serverless-wrapper-plugin

Serverless Wrapper Plugin

This goal of this plugin is to provide an easy way to wrap your serverless functions with a wrapper function, without having to edit all the functions themselves.

One use case for this is for example, if you want to override console.warn to prepend a custom string, to make these warning easier to trace in the logs. You could add this to the begining of every handler in your project, or you could write one wrapper function to perform this override and use the serverless-wrapper-plugin to automatically wrap all your handlers.

Another use case might be, you want to write a process to periodically make requests to your functions to keep them hot. You hadn't planned for this when you wrote all your handlers, so what input do you send to avoid causing either errors or unwanted side-effects? With the serverless-wrapper-plugin you could write a wrapper to intercept the input event, and if it is the dummy "wake up" event, then ignore it and return. If it isn't the dummy event then simply pass it through to the handler as normal.

UPDATE 1.1.0

Support function specific wrapper function, using wrapper.path option, set using relative wrapper path to the root of the project in the s-function.json file.
Support skipping wrapper if function-specific wrapper.path is set to false

Setup

Not compatible with Serverless 1.0.0 and above Support only Node Lambdas

NOTE: If you are using the serverless-webpack-plugin, this plugin must be before the webpack plugin in the list.

Wrapper Function

Firstly you need to write the wrapper function that you want to apply. This wrapper function should have the following form:

function myWrapper(handler, event, context) {
    // Do whatever you like here..
    // ...

    // Call the original handler
    return handler(event, context);
}

module.exports = myWrapper;

The way that this is used is that the handler is transformed to be something like this:

const _handler = require('...path to original handler...');
const _wrapper = require('...path to myWrapper...');

module.exports.handler = function (event, context) {
    return _wrapper(_handler, event, context);
}

Plugin Installation

Install the plugin in the root of your Serverless Project:

npm install serverless-wrapper-plugin --save-dev

Add the plugin to the plugins array in your Serverless Project's s-project.json, as below.

NOTE: If you are using the serverless-webpack-plugin, this plugin must be before the webpack plugin in the list.

"plugins": [
    "serverless-wrapper-plugin"
]

In the custom property of either your s-project.json or s-function.json add a default wrapper property. The path is relative to the project root. This is a fallback if your s-function does not contain a wrapper.path property.

{
    ...
    "custom": {
        "wrapper": {
            "path": "path/relative/to/project-root"
        }
    }
    ...
}

To set a custom wrapper for specific function, add the wrapper function's relative path to s-function.json. If you do not want your function to be wrapped by any wrapper function, event the default one, set the wrapper propertry to false

  ...
  "name": "your-function",
  "runtime": "nodejs4.3",
  "custom": {
    "wrapper" {
      "path": "wrapper.js",
    }
  }
  "handler": "handler.handler",
  ...
  ...
  "name": "your-function",
  "runtime": "nodejs4.3",
  "custom": {
    "wrapper": false
  }
  "handler": "handler.handler",
  ...

Development

A brief note about development of the plugin itself. Part of the function of this plugin is to generate code. To do this it uses the template engine doT.js.

There is a template in lib/wrapped-handler.jst this is pre-compiled for runtime efficiency and saved as lib/wrapped-handler.js.

To re-compiled this template, you can use the convenience script in bin/compile-template.js