Corsa

Async iteration channels in JavaScript.

$ npm install corsa --save

import { channel } from 'corsa'

const { readable, writable } = channel()
writable.write(3)
writable.write(2)
writable.write(1)
writable.end()

for await (const value of readable) {
  console.log(value)
}
console.log('done')

output:
> 3
> 2
> 1
> done

Overview

Corsa is a library for creating buffered readable / writable channels in JavaScript. This library was specifically written to help solve backpressure issues that can occur when dealing with high frequency messaging using traditional event listeners in JavaScript.

Corsa approaches this problem by making the channels sender await and suspend at buffer capacity. This helps to ensure the senders send rate is locked to the throughput allowed by a receiver.

Requires async/await and AsyncIteration support. Tested natively on Node v10.

channel<T>

A channel is a uni-directional pipe for which data can flow. The following code creates an unbounded channel which allows for near infinite buffering of messages between writable and readable. The call to channel returns a channel object, which we destructure into the readable and writable pairs.

const { readable, writable } = channel()

The following creates a bounded channel which allows for sending 5 values before suspending (see bounded vs unbounded)

const { readable, writable }  = channel(5)

Writer<T>

The following code creates an unbounded channel and sends the values 1, 2, 3 followed by a call to end() to signal EOF to a receiver.

const { readable, writable } = channel<number>()

writable.write(1)
writable.write(2)
writable.write(3)
writable.end()

Reader<T>

The Reader<T> is the receiving side of a channel and supports for-await-of for general iteration.

const { readable, writable } = channel()
writable.write(1)
writable.write(2)
writable.write(3)
writable.end()

for await (const value of readable) {
  console.log(value)
}

bounded vs unbounded

By default all channels are unbounded but it is possible to set a fixed buffering size when creating a channel(). When setting a channel size, this will cause a writable to pause at await when sending values. The await at the writable will only occur once the channels buffer has filled with values. The writable will remained suspended until such time a receiver starts pulling values from the channel.

The following code demostrates this behavior with channel bound to a buffer of 5.

const { readable, writable } = channel(5)
await writable.write(1) 
await writable.write(2) 
await writable.write(3) 
await writable.write(4) 
await writable.write(5) // - at capacity, the readable will need to read something.

await writable.write(6) // suspend  <-----+
                        //                | - readable.read() dequeues one element from the
...                     //                |   stream which will cause the writable to resume.
                        //                |  
await readable.read()   // resume   ------+

select

This library provides a simple channel select function similar to multi channel select found in the Go programming language. It allows multiple Reader<T> types to be combined into a singular stream.

import { channel, select } from 'corsa'

function strings() {
  const { readable, writable } = channel<string>()
  setInterval(() => writable.write('hello world'), 100)
  return readable
}

function numbers() {
  const { readable, writable } = channel<number>()
  setInterval(() => writable.write(Math.random()), 200)
  return readable
}

async function start() {
  const readable = select(strings(), numbers())
  for await (const value of readable) {
    console.log(value)
  }
}