What is an executor?

As we mentioned earlier, asynchronous programming allows multiple tasks to run on a single thread (or a thread pool). To do that, we need a scheduler that figures out which task to run at any moment and when to switch tasks. That is what an executor does.

There are two categories for how an executor schedules task:

• In preemptive multitasking, the executor decides when to switch between tasks. It may have an internal timer that forces a task to give up control to the CPU to ensure that each task gets a fair share of the CPU.
• In cooperative multitasking, the executor lets the task run until it voluntarily gives up control back to the scheduler.

In this section, we will build an executor that performs cooperative multitasking. Through that, we will answer the following questions:

• How is a task represented?
• How are the tasks stored in an executor?
• How does a task "give up control" back to the scheduler?

Now, let's look at the API of the executor.