createApp.js

// Ignore paths for logging, metrics and docs
const ignorePaths = ['/ready', '/healthy']

// Fast, minimalist web framework for node.
const express = require('express')

// Helmet helps you secure your Express app by setting various HTTP headers
const helmet = require('helmet')

// Very low overhead Node.js logger. Logs in json use pino-pretty for dev.
const logger = require('pino')()

// Fastest HTTP logger for Node.js in town
const httpLogger = require('pino-http')({ logger, autoLogging: { ignorePaths } })

// Readiness and liveness checks middleware
const health = require('@cloudnative/health-connect')

// Prometheus middleware for standard api metrics
const apiMetrics = require('prometheus-api-metrics')

// Swagger UI for api-docs
const swaggerUi = require('swagger-ui-express')
const yaml = require('js-yaml')
const fs = require('fs')
const fetch = require('node-fetch')
const swaggerDocument = yaml.load(fs.readFileSync('./swagger.yaml', 'utf8'))

module.exports = () => {
  // The app returned by express() is in fact a JavaScript Function, designed to be passed to Node’s HTTP servers as a callback to handle requests.

  const app = express()

  const healthCheck = new health.HealthChecker()

  // Application-level middleware
  app.use(httpLogger)
  app.use(helmet())
  app.use('/healthy', health.LivenessEndpoint(healthCheck))
  app.use('/ready', health.ReadinessEndpoint(healthCheck))
  app.use(apiMetrics({ excludeRoutes: ignorePaths }))
  app.use('/api-docs', swaggerUi.serve, swaggerUi.setup(swaggerDocument))
  app.use('/swagger.json', (req, res) => res.json(swaggerDocument))

  const isReady = async () => {
    await fetchManifest()
    if (manifestCache.length === 0) throw new Error('No manifests found.')
  }

  const readinessCheck = new health.ReadinessCheck('readinessCheck', isReady)
  healthCheck.registerReadinessCheck(readinessCheck)

  const livenessCheck = new health.LivenessCheck('livenessCheck', isReady)
  healthCheck.registerLivenessCheck(livenessCheck)

  const urls = yaml.load(fs.readFileSync('./config/manifests/urls.yaml', 'utf8')).manifests

  let manifestCache = []
  let lastCached
  app.timeout = 30000

  const fetchManifest = async () => {
    if (+new Date() < lastCached + (app.timeout || 30000)) return
    const results = urls.map(url => fetch(url).then(result => {
      if (!result.ok) throw new Error(`Failed to load manifest for url ${result.url} (Status: ${result.status}: ${result.statusText})`)
      return result.json().catch(err => { throw new Error(`Failed to load manifest for url ${result.url}: ${err}`) })
    }))
    manifestCache = (await Promise.all(results)).flat()
    lastCached = +new Date()
  }

  app.get('/api/manifest.json', async (req, res, next) => {
    try {
      await fetchManifest()
      res.json(manifestCache || [])
    } catch (err) {
      next(err)
    }
  })

  app.use(function (err, req, res, next) {
    logger.error(err)
    res.status(500).end()
  })

  return app
}