Social network ( Facebook ) - System Design

Facebook is a social network platform which allows users to create online profiles, connect with friends, create posts, send messages, view post feed, leave comments and likes

Functional requirements:

• add/delete/view friends and their profiles
• view home/user feed
• create/edit/delete posts
• send messages in groups/channels or in private
• posts and messages can contain media
• create/edit/view user profiles

Non-functional requirements:

• DAU 70,000,000
• Availability 99.95%
• Avg. post size 4000 bytes
• Max. post size 10000 bytes
• Avg. post read / day 20
• Avg. post write / day 5
• Avg. profile view / day 10
• Response time send 1 s
• Response time read 5 s
• Post storage age Infinity
• Max group members count 1000
• Max channel subscribers 100,000
• No region preference
• No seasonality

Design overview

For system design I have used C4 model. The C4 model was created as a way to help software development teams describe and communicate software architecture, both during up-front design sessions and when retrospectively documenting an existing codebase. It's a way to create maps of your code, at various levels of detail, in the same way you would use something like Google Maps to zoom in and out of an area you are interested in.

Level 1. System context diagram

Level 2. Core system container diagram

Level 2. Analytics system container diagram

Basic calculations

DAU = 70,000,000

RPS (view post):

Each user reads 10 posts per day RPS = 70,000,000 * 10 / 86400 ~= 8000

RPS (write post):

Each user writes 2 posts per day
RPS = 70,000,000 * 2 / 86400 ~= 1600

RPS (read message):

Each user reads 80 messages per day
RPS = 70,000,000 * 80 / 86400 ~= 64,000

RPS (send message):

Each user sends 20 messages per day
RPS = 70,000,000 * 20 / 86400 ~= 16,000

Total Bandwidth:

Total RPS = (8000 + 1600 + 16000 + 64000) = 89,600
Data per second = 89,600 * 4kb = 358,400 kb/s = 350 mb
Data per day = 350 mb/s * 86400 = 30.2 TB
Data per year = 30.2 TB * 365 = 11 PB

Required memory:

Replication factor of 3
Service operation time = 5 years
Write RPS = 1600 + 16000 = 17600
Data per second = 17600 * 4 = 70mb
Data per day = 70 * 86400 = 6 TB
Data per year = 6 * 365 = 2.2 PB
With replication (3x) = 6.6 PB
With backups (+15%) = 7.6 PB
For 5 years: 38 * 5 = 38 PB