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README.md Updates for FDK for Language instances. Function Development Kit Oct 29, 2019

README.md

Fn Function Development Kit for Node.js

This Function Development Kit makes it easy to deploy Node.js functions to Fn.

Creating a Node Function

Writing a Node.js function is simply a matter of writing a handler function that you pass to the FDK to invoke each time your function is called.

Start by creating a node function with fn init and installing the FDK:

fn init --runtime node nodefunc
cd nodefunc

This creates a simple hello world function in func.js:

var fdk = require('@fnproject/fdk');

fdk.handle(function(input) {
  var name = 'World';
  if (input.name) {
    name = input.name;
  }
  response = {'message': 'Hello ' + name}
  return response
})

The handler function takes the string input that is sent to the function and returns a response string. Using the FDK you don't have to worry about reading input from standard input and writing to standard output to return your response. The FDK let's you focus on your function logic and not the mechanics.

Now run it!

fn run

Now you have a basic running Node function that you can modify and add what you want.

From this point on, it's the same as any other function in any language. deploy the function:

fn deploy --app fnfdk --local

Once deployed, you and invoke the function:

echo -n "Tom" | fn call fdkdemo /hello

or

curl -d "Tom" http://localhost:8080/r/fdkdemo/hello

In both cases you'll get the response:

Hello Tom from Node!

Function Context

Function invocation context details are available through an optional function argument. To receive a context object, simply add a second argument to your handler function. in the following example the call_id is obtained from the context and included in the response message:

var fdk = require('@fnproject/fdk');

fdk.handle(function(input, ctx) {
  var name = 'World';
  if (input) {
    name = input;
  }
  return 'Hello ' + name + ' from Node call ' + ctx.callId + '!';
})

In the case of default format functions the context give you access to all environment variables including those defined through function or app config as well as those automatically provided by Fn like app_name, path, memory, etc.

Asynchronous function responses

You return an asynchronous response from a function by returning a Javascript Promise from the function body:

var fdk = require('@fnproject/fdk');

fdk.handle(function(input, ctx) {
  return new Promise((resolve, reject) => {
     setTimeout(() => resolve("Hello"), 1000);
  });
})

Handling non-json input and output

By default the FDK will convert input with a content-type matching application/json into a JSON object as the function input, if the incoming content type is different from application/json then the input will be the raw string value of the input. In both cases, the raw (string) version of the input is also available in ctx.body.

Likewise by default the output of a function will be treated as a JSON object and converted using JSON.stringify - you can change this behaviour by setting the content type of the response in the context using ctx.responseContentType='application/text-plain'. Changing the content type to non-json will result in the output being treated as a string.

Using HTTP headers and setting HTTP status codes

You can read http headers passed into a function invocation using ctx.protocol.header(key), this returns the first header value of the header matching key (after canonicalization) and ctx.protocol.headers which returns an object containing all headers.

var fdk = require('@fnproject/fdk');

fdk.handle(function(input, ctx) {
  console.log("Authorization header:" , ctx.protocol.header("Authorization"))
  console.log( ctx.protocol.headers) // prints e.g. { "Content-Type": ["application/json"],"Accept":["application/json","text/plain"] } 
})

Outbound headers and the HTTP status code can be modified when the function uses the json request format.

To update the outbound status-code set ctx.protocol.statusCode. To modify outbound headers use ctx.protocol.setHeader(k,v) or ctx.protocol.addHeader(k,v) which set (overwriting existing headers) or add (preserving existing headers) headers to the response respectively.

var fdk = require('@fnproject/fdk');

fdk.handle(function(input, ctx) {
   ctx.protocol.setHeader("Location", "http://example.com")
   ctx.protocol.statusCode = 302
})

Fn and Node.js Dependencies

Fn handles Node.js dependencies in the following way:

  • If a package.json is present without a node_modules directory, an Fn build runs an npm install within the build process and installs your dependencies.
  • If the node_modules is present, Fn assumes you have provided the dependencies yourself and no installation is performed.
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