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Serverless Framework - Part 3: The Guts of a Serverless Service

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This is part of an ongoing series about the Serverless framework. For those following along, part 1 and part 2 have been updated for the current latest version of Serverless 0.3.1.

In this post, we’ll discuss how a Serverless function actually works.

The guts of a serverless function

When we visited the deployed endpoint at the end of part 1, it correctly returned some JSON content.

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{
    message: "Your Serverless function ran successfully!"
}

Where does this message come from? Look at index.js in the component’s lib folder.

nodejscomponent/lib/index.js 
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/**
 * Lib
 */

module.exports.respond = function(event, cb) {

  var response = {
    message: "Your Serverless function ran successfully!"
  };

  return cb(null, response);
};

And it’s the handler.js file in the function’s subfolder which calls it.

nodejscomponent/potd/check/handler.js 
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'use strict';

/**
 * Serverless Module: Lambda Handler
 * - Your lambda functions should be a thin wrapper around your own separate
 * modules, to keep your code testable, reusable and AWS independent
 * - 'serverless-helpers-js' module is required for Serverless ENV var support.  Hopefully, AWS will add ENV support to Lambda soon :)
 */

// Require Serverless ENV vars
var ServerlessHelpers = require('serverless-helpers-js').loadEnv();

// Require Logic
var lib = require('../../lib');

// Lambda Handler
module.exports.handler = function(event, context) {

  lib.respond(event, function(error, response) {
    return context.done(error, response);
  });
};

In our case we’re coding a password checking function. The URI will look something like this:

http://something.amazonaws.com/development/potd/check?password=P455w0rd

We’ll modify lib/index.js to retrieve the value from the query parameter password and return true if the password is correct and false otherwise. But first we need to set up the parameter.

Configuring the function parameter

In each function’s directory, there is a file named s-function.json which allows you to specify the details of the function call. Add a section to the requestTemplates as follows:

nodejscomponent/potd/check/s-function.js 
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{
  "name": "check",
  "handler": "potd/check/handler.handler",
  "timeout": 6,
  "memorySize": 1024,
  "custom": {
    "excludePatterns": [],
    "envVars": []
  },
  "endpoints": [
    {
      "path": "potd/check",
      "method": "GET",
      "authorizationType": "none",
      "apiKeyRequired": false,
      "requestParameters": {},
      "requestTemplates": {
+       "application/json": {
+           "password": "$input.params('password')"
+       }
      },
      "responses": {
        "400": {
          "statusCode": "400"
        },
        "default": {
          "statusCode": "200",
          "responseParameters": {},
          "responseModels": {},
          "responseTemplates": {
            "application/json": ""
          }
        }
      }
    }
  ]
}

Note that the s-function.json file is also where you can configure if the service accepts POST or PUT or DELETE requests. Here we are only interested in GET.

You can easily tailor the requestTemplates in this file to extract whatever parameters you need in your lambda function.

Retrieving the parameter value

Now back in index.js you will find that the function’s event parameter has a property password which is set to the value of the querystring parameter.

nodejscomponent/lib/index.js 
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module.exports.respond = function(event, cb) {

  var parameterValue = event.password; // the querystring parameter
  var response = {
    message: parameterValue
  };

  return cb(null, response);
};

Redeploy.

 $ serverless dash deploy

Visit the URI.

http://something.amazonaws.com/development/potd/check?password=P455w0rd

The response is:

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{
    message: "P455w0rd"
}

Ready for implementation

We now have all the pieces we need. Serverless, Typescript, Mocha and AWS. In the next post I’ll show how to wire up everything get it working.

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