About this interactive
Scalability is being able to grow a service without growing the team to match. When a service needs more capacity, there are two directions to go.
Vertical scaling, or scaling up, makes one machine bigger: more CPU cores, more RAM, more or larger disks, a GPU, or a bigger instance size for a virtual machine. Nothing else changes; the same server does more.
Horizontal scaling, or scaling out, adds more machines: more web servers behind a load balancer, more container copies, more nodes in a cluster. The work is shared among them.
Either can be automated. A cloud database that moves itself to a bigger size at peak is scaling vertically on its own; an autoscaling group that starts extra instances when CPU use climbs, and stops them when demand drops, is scaling horizontally. Horizontal is the more common kind of autoscaling.
To sort a card, count the servers before and after. Same number, bigger: vertical. More of them: horizontal.
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