users and subscribe to communities (subreddits).
  • Content Creation and Interaction:
    • Posting text, links, images, and videos.
    • Commenting on posts and replying to comments.
    • Upvoting and downvoting posts and comments.
  • Community Features:
    • Creation and moderation of communities (subreddits).
    • Joining and leaving communities based on user preferences.
  • Content Discovery and Personalization:
    • Personalized feeds based on user interests and subscriptions.
    • Trending or popular content recommendations.
  • Notifications and Messaging:
    • Email or in-app notifications for new posts, comments, or community updates.
    • Direct messaging between users.

Non-Functional Requirements for Reddit:

  • User Load: The platform should handle increasing user loads, accommodating more users and content over time.
  • Platform: Minimal downtime, ensuring the platform is available and responsive.
  • Content Delivery: Fast content delivery, quick response times for interactions, and minimal latency.
  • Storage: Efficient storage and retrieval of user-generated content, ensuring data integrity and scalability over time.
  • User Interface: Intuitive interface, ease of navigation, and a responsive design across devices.
  • Data: Compliance with data protection laws and regulations concerning user privacy and content moderation.

Capacity Estimation for Reddit System Design

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.

Traffic Estimation of Reddit System Design

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

Storage Estimation of Reddit System 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)

Bandwidth Estimation of Reddit System Design

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.

Uses Case Diagram for Reddit System Design


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

Getting Started and Sharing Content

  • When a user joins, they set up their profile with personal info.
  • They can then share posts with text, links, images, or videos.
  • Before posts go live, our moderation services review them to ensure they align with community rules.
  • Once approved, the posts get circulated within the community, and users are notified about their post’s status.

Engagement and Interactions

  • Users can jump into discussions by adding comments to posts.
  • Every comment goes through a quick check to make sure it fits within the community guidelines.
  • Approved comments show up alongside the posts for everyone to see.

User Involvement and Community Connections

  • People interact with posts and comments by upvoting or downvoting them.
  • These interactions influence the visibility of posts and comments across the platform.

Joining Communities and Messaging

  • Users can join or follow specific communities of interest.
  • Once part of a community, they get updates and notifications about community-related activities.
  • They can also directly message other users using real-time messaging features.

Staying Updated with Notifications

  • Users receive notifications for various activities like someone liking their post or replying to their comment.
  • Our notification services ensure that users promptly receive these updates, keeping them in the loop.

Low Level Design(LLD) in Reddit System Design

The low level components are:

  • Authentication Service:
    • Manages user registration and login functionalities, providing unique user IDs upon successful registration and authenticating users during login sessions.
  • Post Service:
    • Handles content creation by users, ensuring that posts (text, links, images, videos) are submitted for moderation, processed, and published once approved.
  • Comment Service:
    • Enables users to engage in discussions by adding, editing, or deleting comments on posts. Ensures proper association of comments with relevant posts.
  • Subscription Service:
    • Manages user subscriptions to communities or subreddits, maintaining the list of communities a user is part of and providing updates accordingly.
  • Notification Service:
    • Responsible for delivering real-time notifications to users regarding activities like post likes, comment replies, and community updates based on user preferences.
  • Interaction Service:
    • Records and manages user interactions such as upvotes, downvotes, or comments on posts and comments across the platform.
  • Messaging Service:
    • Facilitates direct messaging between users through WebSocket connections, ensuring real-time communication and message exchange.
  • Moderation Service:
    • Monitors user-generated content, ensuring alignment with community guidelines, and managing the approval or rejection of posts and comments.
  • Recommendation Service:
    • Collects user metadata and employs machine learning models to predict and push personalized content to users’ feeds based on their preferences.
  • Cache Service:
    • Stores frequently accessed data, including personalized feeds and trending posts, enhancing retrieval speed and reducing load on the main database.

High Level Design(HLD) of Reddit System Design

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:


Uploading the contents

  • Users authenicate themselves using authentication services.
  • The users then upload contents using Post Services.
  • The data is stored into the database.

Streaming the contents

  • Users authenticate themselves using authentication services.
  • Feed services then creates the feed for each user by using the data from database.
  • The the feeds are then pushed into the CDN.
  • The users then pull their feeds from the CDN.

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.

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.

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.

Post Services

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

Authentication Services

Manages user accounts, authentication, and profile settings.

Feed Services

Provides personalized feeds based on user preferences and interactions.

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.

Microservices Used in Reddit System Design


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.

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.

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.

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:

  • Push Model: This model instantly shares content with followers’ feeds as soon as a high-profile user creates or engages with it, ensuring real-time distribution for immediate access. Its advantages include real-time delivery and reduced latency, yet it can strain resources and face potential overload during high traffic.
  • Pull Model: In this model, content isn’t instantly distributed upon creation; instead, it’s fetched when users access their feeds. While resource-efficient and scalable, it might cause delays in accessing the latest content until users request it.

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:

  • Push Model for Most Users:
    • Utilize the push model for the majority of users, ensuring rapid content access upon login.
  • Pull-On-Demand for High-Follower Users:
    • For users with massive followings, adopt a pull-on-demand approach. Instead of proactively pushing content to all followers, allow their followers to retrieve content as needed, mitigating system overload.

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

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.

  • Batch Model: The model undergoes training at fixed intervals, say every two days, updating its prediction abilities based on slightly older data. While conserving computational power, it may be somewhat outdated, considering users’ rapidly changing tastes or moods.
  • Real-time Model: Contrarily, the real-time model constantly undergoes training, demanding significant computational resources. However, it offers more precise predictions and reduces computation costs for users who visit infrequently.

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:

  • Real-time Communication: Leveraging full-duplex channels enables instantaneous message exchange between users.
  • Low Latency: These connections maintain persistent links, minimizing delays for immediate message delivery.
  • Efficiency: By eliminating the necessity for repetitive HTTP requests, WebSocket connections streamline performance, especially for interactive messaging.
  • Bi-Directional Data Transfer: Supporting both server-to-client and client-to-server messaging, it ensures seamless communication pathways.

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 notification system operates by constantly monitoring user actions and events, triggering notifications based on user preferences and subscribed activities.
  • Efficient notification services enhance user engagement, prompting users to stay updated on relevant discussions, interactions, or community activities within the platform.

Comment Services

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 in Reddit Design

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



userID (Primary Key)
other user-related fields (e.g., Profile Info, Preferences)

  • userID: Unique identifier for each user.
  • username: User’s chosen username.
  • email: User’s email address.
  • password: Hashed password for user authentication.
  • other user-related fields: Additional information like profile details, preferences, etc.



postID (Primary Key)
userID (Foreign Key)
content (Text, Links, Media)
type (Text, Link, Image, Video)
other post-related fields

  • postID: Unique identifier for each post.
  • userID: References the user who created the post.
  • title: Title of the post.
  • content: Text, links, or media content within the post.
  • type: Indicates the format of the post (text, link, image, video).
  • time_stamp: Timestamp for post creation.
  • upvotes: Count of upvotes received by the post.
  • downvotes: Count of downvotes received by the post.
  • other post-related fields: Additional attributes related to the post.



commentID (Primary Key)
postID (Foreign Key)
userID (Foreign Key)
parentCommentID (For nested comments)
other comment-related fields

  • commentID: Unique identifier for each comment.
  • postID: References the post to which the comment is linked.
  • userID: References the user who made the comment.
  • parentCommentID: If it is a nested comment, then this references the parent comment.
  • content: Text content of the comment.
  • timeStamp: Timestamp for comment creation.
  • upvotes: Count of upvotes received by the comment.
  • downvotes: Count of downvotes received by the comment.
  • other comment-related fields: Additional attributes related to the comment.



subredditsID (Primary Key)
other community-related fields

  • subredditID: Unique identifier for each subreddit.
  • name: Name of the subreddit.
  • description: Description or summary of the subreddit’s purpose.
  • createdAt: Timestamp for the creation of the subreddit.
  • other community-related fields: Additional attributes related to the subreddit or community.



subscriptionID (Primary Key)
userID (Foreign Key)
communityID (Foreign Key)

  • subscriptionID: Unique identifier for each subscription.
  • userID: Reference to the user who subscribed.
  • communityID: References the subreddit/community to which the user subscribed.
  • createdAt: Timestamp for when the user subscribed to the community.



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

  • interactionID: Unique identifier for each user interaction.
  • userID: References the user performing the interaction.
  • targetID: References the post or comment being interacted with.
  • interactionType: Indicates the type of interaction (upvote, downvote, comment).
  • timestamp: Timestamp for when the interaction occurred.
  • other interaction-related fields: Additional attributes related to user interactions.

Which Database we should use for Reddit?

The database serves as the repository for user-generated content, encompassing posts, videos, images, comments, upvotes, and downvotes. This data undergoes replication and sharding across multiple databases to ensure redundancy and reliability.

  • Relational Databases for Structured Information:
    • Utilizing platforms like PostgreSQL or MySQL, we’ll organize user profiles, posts, comments, and community data, establishing strong connections among various elements like Users, Posts, Comments, and Communities.
  • Adaptive Storage using NoSQL Databases:
    • Employing NoSQL databases such as MongoDB or Cassandra enables us to manage flexible user-generated content like media attachments. This approach supports various data structures and faster access.

API used for communicating with the servers in Reddit

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.

User Registration


Endpoint: 'POST /api/users/register'

Request For Body

  "username": "example_user",
  "email": "",
  "password": "examplePassword123"

User Login


Endpoint: 'POST /api/users/login'

Request For Body

  "username": "example_user",
  "password": "examplePassword123"

User Profile

User Profile

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

Returns user profile information.

Update User Profile


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

Request for Body

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

Create Post


Endpoint: 'POST /api/posts/create'

Request for Body

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

Add Comment to Post


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

Request Body

  "content": "Comment text"

Upvote Post


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



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

Subscriptions & Feeds:

Follow Subreddit


Endpoint: 'POST /api/subreddits/follow'

Request for Body

  "subreddit": "subreddit_name"

User Feed


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

Retrieves personalized feed based on subscriptions and user interactions.

Further Optimizations in Reddit Design

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

  • Smart Caching: Use clever caching techniques to make things load faster for users. This means keeping the most popular stuff close by, so it doesn’t take forever to show up.
  • Balanced Workload: Spread out the work evenly among all the servers. That way, none of them gets too overwhelmed and slows down, keeping things running smoothly for everyone.
  • Mini-Services: Break things down into smaller pieces that can grow or shrink as needed. This helps handle big crowds without breaking a sweat.
  • Database Tricks: Tweak the database settings to find things faster. It’s like having a really well-organized library; you can find the book you want much quicker.
  • Global Content Magic: Use a fancy system that spreads pictures, videos, and stuff all over the world so that no matter where you are, things load up super fast.
  • Quiet Background Helpers: Make the system do hard stuff behind the scenes so users don’t notice any delays. It’s like doing homework while watching TV—you get things done without missing out on the fun.
  • Keep Getting Better: Always be on the lookout for ways to make things faster and smoother. By listening to feedback and making little improvements, everything just keeps getting better.


In conclusion, this system design for our platform caters to a diverse range of user interactions, content sharing, and community engagement. By implementing robust authentication processes, content moderation, and efficient workflows, we ensure a secure and enriching user experience. Incorporating scalable solutions like database sharding, indexing, and caching allows us to manage increasing volumes of data effectively, maintaining performance and responsiveness as our user base grows. The hybrid push-pull model for fanout services mitigates the celebrity problem, ensuring optimal content delivery without overloading the system. Continuous improvement in recommendation algorithms, real-time messaging, and proactive notifications enhances user engagement. With a strong emphasis on data security, compliance, and user experience, our design lays the foundation for a thriving and sustainable social platform.

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