Understanding Web Load Balancing: Principles, Types, and Implementation

Load balancing (Load Balance) is an application of cluster technology that distributes work tasks across multiple processing units to improve concurrent handling capacity. The most common use case is web load balancing, which can be implemented through DNS round‑robin, IP load balancing, and CDN.

What is Web Load Balancing? It refers to a load‑balancing cluster that horizontally scales services, avoiding vertical upgrades. In this article, web load balancing specifically means techniques that distribute HTTP/HTTPS requests.

Basic Principle All load‑balancing methods create a one‑to‑many mapping: a single entry point maps to multiple processing nodes, achieving a divide‑and‑conquer effect and presenting the cluster as a virtual whole to clients.

Common web load‑balancing techniques include:

DNS round‑robin

CDN acceleration

IP load balancing (hardware or software)

DNS Round‑Robin The simplest method, using multiple A records for a domain to distribute requests. It lacks health checks and only supports weighted round‑robin, making true load balance difficult and exposing real server IPs.

CDN (Content Delivery Network) Distributes static content to many cache nodes and uses DNS to direct users to the nearest node, reducing latency for large static resources.

IP Load Balancing Operates at the network (IP) and transport (TCP/UDP) layers, offering higher performance than application‑layer solutions. Common products include hardware (e.g., F5 BIG‑IP) and software such as LVS, HAProxy, and Nginx (the latter works at layer 7). LVS‑NAT, LVS‑DR, and LVS‑TUN are typical implementations.

When a web system grows from 100 k to millions of daily visits, the pressure increases, requiring business splitting and distributed deployment. Load balancers provide a unified entry point to avoid single‑point failures.

HTTP Load Balancing (Redirect) Uses HTTP 302 redirects to send users to a different URL, often based on geographic proximity. While easy to implement, it adds latency and performs poorly under high traffic.

Reverse Proxy Load Balancing A reverse proxy (e.g., Nginx) forwards HTTP requests to backend servers, operating at layer 7. It can implement session persistence via cookie analysis or external session stores like Redis or Memcached.

DNS Load Balancing Configures multiple IPs for a single domain name, letting DNS resolve to different servers. It is simple and fast but cannot define custom rules and suffers from DNS propagation delays.

CDN‑Based Load Balancing (GSLB) Extends DNS load balancing by using Global Server Load Balancing to map users to the nearest IP based on location, improving performance but increasing deployment complexity.

In IP load balancing, the device rewrites the destination IP/port of incoming packets before forwarding them to internal servers, and rewrites the source address on the return path. This method, often called “four‑layer load balancing,” provides high throughput but requires more complex configuration.

Session handling in reverse‑proxy scenarios can be addressed by:

Configuring the proxy to route the same user to the same backend based on cookies (adds CPU overhead).

Storing session data in an external service such as Redis or Memcached (recommended).

Overall, choosing a load‑balancing solution depends on traffic volume, performance requirements, cost, and operational complexity.