Serverless Middleware

Serverless plugin to allow middleware handlers configured directly in serverless.yaml

Requirements:

• Serverless v3
• AWS provider
• Node.js 14+

Supported runtimes

• [x] nodejs12.x (both Javascript and Typescript)
• [x] nodejs14.x (both Javascript and Typescript)
• [x] nodejs16.x (both Javascript and Typescript)
• [x] nodejs18.x (both Javascript and Typescript)
• [x] nodejs20.x (both Javascript and Typescript)
• [ ] dotnetcore2.1
• [ ] java8
• [ ] java11
• [ ] go1.x
• [ ] python2.7
• [ ] python3.7
• [ ] ruby2.5
• [ ] provided

Installation

Install via npm in the root of your Serverless service:

npm install serverless-middleware --save-dev

Add the plugin to the plugins array in your Serverless serverless.yaml:

plugins:
  - serverless-middleware

How it works

Middleware allows you to set up multiple handlers to be executed sequentially including error handlers that will capture any exception in the chain.

Middlewares are just standard AWS lambda handlers that return a promise (or are async). Handlers using callback will NOT work.

const myMiddleware = async (event, context) => { ... };

Once serverless-middleware is installed you can set the function.middleware property to an array and skip the function.handler property. Each middleware handler can be a string (like a standard handler would be) or an object containing the properties then and/or catch.

For example:

provider:
  name: aws
  runtime: nodejs20.x
  
functions:
  myFunction:
    middleware:
      - auth.authenticate
      - auth.authorize
      - then: myFunction.handler # `then:` is unnecessary here.
      - catch: utils.handlerError
      - # or both can be combined
        then: logger.log
        catch: utils.handlerLoggerError

will result in an execution like:

Promise.resolve()
  .then(require('./auth').authenticate)
  .then(require('./auth').authorize)
  .then(require('./myFunction').handler)
  .catch(require('./utils').handlerError)
  .then(require('./logger').log)
  .catch(require('./utils').handlerLoggerError);

As with standard promises, catch handlers are only executed when there are exceptions. The resulting lambda will return the result returned by the last middleware handler executed.

The event and context objects are passed from handler to handler so you can attach new properties to be accessed by subsequent handlers. context always contains the result of the previous handler in the prev property. The user can also stop at any point in the chain by calling the end method in the context argument. After context.end() is called, no more handlers will be executed.

For example:

const myMiddleware = async (event, context) => {
  if (context.prev === undefined) {
    // Previous middleware handler didn't return. End execution.
    context.end();
    return {
      statusCode: 200,
      body: 'No results',
    };
  }

  ...
};

You can also add pre/pos- middleware handlers and maintain the function.handler. These middleware are just prepended/appended to the main handler.

For example:

provider:
  name: aws
  runtime: nodejs20.x
  
functions:
  myFunction:
    events:
      - http:
          path: my-function
          method: get
    handler: myFunction.handler
    middleware:
      pre:
        - auth.authenticate
        - auth.authorize
      pos:
        - catch: utils.handlerError

You can also add pre/pos- middleware handlers at the package level using the custom.middleware section of serverless.yaml. These middleware are just prepended/appended to all the function middleware handlers chain.

For example:

provider:
  name: aws
  runtime: nodejs20.x

custom:
  middleware:
    pre:
      - auth.authenticate
    pos:
      - catch: utils.handlerError

functions:
  myAnonymousFunction:
    events:
      - http:
          path: my-anonymous-function
          method: get
    handler: myAnonymousFunction.handler
  myFunction:
    events:
      - http:
          path: my-function
          method: get
    handler: myFunction.handler
    middleware:
      pre:
        - auth.authorize

will result in a similar promise chain as above.

Packaging

In most cases, you shouldn't need to change the default packaging configuration. For edge cases, Middleware can be configured to use a specific intermediary folder and to not clear it after creating the serverless package.

These settings are also set in the custom.middleware section of serverless.yaml

custom:
  middleware:
    folderName: my_custom_folder  # defaults to '.middleware'
    cleanFolder: false            # defaults to 'true'

This might be useful if you are using sls package and building your own artifacts.

Migrations

v1.0.0 to 2.0.0

Use function.middleware instead fo function.custom.middleware

v0.0.14 to v0.0.15

Use function.custom.middleware instead fo function.handler

Passing an array to the handler property is not allowed anymore since Serverless is getting stricter with it's types and it also causes issues with Typescript.

So

functions:
  myFunction:
    handler:
      - auth.authenticate
      - auth.authorize
      - then: myFunction.handler # `then:` is unnecessary here.
      - catch: utils.handlerError
      - # or both can be combined
        then: logger.log
        catch: utils.handlerLoggerError

becomes

functions:
  myFunction:
    custom:
      middleware:
        - auth.authenticate
        - auth.authorize
        - then: myFunction.handler # `then:` is unnecessary here.
        - catch: utils.handlerError
        - # or both can be combined
          then: logger.log
          catch: utils.handlerLoggerError

Contribute

Help us to make this plugin better.

• Clone the code
• Install the dependencies with npm install
• Create a feature branch git checkout -b new_feature
• Add your code and add tests if you implement a new feature
• Validate your changes npm run lint and npm test (or npm run test-with-coverage)

License

This software is released under the MIT license. See the license file for more details.