== Async programming

Asynchronous programming generally refers to a style of programming
that allows tasks to be run in the background, while the other works is performed.

Imagine if you will a get-request function that performs a
full http request including connecting & ssl handshakes etc.

[source,cpp]
----
std::string http_get(std:string_view url);

int main(int argc, char * argv[])
{
    auto res = http_get("https://boost.org");
    printf("%s", res.c_str());
    return 0;
}
----

The above code would be traditional synchronous programming. If we want to perform
two requests in parallel we would need to create another thread to run another thread
with synchronous code.

[source,cpp]
----
std::string http_get(std:string_view url);

int main(int argc, char * argv[])
{
    std::string other_res;

    std::thread thr{[&]{ other_res = http_get("https://cppalliance.org"); }};
    auto res = http_get("https://boost.org");
    thr.join();

    printf("%s", res.c_str());
    printf("%s", other_res.c_str());
    return 0;
}
----

This works, but our program will spend most of the time waiting for input.
Operating systems provide APIs that allow IO to be performed asynchronously,
and libraries such as https://www.boost.org/doc/libs/master/doc/html/boost_asio.html[boost.asio]
provide portable ways to manage asynchronous operations.
Asio itself does not dictate a way to handle the completions.
This library (boost.cobalt) provides a way to manage this all through coroutines/awaitables.

[source,cpp]
----
cobalt::promise<std::string> http_cobalt_get(std:string_view url);

cobalt::main co_main(int argc, char * argv[])
{
    auto [res, other_res] =
            cobalt::join(
                http_cobalt_get("https://boost.org"),
                http_cobalt_get("https://cppalliance.org")
            );

    printf("%s", res.c_str());
    printf("%s", other_res.c_str());
    return 0;
}
----

In the above code the asynchronous function to perform the request
takes advantage of the operating system APIs so that the actual IO doesn't block.
This means that while we're waiting for both functions to complete,
the operations are interleaved and non-blocking.
At the same time cobalt provides the coroutine primitives that keep us out of callback hell.


