Scalability
The ability of a system to handle growth in load or data by adding resources or changing architecture without collapsing.
Definition
Scalability is how a system behaves when you turn the load up.
It is about growth in traffic, data, or both.
What it usually means in practice
- Can you handle higher QPS without falling over.
- Can you handle data growth without making every query slower.
Why it matters
Scalability is what keeps “it worked in the demo” from becoming “it died at launch.”
In interviews, it is also one of the easiest ways to show seniority: you talk about load shape and bottlenecks before you propose components.
How to reason about it
- Start with rough numbers: peak QPS, data growth, hot keys.
- Identify the bottleneck: CPU, memory, network, database.
- Decide what you will do when you hit that bottleneck.
Common mistake
Designing for the average and being surprised by peak.
That is why peak-to-average matters.