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Hello World - Haskell sample

A simple web app written in Haskell 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.

Create a new file named stack.yaml and paste the following code:

flags: {}
packages:
- .
extra-deps: []
resolver: lts-10.7

Create a new file named package.yaml and paste the following code

name:                helloworld-haskell
version:             0.1.0.0
dependencies:
- base >= 4.7 && < 5
- scotty
- text

executables:
  helloworld-haskell-exe:
    main:                Main.hs
    source-dirs:         app
    ghc-options:
    - -threaded
    - -rtsopts
    - -with-rtsopts=-N

Create a app folder, then create a new file named Main.hs in that folder and paste the following code. This code creates a basic web server which listens on port 8080:

{-# LANGUAGE OverloadedStrings #-}

import           Data.Maybe
import           Data.Monoid        ((<>))
import           Data.Text.Lazy     (Text)
import           Data.Text.Lazy
import           System.Environment (lookupEnv)
import           Web.Scotty         (ActionM, ScottyM, scotty)
import           Web.Scotty.Trans

main :: IO ()
main = do
  t <- fromMaybe "NOT SPECIFIED" <$> lookupEnv "TARGET"
  scotty 8080 (route t)

route :: String -> ScottyM()
route t = get "/" $ hello t

hello :: String -> ActionM()
hello t = text $ pack ("Hello world: " ++ t)

In your project directory, create a file named Dockerfile and copy the code block below into it.

# Use the existing Haskell image as our base
FROM haskell:8.2.2 as builder

# Checkout our code onto the Docker container
WORKDIR /app
ADD . /app

# Build and test our code, then install the “helloworld-haskell-exe” executable
RUN stack setup
RUN stack build --copy-bins

# Copy the "helloworld-haskell-exe" executable to the image using docker multi stage build
FROM fpco/haskell-scratch:integer-gmp
WORKDIR /root/
COPY --from=builder /root/.local/bin/helloworld-haskell-exe .

# Expose a port to run our application
EXPOSE 8080

# Run the server command
CMD ["./helloworld-haskell-exe"]

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-haskell
  namespace: default
spec:
  runLatest:
    configuration:
      revisionTemplate:
        spec:
          container:
            image: docker.io/{username}/helloworld-haskell
            env:
            - name: TARGET
              value: "Haskell Sample v1"

Build and deploy this 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, enter these commands replacing {username} with your Docker Hub username:

# Build the container on your local machine
docker build -t {username}/helloworld-haskell .

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

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).

To find the IP address for your service, enter kubectl get svc knative-ingressgateway -n istio-system to get the ingress IP for your cluster. If your cluster is new, it may take some time for the service to get assigned an external IP address.

kubectl get svc knative-ingressgateway --namespace 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

For minikube or bare-metal, get IP_ADDRESS by running the following command

echo $(kubectl get node  --output 'jsonpath={.items[0].status.addresses[0].address}'):$(kubectl get svc knative-ingressgateway --namespace istio-system   --output 'jsonpath={.spec.ports[?(@.port==80)].nodePort}')

To find the URL for your service, enter:

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

Now you can make a request to your app and see the result. Replace {IP_ADDRESS} with the address you see returned in the previous step.
```
curl -H "Host: helloworld-haskell.default.example.com" http://{IP_ADDRESS}
Hello world: Haskell Sample v1
```

Removing the sample app deployment

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

kubectl delete --filename service.yaml

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