Scalability Issues Vertical and Horizontal Scaling

Scalability:

When multiple requests are sent to the server, there is a possibility of thread over head,  consumption of memory stacks, blocking, asynchronous I/O resulting in scalability issues.

The scalability issues should be resolved by adding new servers on the existing node or add new additional nodes to
enable the server to handle increasing loads without performance degradation, and in a manner
transparent to the end.

Vertical scaling is achieved by increasing the number of servers running on
a single machine.

Horizontal scaling is achieved by increasing the number of machines in the cluster.

Which one is reliable?

Horizontal scaling is more reliable than the vertical scaling because there are multiple machines involved
in the cluster. In vertical scaling the number of server instances that can be run on one machine are
determined by the CPU usage and the JVM heap memory.

- madhuri chopurala