Middy

What is Middy

Middy is a very simple middleware engine that allows you to simplify your AWS Lambda code when using Node.js.

If you are used to web frameworks like Express, then you will be familiar with the concepts adopted in Middy and you will be able to get started very quickly.

A middleware engine allows you to focus on the strict business logic of your Lambda and then attach additional common elements like authentication, authorization, validation, serialization, etc. in a modular and reusable way by decorating the main business logic.

Install

To install middy, you can use NPM:

npm install --save @middy/core

If you are using TypeScript, you will also want to make sure that you have installed the @types/aws-lambda peer-dependency:

npm install --save-dev @types/aws-lambda

Quick example

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

// import core
const middy = require('@middy/core')

// import some middlewares
const jsonBodyParser = require('@middy/http-json-body-parser')
const httpErrorHandler = require('@middy/http-error-handler')
const validator = require('@middy/validator')

// 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' }
     }
   }
 },
 required: ['creditCardNumber'] // Insert here all required event properties
}

// Let's "middyfy" our handler, then we will be able to attach middlewares to it
const handler = middy(processPayment)
  .use(jsonBodyParser()) // 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 }

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 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:

const middy = require('@middy/core')
const middleware1 = require('sample-middleware1')
const middleware2 = require('sample-middleware2')
const middleware3 = require('sample-middleware3')

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

const handler = middy(originalHandler)

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

module.exports = { handler }

.use() takes a single middleware or an array of middlewares, so you can attach multiple middlewares in a single call:

const middy = require("@middy/core");
const middleware1 = require("sample-middleware1");
const middleware2 = require("sample-middleware2");
const middleware3 = require("sample-middleware3");
const middlewares = [middleware1(), middleware2(), middleware3()]

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

const handler = middy(originalHandler);

handler.use(middlewares)

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.

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 implemented 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())

Promises and error handling

onError middlewares can return promises as well. Here's how Middy handles return values from promise-enabled error handlers:

• If onError promise resolves to a truthy value, this value is treated as an error and passed further down the pipeline.

middleware1 = {
  onError: (handler) => {
    Logger.debug("middleware1");
    return Promise.resolve(handler.error)
  }
}
middleware2 = {
  onError: (handler) => {
    Logger.debug("middleware2");
    return Promise.resolve(handler.error)
  }
}
handler.use(middleware1).use(middleware2);

Here, first middleware1.onError then middleware2.onError will be called.

• If the last onError in the chain returns a promise which resolves to a value, the lambda fails and reports an unmanaged error In the example above, the lambda will fail and report the error returned by middleware2.onError.
• If onError promise resolves to a falsy value (null, undefined, false etc.), the error handling pipeline exits early and the response is returned without an error.

middleware1 = {
  onError: (handler) => {
    handler.response = { error: handler.error };
    return Promise.resolve();
    // Resolves to a falsy value
  }
}
middleware2 = {
  onError: (handler) => {
    return Promise.resolve(handler.error)
  }
}
handler.use(middleware1).use(middleware2);

Here, only middleware1.onError will be called. The rest of the error handlers will be skipped, and the lambda will finish normally and return the response. middleware2.onError will not be called.

• If onError promise rejects, the error handling pipeline exits early and the lambda execution fails.

middleware1 = {
  onError: (handler) => {
    return Promise.reject(handler.error);
  }
}
middleware2 = {
  onError: (handler) => {
    return Promise.resolve(handler.error)
  }
}
handler.use(middleware1).use(middleware2);

Here, only middleware1.onError will be called, and the lambda will fail early, reporting an error. middleware2.onError will not be called.

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 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.

# 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/core')
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/core')

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 layer
• db-manager: Provides seamless connection with database of your choice
• do-not-wait-for-empty-event-loop: Sets callbackWaitsForEmptyEventLoop property to false
• function-shield: Hardens AWS Lambda execution environment
• http-content-negotiation: Parses Accept-* headers and provides utilities for content negotiation (charset, encoding, language and media type) for HTTP requests
• http-cors: Sets HTTP CORS headers on response
• http-error-handler: Creates a proper HTTP response for errors that are created with the http-errors module and represents proper HTTP errors.
• http-event-normalizer: Normalizes HTTP events by adding an empty object for queryStringParameters, multiValueQueryStringParameters or pathParameters if they are missing.
• http-header-normalizer: Normalizes HTTP header names to their canonical format
• http-json-body-parser: 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.
• http-multipart-body-parser: Automatically parses HTTP requests with content type multipart/form-data and converts the body into an object.
• http-partial-response: Filter response objects attributes based on query string parameters.
• http-security-headers: Applies best practice security headers to responses. It's a simplified port of HelmetJS.
• http-urlencode-body-parser: Automatically parses HTTP requests with URL encoded body (typically the result of a form submit).
• http-urlencode-path-parser: Automatically parses HTTP requests with URL encoded path.
• s3-key-normalizer: Normalizes key names in s3 events.
• secrets-manager: Fetches parameters from AWS Secrets Manager.
• ssm: Fetches parameters from AWS Systems Manager Parameter Store.
• validator: Automatically validates incoming events and outgoing responses against custom schemas
• warmup: Warmup middleware that helps to reduce the cold-start issue

Community generated middleware

The following middlewares are created and maintained outside this project. We cannot guarantee for its functionality. If your middleware is missing, feel free to open a Pull Request.

• middy-redis: Redis connection middleware
• middy-extractor: Extracts data from events using expressions
• @keboola/middy-error-logger: middleware that catches thrown exceptions and rejected promises and logs them comprehensibly to the console
• @keboola/middy-event-validator: Joi powered event validation middleware
• middy-reroute: provides complex redirect, rewrite and proxying capabilities by simply placing a rules file into your S3 bucket
• middytohof: Convert Middy middleware plugins to higher-order functions returning lambda handlers
• wrap-ware: A middleware wrapper which works with promises / async
• middy-jsonapi: JSONAPI middleware for middy
• middy-middleware-warmup: A middy plugin to help keep your Lambdas warm during Winter
• @sharecover-co/middy-aws-xray-tracing: AWS X-Ray Tracing Middleware
• @sharecover-co/middy-http-response-serializer: This middleware serializes the response to JSON and wraps it in a 200 HTTP response
• @seedrs/middyjs-middleware: Collection of useful middlewares
• middy-autoproxyresponse: A middleware that lets you return simple JavaScript objects from Lambda function handlers and converts them into LAMBDA_PROXY responses
• jwt-auth: JSON web token authorization middleware based on express-jwt

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.