Horizontal scaling, or scaling out, increases capacity by adding more servers or instances that share the load, usually behind a load balancer.
Why it matters
It can grow almost without a ceiling and improves resilience, since losing one node removes only a fraction of capacity. It is the standard approach for web tiers.
It requires applications that work across multiple nodes: sessions and files must not live only on one machine, and databases are harder to scale out than stateless web servers.
Example
A site runs on 2 identical web servers behind a load balancer. Before a sale, the team adds 4 more. Traffic is spread across 6 servers, and each now handles about one third of the original per-server load.
Common mistakes
- Scaling out the web tier while the single database stays the bottleneck.
- Keeping user sessions in local memory so users get logged out when routed elsewhere.
- Differences between nodes (config drift), which cause confusing, inconsistent behavior.
Related
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