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Hello World - Go sample

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

Prerequisites

  • 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

While you can clone all of the code from this directory, hello world apps are generally more useful if you build them step-by-step. The following instructions recreate the source files from this folder.

  1. 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 (
  "flag"
  "fmt"
  "log"
  "net/http"
  "os"
)

func handler(w http.ResponseWriter, r *http.Request) {
  log.Print("Hello world received a request.")
  target := os.Getenv("TARGET")
  if target == "" {
    target = "NOT SPECIFIED"
  }
  fmt.Fprintf(w, "Hello World: %s!\n", target)
}

func main() {
  flag.Parse()
  log.Print("Hello world sample started.")

  http.HandleFunc("/", handler)
  http.ListenAndServe(":8080", nil)
}

  2. 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.

    # Start from a Debian image with the latest version of Go installed
# and a workspace (GOPATH) configured at /go.
FROM golang

# Copy the local package files to the container's workspace.
ADD . /go/src/github.com/knative/docs/helloworld

# Build the helloworld command inside the container.
# (You may fetch or manage dependencies here,
# either manually or with a tool like "godep".)
RUN go install github.com/knative/docs/helloworld

# Run the helloworld command by default when the container starts.
ENTRYPOINT /go/bin/helloworld

# Document that the service listens on port 8080.
EXPOSE 8080

  3. 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/v1alpha1
kind: Service
metadata:
  name: helloworld-go
  namespace: default
spec:
  runLatest:
    configuration:
      revisionTemplate:
        spec:
          container:
            image: docker.io/{username}/helloworld-go
            env:
            - name: TARGET
              value: "Go 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-go .

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

  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 -f service.yaml

  3. 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).

  4. To find the IP address for your service, use kubectl get svc knative-ingressgateway -n istio-system to get the ingress IP for your cluster. If your cluster is new, it may take sometime for the service to get asssigned an external IP address.

    kubectl get svc knative-ingressgateway -n istio-system

NAME                     TYPE           CLUSTER-IP     EXTERNAL-IP      PORT(S)                                      AGE
knative-ingressgateway   LoadBalancer   10.23.247.74   35.203.155.229   80:32380/TCP,443:32390/TCP,32400:32400/TCP   2d

  5. To find the URL for your service, use

    kubectl get services.serving.knative.dev helloworld-go  -o=custom-columns=NAME:.metadata.name,DOMAIN:.status.domain
NAME                DOMAIN
helloworld-go       helloworld-go.default.example.com

  6. Now you can make a request to your app to see the results. Replace {IP_ADDRESS} with the address you see returned in the previous step.

    curl -H "Host: helloworld-go.default.example.com" http://{IP_ADDRESS}
Hello World: Go Sample v1!

Removing the sample app deployment

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

kubectl delete -f service.yaml