The stylish Node.js middleware engine for AWS Lambda
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 a JSON API to process a payment:
# 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:
npm install middy
or
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 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, fastify, hapi, etc.), this problem has been solved using the middleware pattern.
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:
- Write your Lambda handlers as usual, focusing mostly on implementing the bare business logic for them.
- Import
middy
and all the middlewares you want to use - 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. - 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 case 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 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:
// 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:
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:
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, 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).
Take the following code as an example of a handler written with async/await:
middy(async (event, context) => {
await someAsyncStuff()
await someOtherAsyncStuff()
return ({foo: bar})
})
And here is an example of a middleware written with async/await:
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:
before
: a function that is executed in the before phaseafter
: a function that is executed in the after phaseonError
: 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 access to (and modification of) the currentevent
(request), theresponse
(in the after phase) anderror
(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.
# 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 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:
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 to see more examples on how to write a middleware.
Available middlewares
Currently available middlewares:
cache
: A simple but flexible caching layercors
: Sets CORS headers on responsedoNotWaitForEmptyEventLoop
: Sets callbackWaitsForEmptyEventLoop property to falsehttpContentNegotiation
: ParsesAccept-*
headers and provides utilities for content negotiation (charset, encoding, language and media type) for HTTP requestshttpErrorHandler
: Creates a proper HTTP response for errors that are created with the http-errors module and represents proper HTTP errors.httpEventNormalizer
: Normalizes HTTP events by adding an empty object forqueryStringParameters
andpathParameters
if they are missing.httpHeaderNormalizer
: Normalizes HTTP header names to their canonical formathttpPartialResponse
: Filter response objects attributes based on query string parameters.jsonBodyParser
: Automatically parses HTTP requests with JSON body and converts the body into an object. Also handles gracefully broken JSON if used in combination ofhttpErrorHandler
.s3KeyNormalizer
: Normalizes key names in s3 events.ssm
: Fetches parameters from AWS Systems Manager Parameter Store.validator
: Automatically validates incoming events and outgoing responses against custom schemasurlEncodeBodyParser
: Automatically parses HTTP requests with URL encoded body (typically the result of a form submit).warmup
: Warmup middleware that helps to reduce the cold-start issue
For dedicated documentation on available middlewares check out the Middlewares documentation
Api
- middy(handler) β
middy
- middy :
function
- useFunction β
middy
- middlewareAttachFunction β
middy
- middlewareNextFunction :
function
- middlewareFunction β
void
|Promise
- middlewareObject :
Object
middy
middy(handler) β Middy factory function. Use it to wrap your existing handler to enable middlewares on it.
Kind: global function
Returns: middy
- - a middy
instance
Param | Type | Description |
---|---|---|
handler | function |
your original AWS Lambda function |
function
middy : Kind: global typedef
Param | Type | Description |
---|---|---|
event | Object |
the AWS Lambda event from the original handler |
context | Object |
the AWS Lambda context from the original handler |
callback | function |
the AWS Lambda callback from the original handler |
Properties
Name | Type | Description |
---|---|---|
use | useFunction |
attach a new middleware |
before | middlewareAttachFunction |
attach a new before-only middleware |
after | middlewareAttachFunction |
attach a new after-only middleware |
onError | middlewareAttachFunction |
attach a new error-handler-only middleware |
__middlewares | Object |
contains the list of all the attached middlewares organised by type (before , after , onError ). To be used only for testing and debugging purposes |
middy
useFunction β Kind: global typedef
Type | Description |
---|---|
middlewareObject |
the middleware object to attach |
middy
middlewareAttachFunction β Kind: global typedef
Type | Description |
---|---|
middlewareFunction |
the middleware function to attach |
function
middlewareNextFunction : Kind: global typedef
Param | Type | Description |
---|---|---|
error | error |
An optional error object to pass in case an error occurred |
void
| Promise
middlewareFunction β 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.
Param | Type | Description |
---|---|---|
handler | function |
the original handler function. It will expose properties event , context , response , error and callback that can be used to interact with the middleware lifecycle |
next | middlewareNextFunction |
the callback to invoke to pass the control to the next middleware |
Object
middlewareObject : Kind: global typedef
Properties
Name | Type | Description |
---|---|---|
before | middlewareFunction |
the middleware function to attach as before middleware |
after | middlewareFunction |
the middleware function to attach as after middleware |
onError | middlewareFunction |
the middleware function to attach as error middleware |
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) 2017-2018 Luciano Mammino and the Middy team.