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:

git clone -b "release-0.12" https://github.com/knative/docs knative-docs
cd knative-docs/docs/serving/samples/hello-world/helloworld-r

Before you begin

  • A Kubernetes cluster with Knative installed. Follow the installation instructions if you need to create one.
  • Docker 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:

    #!/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:

      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))
      }
      
    2. Create a new file named Dockerfile and copy the code block below into it.

      # Use the offical 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
      
      # 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 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"]
      
  2. 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.

    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:

    # Build the container on your local machine
    docker build -t {username}/helloworld-r .
    
    # Push the container to docker registry
    docker push {username}/helloworld-r
    
  2. 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:

    kubectl apply --filename service.yaml
    
  3. 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).
  4. Run the following command to find the domain URL for your service:

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

    Example:

    NAME                URL
    helloworld-r        http://helloworld-r.default.1.2.3.4.xip.io
    
  5. 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.

    curl http://helloworld-r.default.1.2.3.4.xip.io
    

    Example:

    curl http://helloworld-r.default.1.2.3.4.xip.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:

kubectl delete --filename service.yaml