How to scale the system: from a single server to a fault-tolerant architecture (load balancers, database)?

Scaling a system from a single server to a fault-tolerant architecture involves several stages:

1. Vertical scaling — increasing resources of a single server (CPU, RAM, disk). Simple, but limited by physical capabilities.

2. Horizontal scaling — adding new servers:

   • Load balancers distribute traffic among multiple application instances, ensuring fault tolerance and scalability.
   • Examples: Nginx, HAProxy, cloud load balancers (AWS ELB).

3. Database scaling:

   • Replication — creating copies of the database for read operations, offloading the primary server.
   • Sharding — dividing data across different servers to increase capacity.
   • Using fault-tolerant clusters (e.g., PostgreSQL with Patroni, MySQL Group Replication).

4. Caching — implementing Redis or Memcached to reduce database load and speed up responses.

5. Monitoring and automatic recovery — setting up monitoring systems (Prometheus, Grafana) and orchestration (Kubernetes) for automatic restart and scaling.

Example architecture:

[Clients]
   |
[Load balancer]
   |
[Multiple application servers]
   |
[Cache (Redis)]
   |
[Database with replication and sharding]

This approach ensures high availability, scalability, and fault tolerance.