Last updated: August 2026
Scalability
Scalability is the ability of a system to handle increasing load by adding resources without degrading performance, reliability, or user experience.
Two approaches
Vertical Scaling ("scaling up") adds more resources to an existing server — more CPU, more memory, a faster disk. This has a natural ceiling; you can't make a single machine infinitely powerful. It's simple to implement but hits a limit.
Horizontal Scaling ("scaling out") distributes the load across multiple machines. A load balancer routes requests to available servers. This is theoretically unlimited but requires the application to be stateless and designed for distribution (shared session storage, message queues, etc.).
Design for scalability
- Stateless services — each request can be handled by any server; no session stored on the server itself
- Caching — reduce database queries and computation by caching frequently accessed data
- Database optimization — use indexing, denormalization, and read replicas to handle high query loads
- Message queues — offload slow operations asynchronously rather than blocking the request
- CDNs — serve static content from edge locations close to users
Vertical vs Horizontal Scaling
| Aspect | Vertical | Horizontal |
|---|---|---|
| Implementation | Upgrade hardware on one machine | Add more machines |
| Cost growth | Exponential (diminishing returns) | Linear (add more of the same) |
| Downtime | Often required for hardware upgrade | Zero (rolling deployment) |
| Complexity | Low (simpler) | High (distributed systems) |
| Limit | Hardware ceiling | Theoretically unlimited |
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