Putting things together

So far, we have learned that an application is composed of one or more processes and that each process has one or more threads. We have also learned that blocking a thread means halting the execution of the code in that thread, and for that reason, a thread that interacts with a user is expected to never be blocked. We also know that a coroutine is basically a lightweight thread that resides in a thread but is not tied to one. The following diagram encapsulates the content of this section so far. Notice how each coroutine is started in one thread but at some point is resumed in a different one:

In a nutshell, concurrency occurs when an application runs in more than one thread at the same time. So in order for concurrency to happen, two or more threads need to be created, and both communication and synchronization of those threads will most likely be needed for correct functioning of the application.