Introduction to Fn with Ruby

Fn is a lightweight Docker-based serverless functions platform you can run on your laptop, server, or cloud. In this introductory tutorial we’ll walk through developing a function using the Ruby programming language (without installing any Ruby tools!) and deploying that function to a local Fn server. We’ll also learn about the core Fn concepts like applications and triggers.

Before you Begin

  • Set aside about 15 minutes to complete this tutorial.
  • Make sure Fn server is up and running by completing the Install and Start Fn Tutorial.
    • Make sure you have set your Fn context registry value for local development. (for example, “fndemouser”. See here.)

As you make your way through this tutorial, look out for this icon. Whenever you see it, it’s time for you to perform an action.

Your First Function

Now that Fn server is up and running, let’s start with a very simple “hello world” function written in Ruby. Don’t worry, you don’t need to know Ruby! as Fn provides the necessary Ruby compiler and tools as a Docker container. Let’s walk through your first function to become familiar with the process and how Fn supports development.

Create your Function

In the terminal type the following.

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fn init --runtime ruby --trigger http rubyfn

The output will be

Creating function at: /rubyfn
Function boilerplate generated.
func.yaml created.

The fn init command creates an simple function with a bit of boilerplate to get you started. The --runtime option is used to indicate that the function we’re going to develop is written in Ruby. A number of other runtimes are also supported. The --trigger option creates a URL for HTTP access to the function. Fn creates the simple function along with several supporting files in the /rubyfn directory.

Review your Function File

With your function created change into the /rubyfn directory.

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cd rubyfn

Now get a list of the directory contents.

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 Gemfile func.rb func.yaml

The func.rb file which contains your actual Ruby function is generated along with several supporting files. To view your Ruby function type:

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cat func.rb
require 'fdk'

def myfunction(context:, input:)
  input_value = input.respond_to?(:fetch) ? input.fetch('name') : input
  name = input_value.to_s.strip.empty? ? 'World' : input_value
  { message: "Hello #{name}!" }

FDK.handle(target: :myfunction)

This function looks for JSON input in the form of {"name": "Bob"} If this JSON example is passed to the function, the function returns {"message":"Hello Bob!"}. If no JSON data is found, or the name field is missing or empty, the function returns {"message":"Hello World!"}.

Understand func.yaml

The fn init command generated a func.yaml function configuration file. Let’s look at the contents:

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cat func.yaml
schema_version: 20180708
name: rubyfn
version: 0.0.1
runtime: ruby
entrypoint: ruby func.rb
- name: rubyfn-trigger
  type: http
  source: /rubyfn-trigger

The generated func.yaml file contains metadata about your function and declares a number of properties including:

  • schema_version–identifies the version of the schema for this function file. Essentially, it determines which fields are present in func.yaml.
  • name–the name of the function. Matches the directory name.
  • version–automatically starting at 0.0.1.
  • runtime–the name of the runtime/language which was set based on the value set in --runtime.
  • entrypoint–the name of the executable to invoke when your function is called, in this case ruby func.rb.
  • triggers–identifies the automatically generated trigger name and source. For example, this function would be executed from the URL

http://localhost:8080/t/appname/rubyfn-trigger. Where appname is the name of the app chosen for your function when it is deployed.

There are other user specifiable properties but these will suffice for this example. Note that the name of your function is taken from the containing folder name. We’ll see this come into play later on.

Other Function Files

The fn init command generated one other file.

  • Gemfile – specifies all the dependencies for your Ruby function.

Deploy Your First Function

With the rubyfn directory containing func.rb and func.yaml you’ve got everything you need to deploy the function to Fn server. This server could be running in the cloud, in your datacenter, or on your local machine like we’re doing here.

Check your Context

Make sure your context is set to default and you are using a demo user. Use the fn list context command to check.

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fn list contexts
*       default	default		http://localhost:8080	fndemouser

If your context is not configured, please see the context installation instructions before proceeding. Your context determines where your function is deployed.

Create an App

Next, functions are grouped together into an application. The application acts as the main organizing structure for multiple functions. To create an application type the following:

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fn create app rubyapp

A confirmation is returned:

Successfully created app:  rubyapp

Now rubyapp is ready for functions to be deployed to it.

Deploy your Function to your App

Deploying your function is how you publish your function and make it accessible to other users and systems. To see the details of what is happening during a function deploy, use the --verbose switch. The first time you build a function of a particular language it takes longer as Fn downloads the necessary Docker images. The --verbose option allows you to see this process.

In your terminal type the following:

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fn --verbose deploy --app rubyapp --local

You should see output similar to:

Deploying rubyfn to app: rubyapp
Bumped to version 0.0.2
Building image fndemouser/rubyfn:0.0.2 
FN_REGISTRY:  fndemouser
Current Context:  default
Sending build context to Docker daemon   5.12kB
Step 1/9 : FROM fnproject/ruby:dev as build-stage
dev: Pulling from fnproject/ruby
cd784148e348: Already exists 
dc557ec385e9: Pull complete 
1da25603bb9b: Pull complete 
4b4f9165fe22: Pull complete 
cb19460a63ec: Pull complete 
Digest: sha256:1d810dd09525d7d5c7c98b60e8138bfda26dc227c68e8861a34c5fb3a1918ac8
Status: Downloaded newer image for fnproject/ruby:dev
 ---> 8f015ef7f730
Step 2/9 : WORKDIR /function
 ---> Running in 05a9394fd0c3
Removing intermediate container 05a9394fd0c3
 ---> a6ede4bf3e4a
Step 3/9 : ADD Gemfile* /function/
 ---> be0ac9361ac2
Step 4/9 : RUN bundle install
 ---> Running in 7cf6c166509e
Don't run Bundler as root. Bundler can ask for sudo if it is needed, and
installing your bundle as root will break this application for all non-root
users on this machine.
Fetching gem metadata from https://rubygems.org/..
Resolving dependencies...
Using bundler 2.0.1
Fetching json 2.1.0
Installing json 2.1.0 with native extensions
Fetching webrick 1.4.2
Installing webrick 1.4.2
Fetching fdk 0.0.18
Installing fdk 0.0.18
Bundle complete! 1 Gemfile dependency, 4 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
Removing intermediate container 7cf6c166509e
 ---> a40fd74b7b33
Step 5/9 : FROM fnproject/ruby
latest: Pulling from fnproject/ruby
cd784148e348: Already exists 
dc557ec385e9: Already exists 
1da25603bb9b: Already exists 
Digest: sha256:c7a22b37d040361c43d91ec11a86a787454bea9f6ff58840ab6c6187c0e7417d
Status: Downloaded newer image for fnproject/ruby:latest
 ---> b8ffc8f5dd00
Step 6/9 : WORKDIR /function
 ---> Running in a102b7cb5e02
Removing intermediate container a102b7cb5e02
 ---> c585eaa8237c
Step 7/9 : COPY --from=build-stage /usr/lib/ruby/gems/ /usr/lib/ruby/gems/
 ---> c35bc61d347b
Step 8/9 : ADD . /function/
 ---> 8e27331240a0
Step 9/9 : ENTRYPOINT ["ruby", "func.rb"]
 ---> Running in aa073542028b
Removing intermediate container aa073542028b
 ---> 661255862190
Successfully built 661255862190
Successfully tagged fndemouser/rubyfn:0.0.2

Updating function rubyfn using image fndemouser/rubyfn:0.0.2...
Successfully created function: rubyfn with fndemouser/rubyfn:0.0.2
Successfully created trigger: rubyfn-trigger
Trigger Endpoint: http://localhost:8080/t/rubyapp/rubyfn-trigger

All the steps to load the current language Docker image are displayed.

Specifying --app rubyapp explicitly puts the function in the application rubyapp.

Specifying --local does the deployment to the local server but does not push the function image to a Docker registry–which would be necessary if we were deploying to a remote Fn server.

The output message Updating function rubyfn using image fndemouser/rubyfn:0.0.2... let’s us know that the function packaged in the image fndemouser/rubyfn:0.0.2.

Note that the containing folder name rubyfn was used as the name of the generated Docker container and used as the name of the function that container was bound to. By convention it is also used to create the trigger name rubyfn-trigger.

Normally you deploy an application without the --verbose option. If you rerun the command a new image and version is created and loaded.

Invoke your Deployed Function

There are two ways to call your deployed function.

Invoke with the CLI

The first is using the Fn CLI which makes invoking your function relatively easy. Type the following:

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fn invoke rubyapp rubyfn

which results in:

{"message":"Hello World!"}

When you invoked rubyapp rubyfn the Fn server looked up the rubyapp application and then looked for the Docker container image bound to the rubyfn function and executed the code. Fn invoke invokes your function directly and independently of any associated triggers. You can always invoke a function even without it having any triggers bound to it. Note the content type was specified as well.

You can also pass data to the invoke command. For example:

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echo -n '{"name":"Bob"}' | fn invoke rubyapp rubyfn --content-type application/json
{"message":"Hello Bob!"}

The JSON data was parsed and since name was set to Bob, that value is passed in the output.

Understand Fn deploy

If you have used Docker before the output of fn --verbose deploy should look familiar–it looks like the output you see when running docker build with a Dockerfile. Of course this is exactly what’s happening! When you deploy a function like this Fn is dynamically generating a Dockerfile for your function, building a container, and then loading it for execution.

NOTE: Fn is actually using two images. The first contains the language interpreter and all the necessary build tools. The second image packages all dependencies and any necessary language runtime components. Using this strategy, the final function image size can be kept as small as possible. Smaller Docker images are naturally faster to push and pull from a repository which improves overall performance. For more details on this technique see Multi-Stage Docker Builds for Creating Tiny Go Images.

When using fn deploy --local, Fn server builds and packages your function into a container image which resides on your local machine.

As Fn is built on Docker you can use the docker command to see the local container image you just generated. You may have a number of Docker images so use the following command to see only those created by fndemouser:

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docker images | grep fndemouser

You should see something like:

fndemouser/rubyfn    0.0.2               d0d71f5a2a23        11 minutes ago      63.9MB

Explore your Application

The Fn CLI provides a couple of commands to let us see what we’ve deployed. fn list apps returns a list of all of the defined applications.

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fn list apps

Which, in our case, returns the name of the application we created when we deployed our rubyfn function:

rubyapp		01CT433P8VNG8G00GZJ000001A

We can also see the functions that are defined by an application. Since functions are exposed via triggers, the fn list triggers <appname> command is used. To list the functions included in “rubyapp” we can type:

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fn list triggers rubyapp
FUNCTION    NAME           ID                           TYPE  SOURCE          ENDPOINT
rubyfn		rubyfn-trigger 01D46F1SZANG8G00GZJ0000003	http  /rubyfn-trigger http://localhost:8080/t/rubyapp/rubyfn-trigger

The output confirms that rubyapp contains a rubyfn function that is implemented by the Docker container fndemouser/rubyfn:0.0.2 which may be invoked via the specified URL.

Invoke with Curl

The other way to invoke your function is via HTTP. The Fn server exposes our deployed function at http://localhost:8080/t/rubyapp/rubyfn-trigger, a URL that incorporates our application and function trigger as path elements.

Use curl to invoke the function:

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curl -H "Content-Type: application/json" http://localhost:8080/t/rubyapp/rubyfn-trigger

The result is once again the same.

{"message":"Hello World!"}

We can again pass JSON data to our function and get the value of name passed to the function back.

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curl -H "Content-Type: application/json" -d '{"name":"Bob"}' http://localhost:8080/t/rubyapp/rubyfn-trigger

The result is once again the same.

{"message":"Hello Bob!"}

Wrap Up

Congratulations! In this tutorial you’ve accomplished a lot. You’ve created your first function, deployed it to your local Fn server and invoked it over HTTP.

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