As website or application traffic increases and stability requirements rise, a single CN2 line server may become insufficient. Load balancing and failover technologies are crucial for ensuring service capacity and continuity. Load balancing distributes traffic across multiple CN2 line servers, while failover ensures that if one server fails, traffic is automatically redirected to a healthy server.

Core Concepts: How Do They Work?

Before configuration, let's clarify two core mechanisms:

The core goal of load balancing is to distribute load and improve performance. It acts like a "traffic control center," where all external user requests first arrive. The center distributes requests according to preset rules (such as round-robin, allocation based on CN2 line server performance, etc.) to multiple backend CN2 line servers (called backend CN2 line servers or real CN2 line servers) that handle the actual business. This ensures that no single backend CN2 line server becomes slow due to overload, while the overall system processing capacity is linearly improved.

The core goal of failover is to ensure uninterrupted service and improve availability. It's more like a "backup power system." The system continuously monitors the health status of each CN2 line server (e.g., network connectivity, service process availability). When a primary CN2 line server fails, the system automatically switches all subsequent traffic to a pre-prepared backup CN2 line server within a very short time (usually seconds), making the failure virtually imperceptible to users.

In actual architectures, these two functions are typically implemented by the same set of components or closely cooperating components. The core components include:

Load Balancer: Provides a unified access point (usually a virtual IP address, or VIP) and performs traffic distribution.

Backend CN2 Line Server Cluster: Multiple CN2 line servers actually running the application.

Health Check Module: Continuously probes the status of backend CN2 line servers on the load balancer.

High Availability Coordination Component (e.g., Keepalived): Enables primary/backup failover between multiple load balancers, preventing the load balancer itself from becoming a single point of failure.

Configuration Practice: From Selection to Deployment

After understanding the principles, we move on to the configuration phase. Mainstream technology choices include Nginx (application-layer focused), HAProxy (professional-grade proxy), and LVS combined with Keepalived (operating system-level high availability solution). The following explanation uses the very popular Nginx and Keepalived + HAProxy combination as an example.

Step 1: Implementing Load Balancing and Failover using Nginx

Nginx defines the backend CN2 line server group through the `upstream` module and has a built-in health check mechanism.

```nginx

http {

upstream backend_servers {

# Define two backend CN2 line servers and configure their weights

server 192.168.1.101:80 weight=3 max_fails=3 fail_timeout=30s;

server 192.168.1.102:80 weight=2 max_fails=3 fail_timeout=30s;

# Optional load balancing algorithm, such as least connections

# least_conn;

}

server {

listen 80;

location / {

# Proxies requests to the backend CN2 line server group

proxy_pass http://backend_servers;

}

}

}

Key configuration explanation:

`weight`: Weight; the larger the number, the higher the proportion of requests allocated. Suitable for CN2 line servers with different processing capabilities.

``` `max_fails` and `fail_timeout`: These are key to achieving automatic failover. `max_fails=3` means that within a `fail_timeout=30s` window, if a CN2 line server experiences three consecutive failures, Nginx will mark it as "unavailable" and stop forwarding new requests to it for the next 30 seconds. After 30 seconds, it will retry the connection, and if successful, it will automatically recover. This completes the automatic removal and recovery of the failed node.

The `backup` parameter: This marks a CN2 line server as a backup CN2 line server. It will only receive traffic when all non-backup CN2 line servers are unavailable, serving as a last resort.

Step Two: Using Keepalived to Achieve High Availability of the Load Balancer Itself

A single Nginx load balancer can also fail, so a "backup" needs to be configured for it. Keepalived utilizes the VRRP protocol to create a group of load balancers (one primary and one backup, or multiple backups). They share a virtual IP (VIP), and clients always access the network through this VIP. Example configuration of the Master load balancer (`/etc/keepalived/keepalived.conf`):

```nginx

global_defs {

router_id LVS_MASTER # Local machine identifier

}

vrrp_instance VI_1 {

state MASTER # Role is master

interface eth0 # Network interface for listening to VRRP advertisements

virtual_router_id 51 # Virtual router ID, must be the same for master and backup

priority 100 # Priority, the master node should be higher than the backup node

advert_int 1 # Heartbeat advertisement interval (seconds)

authentication { # Authentication information, must be the same for master and backup

auth_type PASS

auth_pass your_secure_password

}

virtual_ipaddress { # Define virtual IP (VIP)

192.168.1.100/24 ​​dev eth0

}

}

The configuration of the backup load balancer is similar to that of the primary load balancer; simply change `state` to `BACKUP` and assign a lower `priority` (e.g., 90).

After configuration, start the Keepalived service on both sides. Normally, the VIP will be bound to the `eth0` network interface of the primary node. If the primary node fails, the backup node will detect and take over the VIP within approximately 1-3 seconds, achieving seamless failover.

Step 3: Achieve More Professional Load Balancing with HAProxy

HAProxy is a professional load balancing software with a more refined configuration syntax. A typical configuration snippet is as follows:

``nginx

frontend http_front

bind *:80

default_backend http_back

backend http_back

balance roundrobin # Use round-robin algorithm

option httpchk GET /health_check # Define HTTP health check method

server web1 192.168.1.101:80 check inter 2000 rise 2 fall 3

server web2 192.168.1.102:80 check inter 2000 rise 2 fall 3

In this configuration, `option httpchk` specifies a more precise application-layer health check. `inter 2000` means checking every 2 seconds, `rise 2` means marking the CN2 line server as healthy only after two consecutive successful checks, and `fall 3` means marking it as down after three consecutive failures.

Configuring load balancing and failover for CN2 line servers essentially upgrades a single-point architecture to a clustered architecture with redundancy and scalability. This process begins at the application layer (e.g., Nginx/HAProxy) by configuring backend CN2 line server groups and intelligent health checks to achieve traffic distribution and fault detection. Then, at the system layer (e.g., Keepalived), a master-slave mechanism is established for the load balancer itself to eliminate new single points of failure. The key to success lies in selecting an appropriate algorithm based on business characteristics (e.g., whether session persistence is required) and conducting thorough fault simulation and stress testing before deployment.