README.md.hb

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  <p><strong>The stylish Node.js middleware engine for AWS Lambda</strong></p>
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## TOC

 - [A little appetizer](#a-little-appetizer)
 - [Install](#install)
 - [Requirements](#requirements)
 - [Why?](#why)
 - [Usage](#usage)
 - [How it works](#how-it-works)
 - [Writing a middleware](#writing-a-middleware)
 - [Available middlewares](#available-middlewares)
 - [API](#api)
 - [Contributing](#contributing)
 - [License](#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 a JSON API to process a payment:

```javascript
# handler.js

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

// This is your common handler, in no way different than what you are used to doing 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(urlEncodeBodyParser()) // 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:

```bash
npm install middy
```

or

```bash
yarn add 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](https://babeljs.io/) 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 a 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](http://expressjs.com/),
[fastify](http://fastify.io), [hapi](https://hapijs.com/), etc.), this
problem has been solved using the [middleware pattern](https://www.packtpub.com/mapt/book/web_development/9781783287314/4/ch04lvl1sec33/middleware).

This pattern allows developers to isolate these 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 your 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:

```javascript
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](#inline-middlewares) by using the functions `.before`, `.after` and
`.onError`.

For a more detailed use case and examples check the [Writing a middleware section](#writing-a-middleware) and
the [API section](#api).


## How it works

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

![Middy middleware engine diagram](/img/middy-middleware-engine.png)

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 the opportunity within every step 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.


### Interrupt middleware execution early

Some middlewares might need to stop the whole execution flow and return a response immediately.

If you want to do this you can invoke `handler.callback` in your middleware and return early without invoking `next`.

**Note**: this will totally stop the execution of successive middlewares in any phase (`before` and `after`) and returns
an early response (or an error) directly at the Lambda level. If your middlewares do a specific task on every request
like output serialization or error handling, these won't be invoked in this case.

In this example we can use this capability for building a sample caching middleware:

```javascript

// some function that calculates the cache id based on the current event
const calculateCacheId = (event) => { /* ... */ }
const storage = {}

// middleware
const cacheMiddleware = (options) => {
  let cacheKey
  return ({
    before: (handler, next) => {
      cacheKey = options.calculateCacheId(handler.event)
      if (options.storage.hasOwnProperty(cacheKey)) {
        // exits early and returns the value from the cache if it's already there
        return handler.callback(null, options.storage[cacheKey])
      }

      return next()
    },
    after: (handler, next) => {
      // stores the calculated response in the cache
      options.storage[cacheKey] = handler.response
      next()
    }
  })
}

// sample usage
const handler = middy((event, context, callback) => { /* ... */ })
  .use(cacheMiddleware({
    calculateCacheId, storage
  }))
```


### Handling errors

But what happens when 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.

### Promise support

Middy allows you to return promises (or throw errors) from your handlers (instead of calling `callback()`) and middlewares
(instead of calling `next()`).

Here is an example of a handler that returns a promise:

```javascript
middy((event, context, callback) => {
  return someAsyncStuff()
    .then(() => {
      return someOtherAsyncStuff()
    })
    .then(() => {
      return {foo: bar}
    }
})
```

And here is an example of a middleware that returns a similar promise:

```javascript
const asyncValidator = () => {
  before: (handler) => {
    if (handler.event.body) {
      return someAsyncStuff(handler.event.body)
        .then(() => {
          return {foo: bar}
        })
    }

    return Promise.resolve()
  }
}

handler.use(asyncValidator())
```


### Using async/await

Node.js 8.10 supports [async/await](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function),
allowing you to work with promises in a way that makes handling asynchronous logic easier to reason about and
asynchronous code easier to read.

You can still use async/await if you're running AWS Lambda on Node.js 6.10, but you will need to transpile your
`async/await` code (e.g. using [babel](https://babeljs.io/)).

Take the following code as an example of a handler written with async/await:

```javascript
middy(async (event, context) => {
  await someAsyncStuff()
  await someOtherAsyncStuff()

  return ({foo: bar})
})
```


And here is an example of a middleware written with async/await:

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

      return {foo: bar}
    }

    return
  }
}

handler.use(asyncValidator())
```


## 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:

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

Where:

 - `handler`: is a reference to the current context and it allows access to (and modification of)
   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 has 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.

```javascript
# 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:

```javascript
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 a very small need 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 functions to hook
logic into Middy's control flow.

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

```javascript
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 also exposes 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](/src/middlewares) to see more examples
on how to write a middleware.


## Available middlewares

Currently available middlewares:

 - [`cache`](/docs/middlewares.md#cache): A simple but flexible caching layer
 - [`cors`](/docs/middlewares.md#cors): Sets CORS headers on response
 - [`doNotWaitForEmptyEventLoop`](/docs/middlewares.md#donotwaitforemptyeventloop): Sets callbackWaitsForEmptyEventLoop property to false
 - [`httpContentNegotiation`](/docs/middlewares.md#httpcontentnegotiation): Parses `Accept-*` headers and provides utilities for content negotiation (charset, encoding, language and media type) for HTTP requests
 - [`httpErrorHandler`](/docs/middlewares.md#httperrorhandler): Creates a proper HTTP response for errors that are created with the [http-errors](https://www.npmjs.com/package/http-errors) module and represents proper HTTP errors.
 - [`httpEventNormalizer`](/docs/middlewares.md#httpEventNormalizer): Normalizes HTTP events by adding an empty object for `queryStringParameters` and `pathParameters` if they are missing.
 - [`httpHeaderNormalizer`](/docs/middlewares.md#httpheadernormalizer): Normalizes HTTP header names to their canonical format
 - [`httpPartialResponse`](/docs/middlewares.md#httppartialresponse): Filter response objects attributes based on query string parameters.
 - [`jsonBodyParser`](/docs/middlewares.md#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
 `httpErrorHandler`.
 - [`s3KeyNormalizer`](/docs/middlewares.md#s3keynormalizer): Normalizes key names in s3 events.
 - [`ssm`](/docs/middlewares.md#ssm): Fetches parameters from [AWS Systems Manager Parameter Store](https://docs.aws.amazon.com/systems-manager/latest/userguide/systems-manager-paramstore.html).
 - [`validator`](/docs/middlewares.md#validator): Automatically validates incoming events and outgoing responses against custom schemas
 - [`urlEncodeBodyParser`](/docs/middlewares.md#urlencodebodyparser): Automatically parses HTTP requests with URL encoded body (typically the result of a form submit).
 - [`warmup`](/docs/middlewares.md#warmup): Warmup middleware that helps to reduce the [cold-start issue](https://serverless.com/blog/keep-your-lambdas-warm/)


For dedicated documentation on available middlewares check out the [Middlewares
documentation](/docs/middlewares.md)

## Api

{{> main}}


## Contributing

Everyone is very welcome to contribute to this repository. Feel free to [raise issues](https://github.com/middyjs/middy/issues) or to [submit Pull Requests](https://github.com/middyjs/middy/pulls).


## License

Licensed under [MIT License](LICENSE). Copyright (c) 2017-2018 Luciano Mammino and the [Middy team](https://github.com/middyjs/middy/graphs/contributors).

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