Scalability of system.
Scalability of the system means to handle the task as the system grows in size, by growth can be increased in the users or increased amount of work. Like a number of transactions increased, the system should give the correct response irrespective to the increased user transactions or network growth.
Different ways to scale the distributed system
Horizontal and vertical scaling: Horizontal scaling is achieved by adding more server to the existing servers, where Vertical scaling is adding more more resources like (RAM, CPU, Storage etc.) to the existing server.
With Horizontal scaling much easier by adding more servers to the existing resource pool, where vertical scaling is updating smaller servers with bigger capacity servers which involves the downtime for the system.
Good example for vertical scaling are Cassandra and MongoDB by which we can add more machine to scale. Similarly, good example for vertical scaling is Mysql. Is easy to scale by changing smaller machine with bigger one but it involves the downtime.