# Promise x Tokio = Prokio

An asynchronous runtime compatible with WebAssembly and non-WebAssembly targets.

# Rationale

When designing components and libraries that works on both WebAssembly targets backed by
JavaScript Runtime and non-WebAssembly targets with Native Runtimes. Developers usually face
challenges that requires applying multiple feature flags throughout their application:

1. Select I/O and timers that works with the target runtime.
2. Native Runtimes usually require `Send` futures and WebAssembly types are usually `!Send`.

# Implementation

To alleviate these issues, Prokio implements a single-threaded runtime that executes `?Send`
(`Send` or `!Send`) futures.

On platforms with multi-threading support, prokio spawns multiple independent runtimes proportional
to the CPU core number. When tasks are spawned with a runtime handle, it will randomly select a
worker thread from the internal pool. All tasks spawned with `spawn_local` will run on the same thread
as the thread the task was running. When the runtime runs in a WebAssembly target, all
tasks will be scheduled on the main thread.

This runtime is designed in favour of IO-bounded workload with similar runtime cost.
When running I/O workloads, it would produce a slightly better performance as tasks are
never moved to another thread. However, If a worker thread is busy,
other threads will not be able to steal tasks scheduled on the busy thread.
When you have a CPU-bounded task where CPU time is significantly
more expensive, it should be spawned with a dedicated thread (or Web Worker) and communicates
with the application using channels.

Prokio provides the following components:

1. A Task Scheduler that is capable of running non-Send tasks.
2. A Timer that is compatible with the scheduler backend.
3. Task Synchronisation Mechanisms.

# Runtime Backend

Prokio runtime is implemented with different runtimes depending on the target platform and can use
all features (timers / IO / task synchronisation) from the selected native runtime:

- `wasm-bindgen-futures` (WebAssembly targets)
- `tokio` (non-WebAssembly targets)