Answer.

A fault-tolerant architecture is necessary to ensure continuous operation of IT systems even in the event of failures of individual components. The main principle is to eliminate the single point of failure through redundancy, load balancing, and automatic failover.

A classic fault-tolerant system diagram includes clusters of servers, replicated databases, load balancers, and monitoring systems. For large systems, geo-distribution is applied—placing replicas in different data centers.

An example configuration of nginx with multiple upstreams:

upstream backend {
    server backend1.example.com;
    server backend2.example.com;
    server backend3.example.com;
    least_conn;
}
server {
    listen 80;
    server_name example.com;
    location / {
        proxy_pass http://backend;
    }
}

Key features:

• Use of clusters with automated failure detection
• Traffic balancing and manual/automatic load shifting
• Mandatory monitoring and alerting for quick recovery

Tricky questions.

If the database is replicated, can we always guarantee data consistency between replicas?

No, consistency depends on the chosen replication model (strong/ eventual consistency). For example, in eventual consistency, synchronization delays can lead to "stale" data appearing on some replicas.

Can the load balancer itself fix the backend's unavailability issue?

No, the load balancer can only exclude the non-working server from the pool but cannot fix it. Additional services (e.g., orchestration systems like Kubernetes) are used for automated recovery.

Is it enough to just set up a cluster of servers for fault tolerance?

No, it is also important to monitor the fault tolerance of the network infrastructure, storage, and other stack components. Errors in planning any part can jeopardize the entire system.