Table of Contents
What High Availability Actually Means
High availability (HA) is the practice of designing systems that remain operational despite component failures. A 99.99% uptime SLA allows only 52 minutes of downtime per year. Achieving this requires eliminating every single point of failure in your architecture.
The Foundation: Load Balancing
A load balancer distributes incoming traffic across multiple application servers. If one server fails, the load balancer automatically routes traffic to remaining healthy servers. NGINX, HAProxy, and cloud load balancers are common choices.
Database High Availability
Single database servers are dangerous single points of failure. Solutions include: MySQL Group Replication for synchronous replication, PostgreSQL streaming replication with Patroni for automatic failover, and database proxies like ProxySQL that handle connection routing during failovers.
Health Checks and Automatic Failover
Load balancers detect failed servers through health checks — periodic HTTP requests to a health endpoint. Servers failing health checks are automatically removed from the pool. Keepalived provides IP failover for services that cannot use application-level load balancing.
FAQ
How many servers do I need for high availability?
Minimum: 2 application servers behind a load balancer, 2 database servers with replication. The N+1 rule recommends maintaining capacity to handle peak traffic even with one server removed.
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