# Hello World - R

A simple web app that executes an R script. The R script reads an env
variable `TARGET` and prints `Hello ${TARGET}!`. If the `TARGET` environment
variable is not specified, the script uses `World`.

Follow the steps below to create the sample code and then deploy the app to your
cluster. You can also download a working copy of the sample, by running the
following commands:

```bash
git clone https://github.com/knative/docs.git knative-docs
cd knative-docs/code-samples/community/serving/hello-world/helloworld-r
```

## Before you begin

- A Kubernetes cluster with Knative installed. Follow the
  [Knative installation instructions](https://knative.dev/docs/install/) if you need to
  create one.
- [Docker](https://www.docker.com) installed and running on your local machine,
  and a Docker Hub account configured (we'll use it for a container registry).

## Recreating the sample code

1. Create a new file named `HelloWorld.R` and paste the following script:

   ```R
   #!/usr/bin/Rscript
   TARGET <- Sys.getenv("TARGET", "World")

   message = paste("Hello ", TARGET, "!", sep = "")
   print(message)
   ```

1. Create a new file named `invoke.go` and paste the following code. We use a
  basic web server written in Go to execute the shell script:

  ```go
  package main

  import (
     "fmt"
     "log"
     "net/http"
     "os"
     "os/exec"
  )

  func handler(w http.ResponseWriter, r *http.Request) {
     cmd := exec.CommandContext(r.Context(), "Rscript", "HelloWorld.R")
     cmd.Stderr = os.Stderr
     out, err := cmd.Output()
     if err != nil {
         w.WriteHeader(500)
     }
     w.Write(out)
  }

  func main() {
     http.HandleFunc("/", handler)

     port := os.Getenv("PORT")
     if port == "" {
         port = "8080"
     }

     log.Fatal(http.ListenAndServe(fmt.Sprintf(":%s", port), nil))
  }
  ```

1. Create a new file named `Dockerfile` and copy the code block below into it.

  ```docker
  # Use the official Golang image to create a build artifact.
  # This is based on Debian and sets the GOPATH to /go.
  # https://hub.docker.com/_/golang
  FROM golang:1.12 as builder

  ARG TARGETOS
  ARG TARGETARCH

  # Copy local code to the container image.
  WORKDIR /go/src/github.com/knative/docs/helloworld-r
  COPY invoke.go .

  # Build the command inside the container.
  # (You may fetch or manage dependencies here,
  # either manually or with a tool like "godep".)
  RUN CGO_ENABLED=0 GOOS=linux GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -v -o invoke

  # The official R base image
  # https://hub.docker.com/_/r-base
  # Use a Docker multi-stage build to create a lean production image.
  # https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
  FROM r-base:3.6.0

  # Copy Go binary
  COPY --from=builder /go/src/github.com/knative/docs/helloworld-r/invoke /invoke
  COPY HelloWorld.R .

  # Run the web service on container startup.
  CMD ["/invoke"]
  ```


1. Create a new file, `service.yaml` and copy the following service definition
   into the file. Make sure to replace `{username}` with your Docker Hub
   username.

   ```yaml
   apiVersion: serving.knative.dev/v1
   kind: Service
   metadata:
     name: helloworld-r
     namespace: default
   spec:
     template:
       spec:
         containers:
           - image: docker.io/{username}/helloworld-r
             env:
               - name: TARGET
                 value: "R Sample v1"
   ```

## Building and deploying the sample

Once you have recreated the sample code files (or used the files in the sample
folder) you're ready to build and deploy the sample app.

1. Use Docker to build the sample code into a container. To build and push with
   Docker Hub, run these commands replacing `{username}` with your Docker Hub
   username:

   ```bash
   # Build and push the container on your local machine.
   docker buildx build --platform linux/arm64,linux/amd64 -t "{username}/helloworld-r" --push .
   ```

1. After the build has completed and the container is pushed to docker hub, you
   can deploy the app into your cluster. Ensure that the container image value
   in `service.yaml` matches the container you built in the previous step. Apply
   the configuration using `kubectl`:

   ```bash
   kubectl apply --filename service.yaml
   ```

1. Now that your service is created, Knative performs the following steps:

   - Create a new immutable revision for this version of the app.
   - Network programming to create a route, ingress, service, and load balance
     for your app.
   - Automatically scale your pods up and down (including to zero active pods).

1. Run the following command to find the domain URL for your service:

   ```bash
   kubectl get ksvc helloworld-r  --output=custom-columns=NAME:.metadata.name,URL:.status.url
   ```

   Example:

   ```bash
   NAME                URL
   helloworld-r        http://helloworld-r.default.1.2.3.4.sslip.io
   ```

1. Now you can make a request to your app and see the result. Replace
   the URL below with the URL returned in the previous command.

   ```bash
   curl http://helloworld-r.default.1.2.3.4.sslip.io
   ```

   Example:

   ```bash
   curl http://helloworld-r.default.1.2.3.4.sslip.io
   [1] "Hello R Sample v1!"
   ```

   > Note: Add `-v` option to get more detail if the `curl` command failed.

## Removing the sample app deployment

To remove the sample app from your cluster, delete the service record:

```bash
kubectl delete --filename service.yaml
```
