Before we dive into some practical patterns and examples, it is worth saying a few words about the reasons for scaling an application and how it can be achieved.
We already know that most of the tasks of a typical Node.js application run in the context of a single thread. In Chapter 1, Node.js Design Fundamentals, we learned that this is not really a limitation but rather an advantage, because it allows the application to optimize the usage of the resources necessary to handle concurrent requests, thanks to the non-blocking I/O paradigm. A single thread fully exploited by non-blocking I/O works wonderfully for applications handling a moderate number of requests per second, usually a few hundred per second (this greatly depends on the application). Assuming we are using commodity hardware, the capacity that a single thread can support is limited no matter how powerful a server can be, therefore, if we want to use Node.js for...