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Commonly confused · Cloud Computing

Horizontal vs Vertical Scaling: What's the Difference?

Horizontal scaling adds more instances or nodes and spreads load across them. Vertical scaling adds more CPU, memory, or storage to an existing instance. Horizontal improves resilience and has almost no ceiling; vertical is simpler but hits hardware limits and often needs a restart.

Horizontal Scaling (Scale Out)

Full definition
vs

Vertical Scaling (Scale Up)

Full definition
Horizontal ScalingVertical Scaling
MethodAdd more machinesMake one machine bigger
Upper limitPractically unlimitedLargest available instance size
DowntimeUsually noneOften requires a reboot or resize
Fault toleranceImproved; no single point of failureUnchanged; still one node
Application requirementStateless or distributed design, load balancerWorks with most apps unchanged
Cloud exampleAuto Scaling group, VM scale setChanging an EC2 instance type or Azure VM size
Remember it

Scale out adds more horses; scale up gets a bigger horse.

Exam tip

Look for keywords: 'add more instances behind a load balancer' is horizontal, 'increase RAM on the database server' is vertical. Scale in and scale down are the reverse operations.

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