Data Center

Every popular software should have a Rate Limiter. It prevents DDOS attack, reduces cost and prevents servers from being overloaded.

There are some tricky questions to be considered during implementation of Rate Limiter:

Where to put Rate Limiter: client-side, server-side, gateway?
Algorithms for rate limiting. There are many algorithms with pros and cons: Token bucket, Leaking bucket, Fixed window counter, Sliding window log, Sliding window counter. Your business needs will define the right algorithm.
How are rate limiting rules created?
Where are the rules stored?
How to handle requests that are rate limited?

These questions are disclosed in a very interesting Chapter 4 of the book:

A great generic plan for scaling any app from zero to millions of users.

Single server setup
Selection and usage of database
Vertical scaling vs horizontal scaling approaches. And why you should prefer horizontal
Adding load balancer for horizontal scaling
Adding database replication for horizontal scaling
Adding cache
Adding CDN
Stateless vs Stateful architecture and using external state storage
Adding extra Data Centers
Adding Message queue
Adding Logging, Metrics, and Automation
Scaling database (sharding)
and futher steps…

All of these is carefully but briefly disclosed in the Chapter 1 of the book: