Introduction

Scalability means system's capability of handling more work. 

Consider an example of a website where there is one server handling 1000 users. Due to some reason more and more users are opening the website and suddenly the number of users increased to 50,000. However, the system is not capable of handling more than 50,000 users. In this case if load further increases, the system will fail to serve the users, it may crash.

We can solve this problem primarily in two ways. We can either scale the system Vertically or Horizontally. 

Vertical Scaling

When we increase the capacity of the server to handle more work is considered as vertical scaling. 

Vertical Scaling

Pros

• Easy to maintain as there will be fewer components in the system.

Cons

• Capacity increase comes with additional cost which increases rapidly for large scale systems.
• There is an upper limit till which only you can scale.
• Single Point of Failure. 

Horizontal Scaling

When we increase the number of servers to handle increasing work, is considered as horizontal scaling. By increasing the number of servers we can (evenly) distribute the load amongst these servers to handle more work using the load balancer. 

Horizontal Scaling

Pros

• Can solve single point of failure problem.
• Highly scalable and available.
• Comparatively cheaper as few small servers are cheaper compared to one high end server.

Cons

• Added complexity in deployment and maintenance.