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Micro — Asynchronous HTTP microservices

Build Status Coverage Status Slack Channel XO code style


  • Easy: Designed for usage with async and await (more)
  • Fast: Ultra-high performance (even JSON parsing is opt-in)
  • Micro: The whole project is ~260 lines of code
  • Agile: Super easy deployment and containerization
  • Simple: Oriented for single purpose modules (function)
  • Standard: Just HTTP!
  • Explicit: No middleware - modules declare all dependencies
  • Lightweight: With all dependencies, the package weighs less than a megabyte


Important: Micro is only meant to be used in production. In development, you should use micro-dev, which provides you with a tool belt specifically tailored for developing microservices.

To prepare your microservice for running in the production environment, firstly install micro:

npm install --save micro

Then create an index.js file and populate it with function, that accepts standard http.IncomingMessage and http.ServerResponse objects:

module.exports = (req, res) => {
  res.end('Welcome to Micro')

Micro provides useful helpers but also handles return values – so you can write it even shorter!

module.exports = () => 'Welcome to Micro'

Next, ensure that the main property inside package.json points to your microservice (which is inside index.js in this example case) and add a start script:

  "main": "index.js",
  "scripts": {
    "start": "micro"

Once all of that is done, the server can be started like this:

npm start

And go to this URL: http://localhost:3000 - 🎉

async & await


Micro is built for usage with async/await. You can read more about async / await here

const sleep = require('then-sleep')
module.exports = async (req, res) => {
  await sleep(500)
  return 'Ready!'


The package takes advantage of native support for async and await, which is always as of Node.js 8.0.0! In turn, we suggest either using at least this version both in development and production (if possible), or transpiling the code using async-to-gen, if you can't use the latest Node.js version.

In order to do that, you firstly need to install it:

npm install -g async-to-gen

And then add the transpilation command to the property inside package.json:

  "scripts": {
    "build": "async-to-gen input.js > output.js"

Once these two steps are done, you can transpile the code by running this command:

npm run build

That's all it takes to transpile by yourself. But just to be clear: Only do this if you can't use Node.js 8.0.0! If you can, async and await will just work right out of the box.

Port Based on Environment Variable

When you want to set the port using an environment variable you can use:

micro -p $PORT

Optionally you can add a default if it suits your use case:

micro -p ${PORT:-3000}

${PORT:-3000} will allow a fallback to port 3000 when $PORT is not defined.

Body parsing


For parsing the incoming request body we included an async functions buffer, text and json

const {buffer, text, json} = require('micro')
module.exports = async (req, res) => {
  const buf = await buffer(req)
  // <Buffer 7b 22 70 72 69 63 65 22 3a 20 39 2e 39 39 7d>
  const txt = await text(req)
  // '{"price": 9.99}'
  const js = await json(req)
  // 9.99
  return ''


buffer(req, { limit = '1mb', encoding = 'utf8' })
text(req, { limit = '1mb', encoding = 'utf8' })
json(req, { limit = '1mb', encoding = 'utf8' })
  • Buffers and parses the incoming body and returns it.
  • Exposes an async function that can be run with await.
  • Can be called multiple times, as it caches the raw request body the first time.
  • limit is how much data is aggregated before parsing at max. Otherwise, an Error is thrown with statusCode set to 413 (see Error Handling). It can be a Number of bytes or a string like '1mb'.
  • If JSON parsing fails, an Error is thrown with statusCode set to 400 (see Error Handling)

For other types of data check the examples

Sending a different status code

So far we have used return to send data to the client. return 'Hello World' is the equivalent of send(res, 200, 'Hello World').

const {send} = require('micro')
module.exports = async (req, res) => {
  const statusCode = 400
  const data = { error: 'Custom error message' }
  send(res, statusCode, data)
send(res, statusCode, data = null)
  • Use require('micro').send.
  • statusCode is a Number with the HTTP status code, and must always be supplied.
  • If data is supplied it is sent in the response. Different input types are processed appropriately, and Content-Type and Content-Length are automatically set.
    • Stream: data is piped as an octet-stream. Note: it is your responsibility to handle the error event in this case (usually, simply logging the error and aborting the response is enough).
    • Buffer: data is written as an octet-stream.
    • object: data is serialized as JSON.
    • string: data is written as-is.
  • If JSON serialization fails (for example, if a cyclical reference is found), a 400 error is thrown. See Error Handling.

Programmatic use

You can use Micro programmatically by requiring Micro directly:

const micro = require('micro')
const sleep = require('then-sleep')
const server = micro(async (req, res) => {
  await sleep(500)
  return 'Hello world'
  • This function is exposed as the default export.
  • Use require('micro').
  • Returns a http.Server that uses the provided function as the request handler.
  • The supplied function is run with await. So it can be async
sendError(req, res, error)
  • Use require('micro').sendError.
  • Used as the default handler for errors thrown.
  • Automatically sets the status code of the response based on error.statusCode.
  • Sends the error.message as the body.
  • Stacks are printed out with console.error and during development (when NODE_ENV is set to 'development') also sent in responses.
  • Usually, you don't need to invoke this method yourself, as you can use the built-in error handling flow with throw.
createError(code, msg, orig)
  • Use require('micro').createError.
  • Creates an error object with a statusCode.
  • Useful for easily throwing errors with HTTP status codes, which are interpreted by the built-in error handling.
  • orig sets error.originalError which identifies the original error (if any).

Error Handling

Micro allows you to write robust microservices. This is accomplished primarily by bringing sanity back to error handling and avoiding callback soup.

If an error is thrown and not caught by you, the response will automatically be 500. Important: Error stacks will be printed as console.error and during development mode (if the env variable NODE_ENV is 'development'), they will also be included in the responses.

If the Error object that's thrown contains a statusCode property, that's used as the HTTP code to be sent. Let's say you want to write a rate limiting module:

const rateLimit = require('my-rate-limit')
module.exports = async (req, res) => {
  await rateLimit(req)
  // ... your code

If the API endpoint is abused, it can throw an error with createError like so:

if (tooMany) {
  throw createError(429, 'Rate limit exceeded')

Alternatively you can create the Error object yourself

if (tooMany) {
  const err = new Error('Rate limit exceeded')
  err.statusCode = 429
  throw err

The nice thing about this model is that the statusCode is merely a suggestion. The user can override it:

try {
  await rateLimit(req)
} catch (err) {
  if (429 == err.statusCode) {
    // perhaps send 500 instead?
    send(res, 500)

If the error is based on another error that Micro caught, like a JSON.parse exception, then originalError will point to it. If a generic error is caught, the status will be set to 500.

In order to set up your own error handling mechanism, you can use composition in your handler:

const {send} = require('micro')
const handleErrors = fn => async (req, res) => {
  try {
    return await fn(req, res)
  } catch (err) {
    send(res, 500, 'My custom error!')
module.exports = handleErrors(async (req, res) => {
  throw new Error('What happened here?')


Micro makes tests compact and a pleasure to read and write. We recommend ava, a highly parallel Micro test framework with built-in support for async tests:

const micro = require('micro')
const test = require('ava')
const listen = require('test-listen')
const request = require('request-promise')
test('my endpoint', async t => {
  const service = micro(async (req, res) => {
    micro.send(res, 200, {
      test: 'woot'
  const url = await listen(service)
  const body = await request(url)
  t.deepEqual(JSON.parse(body).test, 'woot')

Look at test-listen for a function that returns a URL with an ephemeral port every time it's called.


  1. Fork this repository to your own GitHub account and then clone it to your local device
  2. Link the package to the global module directory: npm link
  3. Transpile the source code and watch for changes: npm start
  4. Within the module you want to test your local development instance of Micro, just link it to the dependencies: npm link micro. Instead of the default one from npm, node will now use your clone of Micro!

As always, you can run the AVA and ESLint tests using: npm test


Thanks to Tom Yandell and Richard Hodgson for donating the name "micro" on npm!