@jose_santacruz/middy v0.3.3

TOC

• A little appetizer
• Install
• Requirements
• Why?
• Usage
• How it works
• Writing a middleware
• Available middlewares
• API
• Contributing
• License

A little appetizer

Middy is a very simple middleware engine. If you are used to web frameworks like express, than you will be familiar with the concepts adopted in Middy and you will be able to get started very quickly.

But code is better than 10.000 words, so let's jump into an example. Let's assume you are building an JSON API to process a payment:

# handler.js

const middy = require('middy')
const { urlEncodedBodyParser, validator, httpErrorHandler } = require('middy/middlewares')

// This is your common handler, no way different than what you are used to do every day
// in AWS Lambda
const processPayment = (event, context, callback) => {
  // we don't need to deserialize the body ourself as a middleware will be used to do that
  const { creditCardNumber, expiryMonth, expiryYear, cvc, nameOnCard, amount } = event.body

  // do stuff with this data
  // ...

  return callback(null, { result: 'success', message: 'payment processed correctly'})
}

// Notice that in the handler you only added base business logic (no deserilization, validation or error handler), we will add the rest with middlewares

const inputSchema = {
  type: 'object',
  properties: {
    body: {
      type: 'object',
      properties: {
        creditCardNumber: { type: 'string', minLength: 12, maxLength: 19, pattern: '\d+' },
        expiryMonth: { type: 'integer', minimum: 1, maximum: 12 },
        expiryYear: { type: 'integer', minimum: 2017, maximum: 2027 },
        cvc: { type: 'string', minLength: 3, maxLength: 4, pattern: '\d+' },
        nameOnCard: { type: 'string' },
        amount: { type: 'number' }
      }
    }
  }
}

// Let's "middyfy" our handler, then we will be able to attach middlewares to it
const handler = middy(processPayment)
  .use(urlEncodedBodyParser()) // parses the request body when it's a JSON and converts it to an object
  .use(validator({inputSchema})) // validates the input
  .use(httpErrorHandler()) // handles common http errors and returns proper responses

module.exports = { handler }

Install

As simple as:

npm install middy

Requirements

Middy has been built to work by default from Node >= 6.10.

If you need to run it in earlier versions of Node (eg. 4.3) then you will have to transpile middy's code yourself using babel or a similar tool.

Why?

One of the main strengths of serverless and AWS Lambda is that, from a developer perspective, your focus is mostly shifted toward implementing business logic.

Anyway, when you are writing an handler, you still have to deal with some common technical concerns outside business logic, like input parsing and validation, output serialization, error handling, etc.

Very often, all this necessary code ends up polluting the pure business logic code in your handlers, making the code harder to read and to maintain.

In other contexts, like generic web frameworks (express, fastify, hapi, etc.), this problem has been solved using the middleware pattern.

This pattern allows developers to isolate this common technical concerns into "steps" that decorate the main business logic code. Middleware functions are generally written as independent modules and then plugged in into the application in a configuration step, thus not polluting the main business logic code that remains clean, readable and easy to maintain.

Since we couldn't find a similar approach for AWS Lambda handlers, we decided to create middy, our own middleware framework for serverless in AWS land.

Usage

As you might have already got from our first example here, using middy is very simple and requires just few steps:

1. Write your Lambda handlers as usual, focusing mostly on implementing the bare business logic for them.
2. Import middy and all the middlewares you want to use
3. Wrap you handler in the middy() factory function. This will return a new enhanced instance of your original handler, to which you will be able to attach the middlewares you need.
4. Attach all the middlewares you need using the function .use(somemiddleware())

Example:

const middy = require('middy')
const { middleware1, middleware2, middleware3 } = require('middy/middlewares')

const originalHandler = (event, context, callback) => { /* your business logic */ }

const handler = middy(originalHandler)

handler
  .use(middleware1())
  .use(middleware2())
  .use(middleware3())

module.exports = { handler }

You can also attach inline middlewares by using the functions .before, .after and .onError.

For a more detailed use cases and examples check the Writing a middleware section and the API section.

How it works

Middy implements the classic onion-like middleware pattern, with some peculiar details.

When you attach a new middleware this will wrap the business logic contained in the handler in two separate steps.

When another middleware is attached this will wrap the handler again and it will be wrapped by all the previously added middlewares in order, creating multiple layers for interacting with the request (event) and the response.

This way the request-response cycle flows through all the middlewares, the handler and all the middlewares again, giving to every step, the opportunity to modify or enrich the current request, context or the response.

Execution order

Middlewares have two phases: before and after.

The before phase, happens before the handler is executed. In this code the response is not created yet, so you will have access only to the request.

The after phase, happens after the handler is executed. In this code you will have access to both the request and the response.

If you have three middlewares attached as in the image above this is the expected order of execution:

• middleware1 (before)
• middleware2 (before)
• middleware3 (before)
• handler
• middleware3 (after)
• middleware2 (after)
• middleware1 (after)

Notice that in the after phase, middlewares are executed in inverted order, this way the first handler attached is the one with the highest priority as it will be the first able to change the request and last able to modify the response before it gets sent to the user.

Handling errors

But what happens in case there is an error?

When there is an error, the regular control flow is stopped and the execution is moved back to all the middlewares that implements a special phase called onError, following the order they have been attached.

Every onError middleware can decide to handle the error and create a proper response or to delegate the error to the next middleware.

When a middleware handles the error and creates a response, the execution is still propagated to all the other error middlewares and they have a chance to update or replace the response as needed. At the end of the error middlewares sequence, the response is returned to the user.

If no middleware manages the error, the lambda execution fails reporting the unmanaged error.

Async Middlewares

Middy supports middlewares that return promises instead that directly calling the callback:

const asyncValidator = () => {
  before: (handler) => {
    if (handler.event.body) {
      return new Promise((resolve, reject) => {
        // async validation logic
      })
    }

    return Promise.resolve()
  }
}

handler.use(asyncValidator())

Thanks to this behaviour you can define middlewares using async functions:

const asyncValidator = () => {
  before: async (handler) => {
    if (handler.event.body) {
      return await asyncValidate(handler.event.body)
    }

    return
  }
}

handler.use(asyncValidator())

Of course, since AWS lambda runs on Node.js 6.10, you will need to transpile your async/await code (e.g. using babel).

Writing a middleware

A middleware is an object that should contain at least 1 of 3 possible keys:

1. before: a function that is executed in the before phase
2. after: a function that is executed in the after phase
3. onError: a function that is executed in case of errors

before, after and onError functions need to have the following signature:

function (handler, next) {
  // ...
}

Where:

• handler: is a reference to the current context and it allows to access (and modify) the current event (request), the response (in the after phase) and error (in case of an error).
• next: is a callback function that needs to be invoked when the middleware finished its job so that the next middleware can be invoked

Configurable middlewares

In order to make middlewares configurable they are generally exported as a function that accepts a configuration object. This function should then return the middleware object with before, after and onError as keys.

E.g.

# myMiddleware.js

const myMiddleware = (config) => {
  // might set default options in config
  return ({
    before: (handler, next) => {
      // might read options from `config`
    },
    after: (handler, next) => {
      // might read options from `config`
    },
    onError: (handler, next) => {
      // might read options from `config`
    }
  })
}

module.exports = myMiddleware

With this convention in mind, using a middleware will always look like the following example:

const middy = require('middy')
const myMiddleware = require('myMiddleware')

const handler = middy((event, context, callback) => {
  // do stuff
})

handler.use(myMiddleware({
  option1: 'foo',
  option2: 'bar'
}))

module.exports = { handler }

Inline middlewares

Sometimes you want to create handlers that serve very small needs and that are not necessarily re-usable. In such cases you probably will need to hook only into one of the different phases (before, after or onError).

In these cases you can use inline middlewares which are shortcut function to hook logic into Middy's control flow.

Let's see how inline middlewares work with a simple example:

const middy = require('middy')

const handler = middy((event, context, callback) => {
  // do stuff
})

handler.before((handler, next) => {
  // do something in the before phase
  next()
})

handler.after((handler, next) => {
  // do something in the after phase
  next()
})

handler.onError((handler, next) => {
  // do something in the on error phase
  next()
})

module.exports = { handler }

As you can see above, a middy instance exposes also the before, after and onError methods to allow you to quickly hook-in simple inline middlewares.

More details on creating middlewares

Check the code for existing middlewares to have more examples on how to write a middleware.

Available middlewares

Currently available middlewares:

• cors: sets CORS headers on response
• httpErrorHandler: creates a proper HTTP response for errors that are created with the http-errors module and represents proper HTTP errors.
• jsonBodyParser: automatically parses HTTP requests with JSON body and converts the body into an object. Also handles gracefully broken JSON if used in combination of httpErrorHanler.
• s3KeyNormalizer: normalizes key names in s3 events.
• urlencodeBodyParser: automatically parses HTTP requests with URL encoded body (typically the result of a form submit).
• validator: automatically validates incoming events and outgoing responses against custom schemas
• doNotWaitForEmptyEventLoop: sets callbackWaitsForEmptyEventLoop property to false

For a dedicated documentation on those middlewares check out the Middlewares documentation

Api

Functions

Typedefs

middy(handler) ⇒ middy

Middy factory function. Use it to wrap your existing handler to enable middlewares on it.

Kind: global function
Returns: middy - - a middy instance

middy : function

Kind: global typedef

Properties

useFunction ⇒ middy

Kind: global typedef

middlewareAttachFunction ⇒ middy

Kind: global typedef

middlewareNextFunction : function

Kind: global typedef

middlewareFunction ⇒ void | Promise

Kind: global typedef
Returns: void | Promise - - A middleware can return a Promise instead of using the next function as a callback. In this case middy will wait for the promise to resolve (or reject) and it will automatically propagate the result to the next middleware.

middlewareObject : Object

Kind: global typedef
Properties

Contributing

Everyone is very welcome to contribute to this repository. Feel free to raise issues or to submit Pull Requests.

License

Licensed under MIT License. Copyright (c) 2017 Planet9.