A traditional application builds a tightly integrated server, where each server has a specific responsibility. Applications depend upon other servers for completeness of functionality. As shown in the following diagram, in a tightly coupled application, the web server fleet has a direct dependency on all application servers, and vice versa:
In the preceding architecture diagram, if one application server goes down, then all web servers will start receiving errors, as the request will route to an unhealthy application server, which may cause a complete system failure. In this case, if you want to scale by adding and removing servers, it requires lots of work, as all connections need to be set up appropriately.
With loose coupling, you can add an intermediate layer such as a load balancer or a queue, which automatically handles failures or scaling for you. In the following architecture diagram, there is a load balancer between the web server...
