Database design

Read is crucial for the system, and there is a significant join case, so we can consider a key-value database like DynamoDB for all entities.

Users
1. user_id is the partition key for easily even partition
Friendships
1. key is following_user:followed_user
2. partition key is following_user
3. sort key is followed_user
4. To easily fetch who follows a user, we can create a GSI: partition key is followed_user and sort key is following_user
Posts
1. post_id is the partition key for easily even partition
2. created_by and created_at are the sort keys
3. To easily fetch all feeds by user_id and sort by reverse chronological order, we can create a GSI: partition key is created_by and sort keys are post_id and created_at
Comments
1. post_id is the partition key for easily even partition
2. comment_id and created_at are the sort keys
Likes
1. post_id is the partition key for easily even partition

We normalized comments and likes instead of putting them in posts entity, because this approach can allow us to manage delete and update easily.