Requirements

Functional Requirements:

• Allow users to post, edit, and delete advertisements with title, description, category, price, and location.
• Allow users to browse and view listings organized by categories and subcategories (jobs, housing, services, for sale, etc.).
• Enable users to search for listings using keywords, and filter by category, price range, location, and date posted.
• Enable user registration and authentication with email/password or social login to post and manage their listings.
• Allow users to upload photos for their listings; store, resize, and serve images via CDN.

Non-Functional Requirements:

• High Avaliability 99.99% Uptime
• Horizontal Scalability
• Low Latency

API Design

POST /api/v1/createAdvert

GET /api/v1/getAdvert?filters&keywords

PUT /api/v1/editAdvert

DELETE /api/v1/deleteAdver

POST /api/v1/uploadContent

POST /api/v1/Login - we can use oauth

POST /api/v1/signup - for registration

High-Level Design

Database Design:

We will be going to use mongoDB as our database which has the following tables:

Advertisement:

ID - primary Key

Title - Text

Body - Text

AttachmentID - foreign key

CreatedAt - Timestamp

UpdatedAt - Timestamp

Attachments:

ID - Primary Key

Presigned URL - Text

READ PATH:

Client -> CDN -> API Gateway -> Load Balancer -> Advert Service -> Redis -> MongoDB -> Object Store

Write PATH:

Client -> CDN -> API Gateway -> Load Balancer -> Advert Service -> Mongo DB -> Object Store

Advert service handles all the apis related to advert including the search functionality. Along with the indexing done in Mongo using ID and title for better search capabilities.

The loadbalancer is integrated into the api gateway api paths through a vpc endpoint. Making it possible for communications.

When ever a request comes to an api it will go through the load balancer from there it will go to the respective service. From there if it is a read operation then it will try to get it from Redis if the cache hit is a miss only then it will go to the Mongo DB.

We are going to use an object store here like an s3 service to store the content like images/videos of the advert. We will be using either a presigned url to show display these images or it can be directly integrated into the CDN.

During Burst Traffic the servers/compute where the services are running scale by using simple scaling.

The load is balanced using load balancer

Even before coming to the Redis most of it is taken care off by the CDN.

For write it will go through api gateway and load balancer from there to the respective service which would push it to kafka queue from there the write happens to Redis and Mongo.

This keeps the Cache up to date and stale free.

We have a cleanup service which cleans up the cache in redis and invalidate cdn after ttl expiry.

Detailed Component Design

CDN - We host our frontend in this. I will be able to handle millions of request and is available on edge. It even acts as the first layer of cache where most of the read requests are taken care off. Also we can integrate our object store with this to serve the content files easily.

API Gateway routes the traffic based on the api that is getting hit. This also helps in rate limiting/throttling of requests which prevents any DDoS and Brute force attacks.

Load Balancer this is configured with API gateway and is used to distribute the load across the servers hosting our services. Keeping the traffic uniform and capable of handling the spike in traffic.

Kafka Queue - Takes in the events that the services create and pushes the data into the DB and Redis.

Cleanup Service - Service used to keep the redis cluster clean by removing the rows after ttl expiry. Can also invalidate the cache in CDN.