JavaScript at the Edge

import { Image } from '@fly/image'

async function watermarkPicture() {
  const [picture, logo] = await Promise.all([
    loadImage(pictureURL),
    loadImage(logoURL)
  ])

  const meta = picture.metadata()

  const padPct = 0.1
  const padding = {
    top: parseInt(meta.height * padPct),
    bottom: parseInt(meta.height * padPct),
    left: parseInt(meta.width * padPct),
    right: parseInt(meta.width * padPct)
  }

  logo.extend(padding).background({ r: 0, g: 0, b: 0, alpha: 0 })

  picture.overlayWith(logo, { gravity: Image.gravity.southeast })

  const body = await picture.toBuffer()

  return new Response(body.data, {
    headers: {
      "Content-Type": "image/jpg"
    }
  })
}

async function loadImage(url) {
  const resp = await fetch(url)
  if (resp.status != 200) {
    throw new Error("Couldn't load image: " + url)
  }
  const body = await resp.arrayBuffer()

  return new Image(body)
}

import proxy from '@fly/proxy'

// the Glitch App URL
const glitchHost = "fly-example.glitch.me"

/* Deployment instructions */
// fly apps create <app-name>
// fly deploy
// fly hostnames add <your-custom-hostname.com>

// create a proxy fetch function to Glitch
const glitch = proxy(`https://${glitchHost}`,
  {
    headers: {
      'host': glitchHost, // set HOST header to glitchHost
      'x-forwarded-host': false // Don't send x-forwarded-host header
    }
  })

fly.http.respondWith(function (req) {
  // redirect to https
  let resp = requireSSL(req)
  if (resp) return resp

  // proxy to glitch
  return glitch(req)
})

function requireSSL(req) {
  const url = new URL(req.url)

  if (url.protocol != "https:") {
    url.protocol = "https"
    url.port = '443'
    if (app.env === "development") {
      console.log("Skipping SSL redirect in dev mode:", url.toString())
    } else {
      return new Response("Redirecting", {
        status: 301,
        headers: { "Location": url.toString() }
      })
    }
  }
}

export default function balancer(backends: FetchFn[]) {
  const tracked = backends.map((h) => {
    if (typeof h !== "function") {
      throw Error("Backend must be a fetch like function")
    }
    return <Backend>{
      proxy: h,
      requestCount: 0,
      scoredRequestCount: 0,
      statuses: Array<number>(10),
      lastError: 0,
      healthScore: 1,
      errorCount: 0
    }
  })

  const fn = async function fetchBalancer(req: RequestInfo, init?: RequestInit | undefined): Promise<Response> {
    if (typeof req === "string") {
      req = new Request(req)
    }
    const attempted = new Set<Backend>()
    while (attempted.size < tracked.length) {
      let backend: Backend | null = null
      const [backendA, backendB] = chooseBackends(tracked, attempted)

      if (!backendA) {
        return new Response("No backend available", { status: 502 })
      }
      if (!backendB) {
        backend = backendA
      } else {
        // randomize between 2 good candidates
        backend = (Math.floor(Math.random() * 2) == 0) ? backendA : backendB
      }

      const promise = backend.proxy(req, init)
      if (backend.scoredRequestCount != backend.requestCount) {
        // fixup score
        // this should be relatively concurrent with the fetch promise
        score(backend)
      }
      backend.requestCount += 1
      attempted.add(backend)

      let resp: Response
      try {
        resp = await promise
      } catch (e) {
        resp = proxyError
      }
      if (backend.statuses.length < 10) {
        backend.statuses.push(resp.status)
      } else {
        backend.statuses[(backend.requestCount - 1) % backend.statuses.length] = resp.status
      }

      if (resp.status >= 500 && resp.status < 600) {
        backend.lastError = Date.now()
        // always recompute score on errors
        score(backend)

        // clear out response to trigger retry
        if (canRetry(req, resp)) {
          continue
        }
      }

      return resp
    }

    return proxyError
  }

  return Object.assign(fn, { backends: tracked })
}
const proxyError = new Response("couldn't connect to origin", { status: 502 })
export interface FetchFn {
  (req: RequestInfo, init?: RequestInit | undefined): Promise<Response>
}

/**
 * Represents a backend with health and statistics.
 */
export interface Backend {
  proxy: (req: RequestInfo, init?: RequestInit | undefined) => Promise<Response>,
  requestCount: 0,
  scoredRequestCount: 0,
  statuses: number[],
  lastError: number,
  healthScore: number,
  errorCount: 0
}
// compute a backend health score with time + status codes
function score(backend: Backend, errorBasis?: number) {
  if (typeof errorBasis !== "number" && !errorBasis) errorBasis = Date.now()

  const timeSinceError = (errorBasis - backend.lastError)
  const statuses = backend.statuses
  const timeWeight = (backend.lastError === 0 && 0) ||
    ((timeSinceError < 1000) && 1) ||
    ((timeSinceError < 3000) && 0.8) ||
    ((timeSinceError < 5000) && 0.3) ||
    ((timeSinceError < 10000) && 0.1) ||
    0;
  if (statuses.length == 0) return 0
  let requests = 0
  let errors = 0
  for (let i = 0; i < statuses.length; i++) {
    const status = statuses[i]
    if (status && !isNaN(status)) {
      requests += 1
      if (status >= 500 && status < 600) {
        errors += 1
      }
    }
  }
  const score = (1 - (timeWeight * (errors / requests)))
  backend.healthScore = score
  backend.scoredRequestCount = backend.requestCount
  return score
}
function canRetry(req: Request, resp: Response) {
  if (resp && resp.status < 500) return false // don't retry normal boring errors or success
  if (req.method == "GET" || req.method == "HEAD") return true
  return false
}

function chooseBackends(backends: Backend[], attempted?: Set<Backend>) {
  let b1: Backend | undefined
  let b2: Backend | undefined
  for (let i = 0; i < backends.length; i++) {
    const b = backends[i]
    if (attempted && attempted.has(b)) continue;

    if (!b1) {
      b1 = b
      continue
    }
    if (!b2) {
      b2 = b
      continue
    }

    const old1 = b1
    b1 = bestBackend(b, b1)

    if (old1 != b1) {
      // b1 got replaced, make sure it's not better
      b2 = bestBackend(old1, b2)
    } else {
      b2 = bestBackend(b, b2)
    }
  }

  return [b1, b2]
}

function bestBackend(b1: Backend, b2: Backend) {
  if (
    b1.healthScore > b2.healthScore ||
    (b1.healthScore == b2.healthScore && b1.requestCount < b2.requestCount)
  ) {
    return b1
  }
  return b2
}

export const _internal = {
  chooseBackends,
  score
}