System requirements

Functional:

User Management:

• Users can register for new accounts with secure password hashing and email verification.
• Login functionality should be robust with features like session management and two-factor authentication for enhanced security.
• Users should have options to edit their account information and manage preferences.

Advertisement Management:

• Users can create new advertisements with detailed descriptions, categories, images, and pricing information.
• The system should allow users to edit and update their existing advertisements.
• Users can view their own advertisements and those posted by others.
• A clear mechanism for deleting unwanted advertisements is necessary.

Search Functionality:

• Implement a search engine that allows users to find advertisements based on relevant keywords, categories, location filters, and price ranges.
• Consider incorporating search filters based on additional criteria like ad creation date or user ratings (if implemented in the future).

Messaging System:

• Enable a messaging system where users can initiate conversations and exchange messages regarding specific advertisements.
• The messaging system should provide functionalities to track unread messages and maintain conversation history.

Non-Functional:

• Performance: The platform should be designed to handle a high number of concurrent users efficiently. This might involve implementing techniques like load balancing and caching to distribute requests and minimize response times.
• Scalability: The system architecture should allow for horizontal scaling to accommodate increasing user traffic and data volume. This could involve using distributed databases and cloud-based infrastructure.
• Security: User data, including passwords and personal information, should be securely stored using encryption techniques. Additionally, transactions like posting or editing advertisements should be encrypted to prevent data breaches.
• Usability: The user interface (UI) should be intuitive and user-friendly for both novice and experienced users. A clean and well-organized UI with clear navigation elements will enhance user experience.

Capacity estimation

Here's the calculation for storage requirements based on the provided assumptions:

Monthly Data

• Users per month: 1,000,000
• Advertisements per month: 100,000
• Record size per user (assumed): 1 KB
• Record size per advertisement (assumed): 10 KB

Monthly Storage

• User data storage: 1,000,000 users * 1 KB/user = 1,000,000 KB = 1 GB
• Advertisement data storage: 100,000 ads * 10 KB/ad = 1,000,000 KB = 1 GB

Total Monthly Storage

Total storage = User data storage + Advertisement data storage + Media Storage

Total monthly storage = 1 GB + 1 GB

Yearly Storage Requirements (in Gigabytes)

Total Yearly Storage = Total monthly storage * Months in a year

Total Yearly Storage = 2 GB * 12

Total Yearly Storage = 24 GB

Therefore, based on the assumptions, the estimated storage requirement for the system would be around 24 GB per year.

API design

Here's a breakdown of essential APIs for our Craigslist-like platform, categorized by functionality:

• User Management APIs:
• User Registration: Allows users to create new accounts with username, password, email address, and potentially other profile information.
• User Login: Enables users to authenticate with their credentials and obtain a secure access token for subsequent API calls.
• User Profile Management: Provides functionalities for users to update their profile information, contact details, and potentially manage preferences.
• Advertisement Management APIs:
• Create Advertisement: Enables users to create new advertisements with details like title, description, category, location, price, and images (potentially requiring separate image upload API).
• Get User Advertisements: Retrieves a list of advertisements posted by a specific user.
• Get All Advertisements: Retrieves a list of all advertisements based on various filters (category, location, price range, etc.).
• Get Single Advertisement: Retrieves details of a specific advertisement by its unique identifier.
• Update Advertisement: Allows users to edit existing advertisements they have created.
• Delete Advertisement: Enables users to remove their advertisements.
• Search Functionality APIs:
• Search Advertisements: Provides functionalities to search for advertisements based on keywords, categories, location filters, price ranges, and potentially other criteria.
• Messaging System APIs:
• Send Message: Allows users to send messages to other users regarding specific advertisements.
• Get Conversation: Retrieves a conversation thread between two users regarding a particular advertisement.
• Get Inbox: Retrieves a list of messages received by a user.

Database design

Database Choices for Craigslist-like Platform (CAP Theorem Considerations)

1. User Data:

• Database Type: SQL (MySQL/PostgreSQL)
• Reasoning: Structured user data (profiles, credentials) benefits from relational model and ACID transactions for data integrity.
• CAP Theorem Focus: Consistency - Ensuring consistent user data across replicas is crucial for accurate information retrieval.

2. Advertisements, Messages:

• Database Type: NoSQL (MongoDB)
• MongoDB provides flexibility for potentially semi-structured ad data (images, categories) and fast queries is a focus.
• Reasoning: Both options offer high write scalability and flexibility for handling a large volume of advertisements.
• CAP Theorem Focus: Balanced - Both availability for ad browsing and consistency for accurate ad details are important, but the weight can depend on specific needs.

Data Partitioning Strategy

Best Partitioning Strategy: Sharding by User ID

• Reasoning: User ID is a natural partitioning key as users primarily interact with their own data and advertisements. Sharding by User ID distributes load across servers and facilitates horizontal scaling for the user and advertisement data.

Regional or Geographical Partitioning

Consider Regional Partitioning: Yes, for advertisements

• Partitioning: By geographic location (city, state, or region)
• Reasoning: Improves search performance and user experience by delivering geographically relevant advertisements. Users searching for local listings benefit from faster queries that only access the relevant partition.
• Partitioning Field: Use a combination of city and state/region fields from the advertisement data.

Sharding Strategy

Best Sharding Strategy: User ID sharding for user and advertisement data

• Reasoning: As mentioned earlier, user ID provides a well-defined partition key for efficient data distribution and horizontal scaling based on user base growth. This approach keeps user data and related advertisements together on the same shard.

High-level design

Here's a breakdown of the essential components and services required to build your Craigslist-like platform:

1. Frontend Client:

• Technology Stack: Web technologies like HTML, CSS, and JavaScript frameworks (React, Angular) for building a user-friendly interface.
• Responsibilities:
• User registration and login functionalities.
• Advertisement creation, editing, and viewing.
• Search functionality based on keywords, categories, location filters, etc.
• Messaging system for communication between users regarding advertisements.
• User profile management.

2. API Gateway:

• Technology Stack: API gateway software (e.g., Apigee, AWS API Gateway)
• Responsibilities:
• Serves as a single entry point for all API requests from the frontend client.
• Routes requests to appropriate backend services based on their functionalities.
• Implements authentication and authorization mechanisms to secure access to APIs.

3. User Service:

• Technology Stack: Backend programming language (e.g., Python, Java) with a web framework (e.g., Django, Spring Boot).
• Database: Likely a relational database (MySQL/PostgreSQL) for storing user data (profiles, credentials).
• Responsibilities:
• Handles user registration, login, and authentication.
• Manages user profile information and preferences.
• Interacts with the user database to store and retrieve user data.

4. Advertisement Service:

• Technology Stack: Backend programming language with a web framework.
• Database: Potentially a NoSQL database (Cassandra/MongoDB) for scalability and flexibility with ad data (text, images, categories).
• Responsibilities:
• Processes advertisement creation, editing, and deletion requests.
• Stores and retrieves advertisement data from the database.
• Interacts with the search service for indexing and retrieving advertisements based on user queries.

5. Search Service:

• Technology Stack: Search engine technology (e.g., Elasticsearch) for efficient full-text search functionalities.
• Responsibilities:
• Indexes advertisement data from the advertisement service.
• Processes user search queries and retrieves relevant advertisements based on keywords, categories, location filters, etc.

6. Messaging Service:

• Technology Stack: Backend programming language with a web framework and potentially a message queuing system (e.g., RabbitMQ, Kafka) for handling asynchronous message delivery.
• Database: Potentially a NoSQL database (Cassandra) for scalability with message data.
• Responsibilities:
• Enables users to send and receive messages regarding advertisements.
• Stores and retrieves message data from the database.
• Manages message delivery and notification systems.

7. Database Layer:

• Technology Stack: Combination of databases based on data types.
• Responsibilities:
• Stores and manages all platform data securely and efficiently.
• Implements sharding techniques for horizontal scaling as the user base and data volume grow.

8. Security Layer:

• Technology Stack: Secure communication protocols (HTTPS), authentication and authorization mechanisms, intrusion detection/prevention systems.
• Responsibilities:
• Protects user data and platform integrity from unauthorized access and security threats.
• Ensures secure communication channels and data encryption.

9. Administration Panel (Optional):

• Technology Stack: Web interface built with similar technologies as the frontend client.
• Responsibilities:
• Provides functionalities for platform administrators to manage user accounts, content moderation, and system settings (if needed).

10. Notification Service:

• Technology Stack: Backend programming language with a web framework and potentially a message queuing system (e.g., RabbitMQ, Kafka) for handling asynchronous message delivery.
• Responsibilities:
• Triggers notifications based on user actions and platform events (e.g., new message received, reply to message, advertisement matching user search criteria).
• Integrates with various notification channels (email, push notifications) to deliver alerts to users.
• Manages notification preferences for users (allowing them to choose notification channels and frequency).

Request flows

Detailed component design

Let's delve deeper into the individual components of our Craigslist-like platform, exploring their functionalities, scalability considerations, and potential algorithms/data structures:

1. API Gateway:

• Technology Stack: API gateway software (Apigee, AWS API Gateway)
• Functionalities:
• Single entry point for all API requests from the frontend client.
• Routes requests to appropriate backend services based on functionalities.
• Implements authentication and authorization mechanisms for security.
• Scalability: API gateways are generally designed to handle high volumes of requests. They can be scaled horizontally by adding more instances behind a load balancer.
• Algorithms/Data Structures: Routing algorithms like path-based routing or content-based routing can be used to efficiently direct requests to backend services.

2. User Service:

• Technology Stack: Backend programming language (Python, Java) with a web framework (Django, Spring Boot)
• Database: Likely a relational database (MySQL/PostgreSQL)
• Functionalities:
• User registration, login, and authentication.
• User profile information and preference management.
• Interacts with the user database to store and retrieve user data.
• Scalability: Horizontal scaling can be achieved by adding more database servers for the user database or implementing sharding techniques to distribute user data across multiple servers based on a sharding key (e.g., User ID).
• Algorithms/Data Structures: Hash tables or B-Trees can be used for efficient user lookup based on usernames or unique IDs within the user database.

3. Advertisement Service:

• Technology Stack: Backend programming language with a web framework
• Database: Potentially a NoSQL database (Cassandra/MongoDB)
• Functionalities:
• Processes advertisement creation, editing, and deletion requests.
• Stores and retrieves advertisement data from the database.
• Interacts with the search service for indexing and retrieving advertisements.
• Scalability: Horizontal scaling of the advertisement database and service can be achieved by adding more servers. Sharding advertisements by category or location can further improve scalability.
• Algorithms/Data Structures:
• Advertisements can be stored using document-oriented structures like JSON documents in a NoSQL database, allowing for flexible data models.
• Full-text search algorithms can be used within the search service to enable efficient search based on advertisement content (titles, descriptions).

4. Search Service:

• Technology Stack: Search engine technology (e.g., Elasticsearch)
• Functionalities:
• Indexes advertisement data from the advertisement service.
• Processes user search queries and retrieves relevant advertisements based on keywords, categories, location filters, etc.
• Scalability: Search engines like Elasticsearch are designed for horizontal scaling by adding more nodes to the cluster.
• Algorithms/Data Structures:
• Inverted indexes are a core data structure used by search engines, allowing for fast retrieval of documents containing specific keywords.
• Ranking algorithms can be implemented to prioritize advertisements that best match user search queries based on relevance factors.

5. Messaging Service:

• Technology Stack: Backend programming language with a web framework and potentially a message queuing system (e.g., RabbitMQ, Kafka)
• Database: Potentially a NoSQL database (Cassandra)
• Functionalities:
• Enables users to send and receive messages regarding advertisements.
• Stores and retrieves message data from the database.
• Manages message delivery and notification systems.
• Scalability: Horizontal scaling can be applied to the messaging service and database to handle high volumes of messages. Message queuing systems can decouple message sending and receiving, improving overall scalability.
• Algorithms/Data Structures:
• Messages can be stored using document-oriented structures in a NoSQL database.
• Queues (FIFO data structures) can be used within the message queuing system to efficiently manage message delivery and ensure messages are not lost

Messaging service for real-time streaming of messages

Optimizing the messaging service for real-time streaming of messages with reliable delivery and message ordering in a distributed system requires a multi-faceted approach. Here's how you can achieve this:

1. Technology Stack:

• Message Queueing System: Utilize a robust message queuing system like Apache Kafka or RabbitMQ. These systems offer high throughput, low latency, and message persistence for reliable delivery.
• Database: Consider a NoSQL database like Cassandra for storing message history. Cassandra's write scalability and tunable consistency settings can accommodate real-time workloads.

2. Message Delivery Guarantees:

• Configure Reliable Delivery: Ensure the message queuing system delivers messages at least once (ideally exactly once). This can be achieved through producer confirmations and consumer acknowledgments.
• At-Least-Once Delivery: Implement mechanisms to handle potential duplicate messages on the receiving end. Deduplication strategies like message identifiers and idempotent operations can be used.

3. Message Ordering:

• Partitioned Queues: Partition the message queue based on the conversation thread (e.g., advertisement ID). This ensures messages within a conversation are delivered in the correct order within each partition.
• Ordering Guarantees within Partitions: Depending on your messaging system, additional configuration might be needed to guarantee strict ordering within partitions. Kafka offers message ordering guarantees within partitions.

4. Scalability:

• Horizontal Scaling: Scale the message queuing system and database horizontally by adding more nodes to handle increased message volume and user base growth.

5. Real-time Streaming:

• Consumers: Implement message consumers that actively listen for new messages within their assigned queues. Libraries like Kafka consumers can be used for efficient message pulling from topics.
• Push vs. Pull Mechanisms: Consider a combination of push and pull mechanisms. Push notifications can be sent for urgent messages while consumers can also actively pull messages for efficient communication.