The stylish Node.js middleware engine for AWS Lambda
- A little appetizer
- Install
- Requirements
- Why?
- Usage
- How it works
- Writing a middleware
- Available middlewares
- API
- Contributing
- License
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 }
As simple as:
npm install middy
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.
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.
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 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. - 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.
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.
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.
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.
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 to access (and modify) 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 finished its job so that the next middleware can be invoked
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 }
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.
Check the code for existing middlewares to have more examples on how to write a middleware.
Currently available middlewares:
cors
: sets CORS headers on responsehttpErrorHandler
: 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 ofhttpErrorHanler
.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 schemasdoNotWaitForEmptyEventLoop
: sets callbackWaitsForEmptyEventLoop property to false
For a dedicated documentation on those middlewares check out the Middlewares documentation
- middy(handler) β
middy
Middy factory function. Use it to wrap your existing handler to enable middlewares on it.
- middy :
function
- useFunction β
middy
- middlewareAttachFunction β
middy
- middlewareFunction :
function
- middlewareObject :
Object
middy(handler) β middy
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 |
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 |
useFunction β middy
Kind: global typedef
Type | Description |
---|---|
middlewareObject |
the middleware object to attach |
middlewareAttachFunction β middy
Kind: global typedef
Type | Description |
---|---|
middlewareFunction |
the middleware function to attach |
Kind: global typedef
Param | Type | Description |
---|---|---|
handler | function |
the original handler function. It will expose properties event , context , response and error that can be used to interact with the middleware lifecycle |
next | function |
the callback to invoke to pass the control to the next middleware |
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 |
Everyone is very welcome to contribute to this repository. Feel free to raise issues or to submit Pull Requests.
Licensed under MIT License. Copyright (c) 2017 Planet9.