Overview

Adapter library for Koa, a popular HTTP microframework for Node.js. Allows you to write Koa handlers as ƒ(request) -> response, similar to Ring in Clojure. See motivation.

Includes optional support for implicit cancelation via Posterus futures and coroutines/fibers. See motivation.

TOC

• Overview
• TOC
• Usage
• Motivation
• API
  • Request
  • Response
  • toKoaMiddleware
• Futures
• Routing
• Changelog
• Misc

Usage

Shell:

npm install --exact koa-ring

Node:

const Koa = require('koa')
const {toKoaMiddleware} = require('koa-ring')

const app = new Koa()

app.use(toKoaMiddleware(exampleMiddleware(exampleHandler)))

function exampleMiddleware(nextHandler) {
  return async function prevHandler(request) {
    // do whatever
    // can substitute request
    const req = patch(request, {})
    const response = await nextHandler(req)
    // can substitute response
    return response || {status: 404}
  }
}

function exampleHandler(request) {
  // Status and headers are optional
  return {status: 200, headers: {}, body: 'Hello world!'}
}

function patch(left, right) {
  return Object.assign({}, left, right)
}

const PORT = 9756

app.listen(PORT, err => {
  if (err) throw err
  else console.info(`Server listening on port ${PORT}`)
})

With cancelation support:

const Koa = require('koa')
const {toKoaMiddleware} = require('koa-ring/posterus')
const {Future} = require('posterus')

const app = new Koa()

app.use(toKoaMiddleware(handler))

// Implicitly converted to a Posterus fiber by koa-ring
function* handler(request) {
  // Could be a future-based database request, etc
  // This work can be automatically canceled if client disconnects
  // koa-ring automatically calls future.deinit()
  const greeting = yield Future.fromResult('Hello world!')
  return {body: greeting}
}

const PORT = 9756

app.listen(PORT, err => {
  if (err) throw err
  else console.info(`Server listening on port ${PORT}`)
})

See API below.

Motivation

Functional Programming

In Koa, request handlers take a request/response context object, return void and mutate the context to send the response. In other words, Koa is a poor match for the HTTP programming model, which lends itself to plain functions of ƒ(request) -> response.

Advantages of ƒ(request) -> response:

• Easy to rewrite request and/or response at handler level

• Lends itself to function composition

• You can often return response from another source, without writing a single line of context-mutating code

• Returning nothing instead of a response makes it easy to signal "not found" or "noop" to the calling handler

Fortunately, we can fix this. We have the technology to write functions.

Cancelation

JS promises lack cancelation, and are therefore fundamentally broken. Particularly unfit for user-facing programs such as servers. On a server, you want each incoming request to own the work it starts. When the request prematurely ends, this work must be aborted.

• In Erlang, this tends to be the default: you create subprocesses using spawn_link, they're owned by the handler process and die with it.

• In thread-based languages, this also works as long as you don't spawn another thread, as there's no analog of spawn_link.

• In Go, you're out of luck, as there's no support for goroutine cancelation.

• In Node.js, you can achieve this by using cancelable async primitives, such as Posterus futures, and coroutines built on them.

Concrete example:

const {Future} = require('posterus')
// const {fiber} = require('posterus/fiber')

function* koaRingHandler(request) {
  // If client disconnects, this invokes onDeinit, aborting work
  const one = yield expensiveFuture(request)
  // Delegate to another fiber, implicitly owning it;
  // if the client disconnects, both routines are canceled, aborting work
  const other = yield expensiveRoutine(request)
  return {body: other}
}

// Futures can be canceled with `future.deinit()`
function expensiveFuture(...args) {
  const future = new Future()
  const operationId = expensiveOperation(...args, (error, result) => {
    future.settle(error, result)
  })
  return future.finally(function finalize(error) {
    if (error) cancelOperation(operationId)
  })
}

// Routines are started as `const future = fiber(generatorFunction(...args))`
// and canceled as `future.deinit()`
function* expensiveRoutine(...args) {
  const value = yield expensiveWork(...args)
  return value
}

Lack of implicit cancelation leads to incorrect behavior. The client may wish to abort the work it has started; smart clients may cancel unnecessary requests to avoid wasting resources; and so on. Worse, this makes Node.js and Go servers uniquely vulnerable to a certain type of DoS attack: making the server start expensive work and immediately canceling the request to free the attacker's system resources, while the server keeps slogging.

Fortunately, we can fix this. We have tools for implicit ownerhip and cancelation in async operations, such as Posterus.

API

In Koa, every request handler acts as middleware: it controls the execution of the next handler, running code before and after it.

In koa-ring, these are separate concepts. A middleware function creates a request handler function by wrapping the next handler.

// Response shape. Status and headers are optional
const mockResponse = {status: 200, headers: {}, body: 'Hello world!'}

const handler = request => mockResponse

const overwritingMiddleware = nextHandler => async request => {
  const ignoredResponse = await nextHandler(request)
  return mockResponse
}

const noopMiddleware = nextHandler => nextHandler

const endware = () => handler

The resulting handlers have a signature of ƒ(request) -> response and lend themselves to composition and functional transformations of requests/responses.

koa-ring doesn't provide any special tools for middleware. Wrap your handlers into middlewares before passing the final handler to toKoaMiddleware and then to koa.use.

Request

Every handler receives a request, which is a plain JS dict with the following shape:

interface Request {
  url: string
  location: Location
  method: string
  headers: {}
  body: any
  ctx: KoaContext
  // Unimportant fields omitted
}

interface Location {
  pathname: string
  search: string
  query: {}
  // Unimportant fields omitted
}

request.ctx is the Koa context. It provides access to additional information and the underlying objects such as Node request, Node response, network socket, and so on. See the Koa reference.

request.location is the parsed version of request.url. It's very similar to a result of Node's require('url').parse, but with location.query parsed into a dict.

Unlike Koa, koa-ring doesn't use prototype chains. The request is a plain JS dict. Middleware can pass modified copies:

function mountingMiddleware(nextHandler) {
  return function handler(request) {
    const url = request.url.replace(/^\/api(?=\/)/, '')
    return nextHandler(patch(request, {url}))
  }
}

function patch(left, right) {
  return Object.assign({}, left, right)
}

You can override both request and response:

function middleware(handler) {
  return async request => {
    request = patch(request, {metadata: {}})
    let response = await handler(request)
    response = patch(response, {status: response.status || 200})
    return response
  }
}

function patch(left, right) {
  return Object.assign({}, left, right)
}

Response

Handlers return responses. A response is a plain JS dict with the following shape:

interface Response {
  status: number
  headers: {}
  body: any
}

Every field is optional. It's ok to return nothing; koa-ring will just run the next Koa middleware.

Handlers in a middleware can easily override each other's responses:

const middleware = next => async request => {
  const response = await next(request)
  return response || {status: 404}
}

toKoaMiddleware

Converts a koa-ring handler into a Koa middleware. You should compose all your handlers and apply koa-ring-style middlewares before passing the resulting handler to toKoaMiddleware. You only need one per application.

const Koa = require('koa')
const {toKoaMiddleware} = require('koa-ring')

const app = new Koa()

// Adds `request.body`
app.use(require('koa-bodyparser')())

const echo = request => request

app.use(toKoaMiddleware(echo))

See below for the future-based version with cancelation.

Futures

See motivation for supporting futures.

To use koa-ring with Posterus futures and coroutines, use the optional koa-ring/posterus module.

const Koa = require('koa')
const {toKoaMiddleware} = require('koa-ring/posterus')

const app = new Koa()

app.use(toKoaMiddleware(handler))

function* handler(request) {
  const response = yield someFuture(request)
  return response
}

Routing

By preparsing request.url into request.location, koa-ring makes manual routing much easier. You might not need a library:

function mainHandler(request) {
  const {location: {pathname}} = request
  if (/^[/]api[/]/.test(pathname)) return apiHandler(request)
  return viewHandler(request)
}

function apiHandler(request) {
  const {method, location: {pathname}} = request

  if (method === 'GET' && pathname === '/api/users') {
    return userHandler(request)
  }

  if (method === 'POST' && pathname === '/api/login') {
    return loginHandler(request)
  }

  // ...
}

Changelog

0.3.1

Updated to a newer version of Posterus (bugfix).

0.3.0

Breaking: replaced routing utils with URL preparsing. Tentative.

• removed match
• removed mount
• added request.location

Instead of using multiple functions hidden behind routes, you're supposed to route imperatively, with a series of if/else, by looking at the conveniently-parsed request.location.

This is tentative, likely to be followed by more changes as I'm experimenting with the idea.

Misc

I'm receptive to suggestions. If this library almost satisfies you but needs changes, open an issue or chat me up. Contacts: https://mitranim.com/#contacts