Requirements

Functional Requirements:

1. User can post and advertisement.
2. Owner user can modify, delete the ads
3. Ad owner can see the other user responses, interest etc.
4. Other uses can view the advertisement. they can show the interest to the ad and send an inquiry for the product.

Non-Functional Requirements:

1. Consistency: The system can be eventual consistent where there can be little lag between viewing the posted ads to the user or and modification published for the users.
2. Availability: The System must be highly available.
3. Latency: User should be able to get the details of a specific ad within an 500ms-1sec to get the better user experience.

Capacity Estimation

1. Lets assume there are 100 million users using this system and 1 million users are the daily active user
2. Lets assume 70-80% of the users are using a system to view the advertisement. and 20-30% users are posting the advertisement.
3. Lets assume each user view on an average of 10 post daily so the read estimate of the system will be 7 million read per day so it will be around 80-100 QPS
4. If each user post 5 advertise per day then there will be 1.5million writes per day which is 20-25 QPS.

API Design

1. User Authentication
2. Post an advertisement
   1. User can post and advertisement where user needs to provide the complete details about the product such as title, description, unit, price, features etc.
   2. HTTP POST /post-advertisement
3. View Advertisement
4. Search Advertisement
5. Modify/delete advertisement.
6. Show interest/inquire for an advertisement.

High-Level Design

There are following services

1. Manage Advertisement service
   1. This service is responsible for posting, update, delete an advertisement.
   2. the new post will be store in the cache along with the database as the newer post are most probably access/read by many users.
   3. The Cache TTL will be 24 hours and the evection policy will be Least Frequently used. the post which is least frequently used will be removed from the cache. the hot post will remain in the memory longer.
   4. On modification of the post the cache entry will be delete from the memory.
2. View Advertisement service
   1. This service is responsible to view the advertisement posted by another user
   2. Search the advertisement and also can take a actions such as send message, inquire etc on the advertisement.
3. Notification Service
   1. Users can get the notification about the inquiry, or the new post uploaded.

Database Design

Define the data model. Identify the main entities, their attributes, and relationships. Consider the choice of database type (SQL vs NoSQL) and justify your decision based on access patterns...

Detailed Component Design

Deep dive into 2-3 key components. Explain how they work, how they scale, discuss tradeoffs, capacity, and any relevant algorithms or data structures.