Architecture and Components

Clients and Web Server

Web Browser/ Mobile apps are the service provider which will communicate to our backend services using HTTP/HTTPS protocol. 

We would need multiple web-server to ensure high availability and scaling horizontally, A load balancer will be required to distribute all requests evenly to different servers.

Requests and Storage

Every user uses shorteners when they are going to use URL multiple times and redirection also happens most of the time, so Read operations on our DB are much more than write operations, on an average considerably it’s 1:100, So our system is going to be read-heavy system. 
Let’s say we get 20 URL generation requests per second So on average we will get 30 days * 24H * 3600sec * 20 = ~50M write requests per month. And 5 Billion read requests per month, So we would be needing good storage and caching system for this.

We assumed we will be storing 50M URLs every month and each object will take around 100 bytes, So 1 TB storage is enough for us for storing data for 5 years at the current rate. 

Caching

We would be requiring Cache for popular URLs let’s say 20% of total requests. So for that purpose, we would need 20 GB of memory for caching. By introducing Caching we can also scale our read queries. We can simply implement an LRU(least recently used) approach for evicting cache policy.
We also need to update Cache if many requests start coming for any new URL, so we need to update the cache for that new entry.