Hello World - Go

Hello World - Go

A simple web app written in Go that you can use for testing. It reads in an env variable TARGET and prints Hello ${TARGET}!. If TARGET is not specified, it will use World as the TARGET.

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 "{{< branch >}}" https://github.com/knative/docs knative-docs
cd knative-docs/docs/serving/samples/hello-world/helloworld-go

Before you begin

A Kubernetes cluster with Knative installed and DNS configured. 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

Create a new file named helloworld.go and paste the following code. This code creates a basic web server which listens on port 8080:

package main
import (
  "fmt"
  "log"
  "net/http"
  "os"
)
func handler(w http.ResponseWriter, r *http.Request) {
  log.Print("helloworld: received a request")
  target := os.Getenv("TARGET")
  if target == "" {
    target = "World"
  }
  fmt.Fprintf(w, "Hello %s!\n", target)
}
func main() {
  log.Print("helloworld: starting server...")
  http.HandleFunc("/", handler)
  port := os.Getenv("PORT")
  if port == "" {
    port = "8080"
  }
  log.Printf("helloworld: listening on port %s", port)
  log.Fatal(http.ListenAndServe(fmt.Sprintf(":%s", port), nil))
}

In your project directory, create a file named Dockerfile and copy the code block below into it. For detailed instructions on dockerizing a Go app, see Deploying Go servers with 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:1.13 as builder
# Create and change to the app directory.
WORKDIR /app
# Retrieve application dependencies using go modules.
# Allows container builds to reuse downloaded dependencies.
COPY go.* ./
RUN go mod download
# Copy local code to the container image.
COPY . ./
# Build the binary.
# -mod=readonly ensures immutable go.mod and go.sum in container builds.
RUN CGO_ENABLED=0 GOOS=linux go build -mod=readonly -v -o server
# Use the official Alpine image for a lean production container.
# https://hub.docker.com/_/alpine
# https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
FROM alpine:3
RUN apk add --no-cache ca-certificates
# Copy the binary to the production image from the builder stage.
COPY --from=builder /app/server /server
# Run the web service on container startup.
CMD ["/server"]

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-go
  namespace: default
spec:
  template:
    spec:
      containers:
        - image: docker.io/{username}/helloworld-go
          env:
            - name: TARGET
              value: "Go Sample v1"

Use the go tool to create a go.mod manifest.

go mod init github.com/knative/docs/docs/serving/samples/hello-world/helloworld-go

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.

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-go .
# Push the container to docker registry
docker push {username}/helloworld-go

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
```
Now that your service is created, Knative will perform 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).

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

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

Example:

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

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-go.default.1.2.3.4.xip.io
Hello Go 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