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Beginner 1 minute
Scalability
Scalability describes how a system responds when demand increases: more users, requests, data, or background jobs. It is broader than making one server faster. You identify the bottleneck, add capacity where it matters, and measure whether the system still meets its latency and reliability goals. Good designs scale the busy path without making every path expensive.
Key idea
Scale the bottleneck, and define what 'more' means: traffic, storage, users, or work per second.
See it in one picture
Follow the arrowsMore usersBottleneck
Make one machine stronger
CapacityHealthy service
Add more machines
Real-world example
A photo service may need more upload bandwidth during a holiday, more storage over months, and more image-processing workers when a viral post arrives.
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