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Ktor makes heavy use of Kotlin 1.3 stable coroutines.

Coroutines is a basic Kotlin mechanism (also called suspend functions), that among other things allows to do asynchronous programming that is linearly written like plain code instead of the traditional callback-based approach.

Other modern languages expose a similar, but more specific mechanism called await-async. The Kotlin approach is more generic and flexible and it is less verbose and less error-prone since the default behaviour when calling an asynchronous (suspend) function is to suspend the caller too.

Ktor uses a standard library from JetBrains called kotlinx.coroutines.

Since Ktor is fully asynchronous and intensively uses coroutines, it is a good idea to familiarize yourself with those concepts.