Rendering a typical web page may require calls to many different APIs (or DB tables) to gather the data required for it. Due to the style of object-oriented programming encouraged by C# (and many other languages), these API calls are often performed in series. However, if the result of one call does not affect another, then this is suboptimal, and the calls could be performed in parallel. We'll cover DB tables later in this chapter, as there are better approaches for them.
Concurrent calls can be more pertinent if you implement a microservices architecture (as opposed to the traditional monolith, or big ball of mud), and have lots of different distributed endpoints. Message queues are sometimes a better choice than HTTP ...
