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Loose coupling (also called decoupling) is a design principle in which application components are broken up in such a way that they can run on different servers. With this approach, redundant application components can be deployed to achieve high availability and scalability.

Let's take a look at a familiar example. Most database-backed web applications decouple the web component from the database so that they can run on separate servers. This makes it possible to run redundant web servers for scaling and high availability.

But loose coupling introduces a new challenge: If there are multiple web servers that users can access, how do you distribute traffic among them? And what if one of the servers fails? The answer is load balancing. A load balancer accepts connections from users and distributes those connections to web servers, typically in a round-robin fashion. When a load balancer sits in front of web servers, users connect to an IP address of the load balancer instead of an IP address of one of the web servers.

Other load balancing functions may include SSL/TLS termination, compression, and session tracking. Load balancers can integrate with autoscaling and server health checks so that if a web server becomes unreachable, the load balancer will detect the failure and trigger an automatic replacement or recovery action. With load balancing, you can effortlessly achieve redundancy and scalability, as shown in Figure 1.19.

FIGURE 1.19 Load balancing web servers