[Revolt](https://revolt.run/) allows such concurrent I/O operations. We keep the cognitive load low by avoiding callbacks.
Our APIs can be used like any other library, except that things _also_ work concurrently, because we use non-blocking I/O under the hood.
Run things concurrently using `Amp\async()` and await the result using `Future::await()` where and when you need it!
There have been various techniques for implementing concurrency in PHP over the years, e.g. callbacks and generators shipped in PHP 5.
These approaches suffered from the ["What color is your function"](https://journal.stuffwithstuff.com/2015/02/01/what-color-is-your-function/) problem, which we solved by shipping Fibers with PHP 8.1.
They allow for concurrency with multiple independent call stacks.
Fibers are cooperatively scheduled by the [event-loop](https://revolt.run), which is why they're also called coroutines.
It's important to understand that only one coroutine is running at any given time, all other coroutines are suspended in the meantime.
You can compare coroutines to a computer running multiple programs using a single CPU core.
Each program gets a timeslot to execute.
Coroutines, however, are not preemptive.
They don't get their fixed timeslot.
They have to voluntarily give up control to the event loop.
Any blocking I/O function blocks the entire process while waiting for I/O.
You'll want to avoid them.
If you haven't read the installation guide, have a look at the [Hello World example](https://v3.amphp.org/installation#hello-world) that demonstrates the effect of blocking functions.
The libraries provided by AMPHP avoid blocking for I/O.
> Cancellations are advisory only. A DNS resolver might ignore cancellation requests after the query has been sent as the response has to be processed anyway and can still be cached. An HTTP client might continue a nearly finished HTTP request to reuse the connection, but might abort a chunked encoding response as it cannot know whether continuing is actually cheaper than aborting.
Cancellation is often optional, which is usually implemented by making the parameter nullable.
To avoid guards like `if ($cancellation)`, a `NullCancellation` can be used instead.
```php
$cancellation ??= new NullCancellationToken();
```
#### CompositeCancellation
A `CompositeCancellation` combines multiple independent cancellation objects. If any of these cancellations is cancelled, the `CompositeCancellation` itself will be cancelled.
